Multi-Threading blocks

[supervoidcoder]

Multi Threading blocks are blocks that run in a separate thread using Web Workers. This feature should:

• be abstracted so that kids can use it just fine
• be compatible with both the Scratch Interpreter and Compiler
• not break existing projects (we aren't removing anything)
• hopefully make stuff faster or something
• we can use broadcast-esque blocks as an abstraction around postmessage

[traycerai]

Plan

Observations

The scratch-vm codebase has a sophisticated architecture with both an interpreter and compiler for executing Scratch blocks. Key findings:

1. Existing Worker Infrastructure: The codebase already has a robust Web Worker system for extensions using worker-loader, central-dispatch.js, worker-dispatch.js, and shared-dispatch.js for cross-worker communication via postMessage.

2. Dual Execution Modes: Blocks can execute via interpreter (file:src/engine/execute.js, file:src/engine/sequencer.js) or compiler (file:src/compiler/jsgen.js, file:src/compiler/jsexecute.js).

3. Broadcast Mechanism: The existing broadcast system (file:src/blocks/scratch3_event.js) uses runtime.startHats() to trigger hat blocks and supports broadcastAndWait which waits for spawned threads to complete.

4. Thread Management: Threads are managed by file:src/engine/thread.js and file:src/engine/sequencer.js, with support for yielding, warp mode, and status tracking.

Approach

The implementation will leverage the existing extension worker infrastructure to create multi-threading blocks. The approach uses broadcast-style blocks as an abstraction layer over Web Workers, making it kid-friendly while maintaining compatibility with both interpreter and compiler modes. New blocks will be added as a built-in extension that spawns worker threads, communicates via the existing dispatch system, and integrates seamlessly with the current execution model without breaking existing projects.

Implementation Steps

1. Create Multi-Threading Extension Structure

Create a new built-in extension at file:src/extensions/scratch3_multithreading/index.js that defines the multi-threading blocks:

• Block Types to Implement:

  • multithreading_runInWorker (command block): Start a script in a worker thread
  • multithreading_runInWorkerAndWait (command block): Start a script and wait for completion
  • multithreading_sendMessage (command block): Send data to worker thread
  • multithreading_whenMessageReceived (hat block): Trigger when message received from main/worker
  • multithreading_receivedMessage (reporter block): Get the last received message data
  • multithreading_isWorkerRunning (boolean block): Check if worker thread is active
  • multithreading_stopWorker (command block): Terminate a worker thread

• Use broadcast-esque naming and behavior to make it intuitive for kids

• Register with block type definitions similar to file:src/blocks/scratch3_event.js

2. Implement Worker Thread Manager

Create file:src/extensions/scratch3_multithreading/worker-manager.js to handle worker lifecycle:

• Maintain a registry of active worker threads (Map of worker ID to Worker instance)
• Create worker threads using the existing worker infrastructure pattern from file:src/extension-support/extension-worker.js
• Implement worker creation that loads a specialized script execution worker
• Handle worker termination and cleanup
• Track worker state (running, idle, terminated)
• Implement message queuing for communication between main thread and workers

3. Create Script Execution Worker

Create file:src/extensions/scratch3_multithreading/script-worker.js as the worker entry point:

• Initialize a minimal runtime environment within the worker
• Load necessary VM components (blocks, compiler, interpreter)
• Set up dispatch communication using file:src/dispatch/worker-dispatch.js pattern
• Implement script execution in worker context:
  • Parse and compile/interpret block scripts
  • Execute scripts independently from main thread
  • Handle variable scope (worker-local vs shared)
• Implement message passing handlers for communication with main thread
• Handle worker-side hat block triggering when messages received

4. Integrate with Dispatch System

Extend the existing dispatch system for worker-to-main communication:

• Register multi-threading service in file:src/dispatch/central-dispatch.js
• Implement message routing between main thread and worker threads
• Use the existing postMessage infrastructure from file:src/dispatch/shared-dispatch.js
• Create service methods for:
  • Starting scripts in workers
  • Sending messages to workers
  • Receiving messages from workers
  • Querying worker status
• Ensure proper serialization of Scratch values (numbers, strings, lists)

5. Implement Interpreter Support

Add interpreter execution support in the extension:

• Implement block primitives in file:src/extensions/scratch3_multithreading/index.js that work with file:src/engine/execute.js
• Handle runInWorker block:
  • Extract the script/procedure to run
  • Serialize block data
  • Send to worker via dispatch
  • Return immediately (non-blocking)
• Handle runInWorkerAndWait block:
  • Similar to broadcastAndWait in file:src/blocks/scratch3_event.js
  • Track started worker threads
  • Use util.yield() and util.yieldTick() for waiting
  • Check worker completion status each iteration
• Implement message sending/receiving with proper thread synchronization

6. Implement Compiler Support

Add compiler support for multi-threading blocks:

• Add opcodes to file:src/compiler/enums.js:
  • MULTITHREADING_RUN_IN_WORKER
  • MULTITHREADING_RUN_IN_WORKER_AND_WAIT
  • MULTITHREADING_SEND_MESSAGE
  • MULTITHREADING_RECEIVED_MESSAGE
• Implement IR generation in file:src/compiler/irgen.js for multi-threading blocks
• Add JavaScript generation in file:src/compiler/jsgen.js:
  • Generate code that calls runtime functions for worker management
  • For runInWorkerAndWait, generate yield* pattern similar to broadcast
  • Ensure proper yielding and thread waiting
• Add runtime functions in file:src/compiler/jsexecute.js:
  • startWorkerScript() - initiate worker execution
  • waitWorker() - generator function for waiting on worker completion
  • sendWorkerMessage() - send message to worker
  • getWorkerMessage() - retrieve received message

7. Implement Variable Scoping

Design and implement variable scoping for worker threads:

• Worker-Local Variables: Each worker has its own variable scope
• Shared Variables: Implement optional shared variable mechanism
  • Use SharedArrayBuffer for numeric variables (if available)
  • Use message passing for other types
  • Add blocks for marking variables as shared
• Handle variable synchronization challenges:
  • Implement copy-on-write semantics for safety
  • Provide explicit sync blocks if needed
• Ensure lists and variables are properly isolated by default

8. Add Hat Block Support

Implement hat block triggering in worker threads:

• Modify file:src/engine/runtime.js to support worker-context hat blocks
• When whenMessageReceived hat is triggered:
  • In main thread: start threads normally via runtime.startHats()
  • In worker: implement worker-local hat triggering
• Store received message data in thread context for reporter block access
• Ensure hat blocks work in both interpreter and compiler modes

9. Handle Thread Lifecycle and Cleanup

Implement proper lifecycle management:

• Track worker threads in file:src/engine/runtime.js alongside regular threads
• Implement cleanup when:
  • Project stops (green flag stop)
  • Worker explicitly stopped
  • Worker script completes
  • Project unloaded
• Prevent memory leaks by properly terminating workers
• Handle worker errors gracefully:
  • Catch worker exceptions
  • Report errors to main thread
  • Provide error reporter blocks

10. Register Extension in VM

Integrate the extension into the VM:

• Add to file:src/index.js exports if needed
• Register in file:src/extension-support/extension-manager.js as a built-in extension
• Add to the default built-in extensions list
• Ensure it loads automatically with the VM
• Add to compiler's extension registry in file:src/engine/runtime.js

11. Implement Safety and Limitations

Add safety measures for kid-friendly usage:

• Limit maximum number of concurrent workers (e.g., 4-8)
• Implement worker timeout mechanisms
• Prevent infinite loops from blocking workers
• Add memory usage monitoring if possible
• Provide clear error messages for common issues
• Ensure workers can't access sensitive APIs

12. Testing and Validation

Create comprehensive tests:

• Test interpreter mode execution
• Test compiler mode execution
• Test message passing in both directions
• Test worker lifecycle (start, stop, cleanup)
• Test with multiple concurrent workers
• Test variable scoping and isolation
• Test hat block triggering
• Test compatibility with existing projects (ensure nothing breaks)
• Test performance improvements for parallelizable tasks

Architecture Diagram

sequenceDiagram
    participant User as Scratch Blocks
    participant Runtime as Runtime/Sequencer
    participant MTExt as MultiThreading Extension
    participant WM as Worker Manager
    participant Dispatch as Dispatch System
    participant Worker as Script Worker
    participant WorkerRT as Worker Runtime

    User->>Runtime: Execute "run in worker" block
    Runtime->>MTExt: Call block primitive
    MTExt->>WM: Create/get worker thread
    WM->>Worker: Spawn new Worker
    Worker->>WorkerRT: Initialize mini-runtime
    WorkerRT->>Dispatch: Register worker service
    MTExt->>Dispatch: Send script to worker
    Dispatch->>Worker: postMessage(script)
    Worker->>WorkerRT: Execute script
    
    Note over User,WorkerRT: Message Passing
    User->>Runtime: Execute "send message" block
    Runtime->>MTExt: Send message
    MTExt->>Dispatch: Route message
    Dispatch->>Worker: postMessage(data)
    Worker->>WorkerRT: Trigger "when message received" hat
    WorkerRT->>WorkerRT: Execute hat script
    
    Note over User,WorkerRT: Wait for Completion
    User->>Runtime: Execute "run in worker and wait"
    Runtime->>MTExt: Start worker and wait
    MTExt->>WM: Track worker thread
    loop Check completion
        Runtime->>MTExt: Check if worker done
        MTExt->>WM: Query worker status
        WM-->>MTExt: Status
        MTExt-->>Runtime: Yield/Continue
    end
    Worker->>Dispatch: Script complete
    Dispatch->>MTExt: Worker finished
    MTExt->>Runtime: Resume execution

Loading

Block Design Examples

Block	Type	Description
run [SCRIPT] in worker thread [WORKER_ID]	Command	Starts a script/procedure in a worker thread
run [SCRIPT] in worker [WORKER_ID] and wait	Command	Starts script and waits for completion
send [MESSAGE] to worker [WORKER_ID]	Command	Sends data to worker thread
when I receive message from [SOURCE]	Hat	Triggers when message received
received message	Reporter	Returns the last received message data
worker [WORKER_ID] is running?	Boolean	Checks if worker is active
stop worker [WORKER_ID]	Command	Terminates a worker thread

Key Considerations

• Backward Compatibility: Extension is additive only; existing projects unaffected
• Abstraction Level: Blocks use familiar broadcast-like terminology
• Performance: Actual performance gains depend on workload parallelizability
• Browser Support: Web Workers are widely supported; graceful degradation if unavailable
• Serialization: Only serializable data (numbers, strings, arrays) can pass between threads
• Debugging: Provide clear error messages and status indicators

Import In IDE

🤖 Prompt for AI Agents

## Observations

The scratch-vm codebase has a sophisticated architecture with both an interpreter and compiler for executing Scratch blocks. Key findings:

1. **Existing Worker Infrastructure**: The codebase already has a robust Web Worker system for extensions using `worker-loader`, `central-dispatch.js`, `worker-dispatch.js`, and `shared-dispatch.js` for cross-worker communication via `postMessage`.

2. **Dual Execution Modes**: Blocks can execute via interpreter (`file:src/engine/execute.js`, `file:src/engine/sequencer.js`) or compiler (`file:src/compiler/jsgen.js`, `file:src/compiler/jsexecute.js`).

3. **Broadcast Mechanism**: The existing broadcast system (`file:src/blocks/scratch3_event.js`) uses `runtime.startHats()` to trigger hat blocks and supports `broadcastAndWait` which waits for spawned threads to complete.

4. **Thread Management**: Threads are managed by `file:src/engine/thread.js` and `file:src/engine/sequencer.js`, with support for yielding, warp mode, and status tracking.

## Approach

The implementation will leverage the existing extension worker infrastructure to create multi-threading blocks. The approach uses broadcast-style blocks as an abstraction layer over Web Workers, making it kid-friendly while maintaining compatibility with both interpreter and compiler modes. New blocks will be added as a built-in extension that spawns worker threads, communicates via the existing dispatch system, and integrates seamlessly with the current execution model without breaking existing projects.

## Implementation Steps

### 1. Create Multi-Threading Extension Structure

Create a new built-in extension at `file:src/extensions/scratch3_multithreading/index.js` that defines the multi-threading blocks:

- **Block Types to Implement**:
  - `multithreading_runInWorker` (command block): Start a script in a worker thread
  - `multithreading_runInWorkerAndWait` (command block): Start a script and wait for completion
  - `multithreading_sendMessage` (command block): Send data to worker thread
  - `multithreading_whenMessageReceived` (hat block): Trigger when message received from main/worker
  - `multithreading_receivedMessage` (reporter block): Get the last received message data
  - `multithreading_isWorkerRunning` (boolean block): Check if worker thread is active
  - `multithreading_stopWorker` (command block): Terminate a worker thread

- Use broadcast-esque naming and behavior to make it intuitive for kids
- Register with block type definitions similar to `file:src/blocks/scratch3_event.js`

### 2. Implement Worker Thread Manager

Create `file:src/extensions/scratch3_multithreading/worker-manager.js` to handle worker lifecycle:

- Maintain a registry of active worker threads (Map of worker ID to Worker instance)
- Create worker threads using the existing worker infrastructure pattern from `file:src/extension-support/extension-worker.js`
- Implement worker creation that loads a specialized script execution worker
- Handle worker termination and cleanup
- Track worker state (running, idle, terminated)
- Implement message queuing for communication between main thread and workers

### 3. Create Script Execution Worker

Create `file:src/extensions/scratch3_multithreading/script-worker.js` as the worker entry point:

- Initialize a minimal runtime environment within the worker
- Load necessary VM components (blocks, compiler, interpreter)
- Set up dispatch communication using `file:src/dispatch/worker-dispatch.js` pattern
- Implement script execution in worker context:
  - Parse and compile/interpret block scripts
  - Execute scripts independently from main thread
  - Handle variable scope (worker-local vs shared)
- Implement message passing handlers for communication with main thread
- Handle worker-side hat block triggering when messages received

### 4. Integrate with Dispatch System

Extend the existing dispatch system for worker-to-main communication:

- Register multi-threading service in `file:src/dispatch/central-dispatch.js`
- Implement message routing between main thread and worker threads
- Use the existing `postMessage` infrastructure from `file:src/dispatch/shared-dispatch.js`
- Create service methods for:
  - Starting scripts in workers
  - Sending messages to workers
  - Receiving messages from workers
  - Querying worker status
- Ensure proper serialization of Scratch values (numbers, strings, lists)

### 5. Implement Interpreter Support

Add interpreter execution support in the extension:

- Implement block primitives in `file:src/extensions/scratch3_multithreading/index.js` that work with `file:src/engine/execute.js`
- Handle `runInWorker` block:
  - Extract the script/procedure to run
  - Serialize block data
  - Send to worker via dispatch
  - Return immediately (non-blocking)
- Handle `runInWorkerAndWait` block:
  - Similar to `broadcastAndWait` in `file:src/blocks/scratch3_event.js`
  - Track started worker threads
  - Use `util.yield()` and `util.yieldTick()` for waiting
  - Check worker completion status each iteration
- Implement message sending/receiving with proper thread synchronization

### 6. Implement Compiler Support

Add compiler support for multi-threading blocks:

- Add opcodes to `file:src/compiler/enums.js`:
  - `MULTITHREADING_RUN_IN_WORKER`
  - `MULTITHREADING_RUN_IN_WORKER_AND_WAIT`
  - `MULTITHREADING_SEND_MESSAGE`
  - `MULTITHREADING_RECEIVED_MESSAGE`
- Implement IR generation in `file:src/compiler/irgen.js` for multi-threading blocks
- Add JavaScript generation in `file:src/compiler/jsgen.js`:
  - Generate code that calls runtime functions for worker management
  - For `runInWorkerAndWait`, generate `yield*` pattern similar to broadcast
  - Ensure proper yielding and thread waiting
- Add runtime functions in `file:src/compiler/jsexecute.js`:
  - `startWorkerScript()` - initiate worker execution
  - `waitWorker()` - generator function for waiting on worker completion
  - `sendWorkerMessage()` - send message to worker
  - `getWorkerMessage()` - retrieve received message

### 7. Implement Variable Scoping

Design and implement variable scoping for worker threads:

- **Worker-Local Variables**: Each worker has its own variable scope
- **Shared Variables**: Implement optional shared variable mechanism
  - Use SharedArrayBuffer for numeric variables (if available)
  - Use message passing for other types
  - Add blocks for marking variables as shared
- Handle variable synchronization challenges:
  - Implement copy-on-write semantics for safety
  - Provide explicit sync blocks if needed
- Ensure lists and variables are properly isolated by default

### 8. Add Hat Block Support

Implement hat block triggering in worker threads:

- Modify `file:src/engine/runtime.js` to support worker-context hat blocks
- When `whenMessageReceived` hat is triggered:
  - In main thread: start threads normally via `runtime.startHats()`
  - In worker: implement worker-local hat triggering
- Store received message data in thread context for reporter block access
- Ensure hat blocks work in both interpreter and compiler modes

### 9. Handle Thread Lifecycle and Cleanup

Implement proper lifecycle management:

- Track worker threads in `file:src/engine/runtime.js` alongside regular threads
- Implement cleanup when:
  - Project stops (green flag stop)
  - Worker explicitly stopped
  - Worker script completes
  - Project unloaded
- Prevent memory leaks by properly terminating workers
- Handle worker errors gracefully:
  - Catch worker exceptions
  - Report errors to main thread
  - Provide error reporter blocks

### 10. Register Extension in VM

Integrate the extension into the VM:

- Add to `file:src/index.js` exports if needed
- Register in `file:src/extension-support/extension-manager.js` as a built-in extension
- Add to the default built-in extensions list
- Ensure it loads automatically with the VM
- Add to compiler's extension registry in `file:src/engine/runtime.js`

### 11. Implement Safety and Limitations

Add safety measures for kid-friendly usage:

- Limit maximum number of concurrent workers (e.g., 4-8)
- Implement worker timeout mechanisms
- Prevent infinite loops from blocking workers
- Add memory usage monitoring if possible
- Provide clear error messages for common issues
- Ensure workers can't access sensitive APIs

### 12. Testing and Validation

Create comprehensive tests:

- Test interpreter mode execution
- Test compiler mode execution
- Test message passing in both directions
- Test worker lifecycle (start, stop, cleanup)
- Test with multiple concurrent workers
- Test variable scoping and isolation
- Test hat block triggering
- Test compatibility with existing projects (ensure nothing breaks)
- Test performance improvements for parallelizable tasks

## Architecture Diagram

```mermaid
sequenceDiagram
    participant User as Scratch Blocks
    participant Runtime as Runtime/Sequencer
    participant MTExt as MultiThreading Extension
    participant WM as Worker Manager
    participant Dispatch as Dispatch System
    participant Worker as Script Worker
    participant WorkerRT as Worker Runtime

    User->>Runtime: Execute "run in worker" block
    Runtime->>MTExt: Call block primitive
    MTExt->>WM: Create/get worker thread
    WM->>Worker: Spawn new Worker
    Worker->>WorkerRT: Initialize mini-runtime
    WorkerRT->>Dispatch: Register worker service
    MTExt->>Dispatch: Send script to worker
    Dispatch->>Worker: postMessage(script)
    Worker->>WorkerRT: Execute script
    
    Note over User,WorkerRT: Message Passing
    User->>Runtime: Execute "send message" block
    Runtime->>MTExt: Send message
    MTExt->>Dispatch: Route message
    Dispatch->>Worker: postMessage(data)
    Worker->>WorkerRT: Trigger "when message received" hat
    WorkerRT->>WorkerRT: Execute hat script
    
    Note over User,WorkerRT: Wait for Completion
    User->>Runtime: Execute "run in worker and wait"
    Runtime->>MTExt: Start worker and wait
    MTExt->>WM: Track worker thread
    loop Check completion
        Runtime->>MTExt: Check if worker done
        MTExt->>WM: Query worker status
        WM-->>MTExt: Status
        MTExt-->>Runtime: Yield/Continue
    end
    Worker->>Dispatch: Script complete
    Dispatch->>MTExt: Worker finished
    MTExt->>Runtime: Resume execution
```

## Block Design Examples

| Block | Type | Description |
|-------|------|-------------|
| `run [SCRIPT] in worker thread [WORKER_ID]` | Command | Starts a script/procedure in a worker thread |
| `run [SCRIPT] in worker [WORKER_ID] and wait` | Command | Starts script and waits for completion |
| `send [MESSAGE] to worker [WORKER_ID]` | Command | Sends data to worker thread |
| `when I receive message from [SOURCE]` | Hat | Triggers when message received |
| `received message` | Reporter | Returns the last received message data |
| `worker [WORKER_ID] is running?` | Boolean | Checks if worker is active |
| `stop worker [WORKER_ID]` | Command | Terminates a worker thread |

## Key Considerations

- **Backward Compatibility**: Extension is additive only; existing projects unaffected
- **Abstraction Level**: Blocks use familiar broadcast-like terminology
- **Performance**: Actual performance gains depend on workload parallelizability
- **Browser Support**: Web Workers are widely supported; graceful degradation if unavailable
- **Serialization**: Only serializable data (numbers, strings, arrays) can pass between threads
- **Debugging**: Provide clear error messages and status indicators

---

Execution Information

Branch: develop
Commit: 89175c7

---

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[supervoidcoder]

@coderabbitai can you do a deep dive into the codebase and see how to do this?

[coderabbitai]

Tip

For best results, initiate chat on the files or code changes.

🧩 Analysis chain

@supervoidcoder I'll do a comprehensive analysis of the codebase to understand how to implement multi-threading blocks with Web Workers. Let me explore the architecture.

---

🏁 Script executed:

#!/bin/bash

# First, let's understand the repository structure
echo "=== Repository Structure ==="
fd -t f -e js -e json --max-depth 2 | head -30

echo -e "\n=== Key Configuration Files ==="
cat package.json | jq '{name, description, main, scripts}'

echo -e "\n=== Main Entry Points ==="
ls -la src/ 2>/dev/null | head -20

echo -e "\n=== Looking for Interpreter-related Files ==="
fd -t f "sequencer|execute|thread" src/

echo -e "\n=== Looking for Compiler-related Files ==="
fd -t f "compiler" src/

echo -e "\n=== Looking for Broadcast Implementation ==="
rg -l "broadcast" src/ --type js | head -10

Length of output: 2869

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🏁 Script executed:

#!/bin/bash

# Explore the engine and block execution
echo "=== Engine and VM Core Files ==="
fd -t f "engine|vm|runtime" src/ -e js

echo -e "\n=== Extension and Block System ==="
fd -t f "extension|block" src/ -e js | head -15

echo -e "\n=== Looking for Worker-related Code ==="
rg -l "Worker|worker" src/ --type js

Length of output: 1219

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🏁 Script executed:

#!/bin/bash

# Let's examine the main VM file structure
echo "=== Main VM Index ==="
cat src/index.js 2>/dev/null | head -50

echo -e "\n=== Virtual Machine Core ==="
cat src/virtual-machine.js 2>/dev/null | head -100

Length of output: 3722

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#!/bin/bash

# Examine the sequencer/thread execution
echo "=== Sequencer Implementation ==="
ls -la src/engine/

echo -e "\n=== Thread Execution ==="
fd -t f "thread" src/engine/

# Look at how threads work currently
cat src/engine/thread.js 2>/dev/null | head -100

Length of output: 4038

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🏁 Script executed:

#!/bin/bash

# Examine the runtime and how blocks are executed
echo "=== Runtime Implementation ==="
cat src/engine/runtime.js 2>/dev/null | head -150

echo -e "\n=== Sequencer Execution ==="
cat src/engine/sequencer.js 2>/dev/null | head -150

Length of output: 10930

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🏁 Script executed:

#!/bin/bash

# Look at broadcast implementation for reference
echo "=== Broadcast in Runtime ==="
rg -n -C5 "startHats.*broadcast" src/engine/runtime.js

echo -e "\n=== Broadcast Block Implementation ==="
rg -n -C10 "event_broadcast" src/blocks/

echo -e "\n=== Event Blocks ==="
cat src/blocks/scratch3_event.js 2>/dev/null | head -200

Length of output: 6276

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#!/bin/bash

# Look for compiler implementation
echo "=== Compiler Files ==="
fd -t f "compiler" . -e js

echo -e "\n=== JSGenerator/Compiler ==="
ls -la src/compiler/ 2>/dev/null

# Check if there's a jsexecute or similar
rg -l "jsexecute|IRGenerator|JSGenerator" src/ --type js

Length of output: 1191

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🏁 Script executed:

#!/bin/bash

# Examine extension system to understand how to add new blocks
echo "=== Extension Manager ==="
cat src/extension-support/extension-manager.js 2>/dev/null | head -200

echo -e "\n=== Looking at a Sample Extension ==="
ls src/extensions/ | head -10

Length of output: 8032

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#!/bin/bash

# Look at a complete extension for reference
echo "=== Sample Extension (pen) ==="
cat src/extensions/scratch3_pen/index.js 2>/dev/null | head -150

echo -e "\n=== Checking Block Types ==="
rg -n "BlockType\." src/extension-support/extension-manager.js | head -20

Length of output: 6884

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🏁 Script executed:

#!/bin/bash

# Check how execute is called and how blocks run
echo "=== Execute Method in Sequencer ==="
rg -n -C10 "\.execute\(" src/engine/sequencer.js

echo -e "\n=== Block Execution Utilities ==="
cat src/engine/execute.js 2>/dev/null | head -200

Length of output: 7957

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Let me continue exploring the worker infrastructure and compiler architecture to understand how to implement this feature.

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#!/bin/bash

# Examine the dispatch system for worker communication
echo "=== Central Dispatch ==="
cat src/dispatch/central-dispatch.js 2>/dev/null | head -200

echo -e "\n=== Worker Dispatch ==="
cat src/dispatch/worker-dispatch.js 2>/dev/null | head -200

echo -e "\n=== Shared Dispatch ==="
cat src/dispatch/shared-dispatch.js 2>/dev/null | head -150

Length of output: 17658

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#!/bin/bash

# Look at extension worker implementation
echo "=== Extension Worker ==="
cat src/extension-support/extension-worker.js 2>/dev/null | head -200

echo -e "\n=== Extension Worker Context ==="
cat src/extension-support/tw-extension-worker-context.js 2>/dev/null | head -150

Length of output: 3970

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#!/bin/bash

# Examine the compiler's IR generation
echo "=== Compiler IR Generation ==="
cat src/compiler/irgen.js 2>/dev/null | head -250

echo -e "\n=== Compiler Intermediate ==="
cat src/compiler/intermediate.js 2>/dev/null | head -150

Length of output: 13441

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#!/bin/bash

# Look at JS execution in compiler
echo "=== Compiler JS Execute ==="
cat src/compiler/jsexecute.js 2>/dev/null | head -250

Length of output: 8299

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🏁 Script executed:

#!/bin/bash

# Examine how blocks are registered and their opcodes
echo "=== Runtime Block Registration ==="
rg -n -C10 "_registerExtensionPrimitives|registerPrimitives" src/engine/runtime.js | head -100

echo -e "\n=== Block Type Definitions ==="
cat src/extension-support/block-type.js 2>/dev/null

Length of output: 2244

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🏁 Script executed:

#!/bin/bash

# Look at how threads interact with the runtime
echo "=== Runtime startHats Method ==="
rg -n -C15 "startHats.*\{" src/engine/runtime.js | head -100

echo -e "\n=== Runtime Thread Management ==="
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Let me explore the broadcast/hat mechanism and thread synchronization more deeply.

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@supervoidcoder I've completed a comprehensive deep dive into the codebase. Here's my analysis and implementation plan for multi-threading blocks using Web Workers:

Architecture Overview

The scratch-vm codebase already has excellent infrastructure for this feature:

1. Existing Worker Infrastructure

• Central/Worker Dispatch System (src/dispatch/): Handles cross-context communication via postMessage
• Extension Workers (src/extension-support/extension-worker.js): Extensions already run in workers
• Shared Dispatch (src/dispatch/shared-dispatch.js): Abstract base for communication between main thread and workers

2. Broadcast Pattern as Model

The broadcast blocks (src/blocks/scratch3_event.js) are the perfect model:

• broadcast → triggers hats immediately
• broadcastAndWait → waits for triggered threads to complete
• event_whenbroadcastreceived → hat block that responds to broadcasts

This maps perfectly to worker communication:

• "send message to worker" → postMessage to worker
• "when I receive message from worker" → hat triggered by worker response

3. Dual Execution Modes

Interpreter Mode (src/engine/execute.js, src/engine/sequencer.js):

• Executes blocks step-by-step
• Handles yields, promises, thread states

Compiler Mode (src/compiler/):

• Generates JavaScript from intermediate representation (IR)
• Compiles scripts to generator functions
• Need to add new IR opcodes for worker operations

Implementation Plan

Phase 1: Core Worker Manager

Create src/engine/worker-manager.js:

class WorkerManager {
    constructor(runtime) {
        this.runtime = runtime;
        this.workers = new Map(); // workerId -> Worker
        this.workerMessages = new Map(); // workerId -> message queue
        this.nextWorkerId = 1;
    }

    createWorker(code) {
        const workerId = this.nextWorkerId++;
        const blob = new Blob([code], { type: 'application/javascript' });
        const workerUrl = URL.createObjectURL(blob);
        const worker = new Worker(workerUrl);
        
        // Listen for messages from worker
        worker.onmessage = (e) => {
            this.handleWorkerMessage(workerId, e.data);
        };
        
        this.workers.set(workerId, worker);
        this.workerMessages.set(workerId, []);
        return workerId;
    }

    sendMessage(workerId, message, data) {
        const worker = this.workers.get(workerId);
        if (worker) {
            worker.postMessage({ message, data });
        }
    }

    handleWorkerMessage(workerId, data) {
        // Trigger hat blocks with this message
        this.runtime.startHats('multithread_whenreceived', {
            WORKER_ID: workerId,
            MESSAGE: data.message,
            DATA: data.data
        });
    }

    terminateWorker(workerId) {
        const worker = this.workers.get(workerId);
        if (worker) {
            worker.terminate();
            this.workers.delete(workerId);
            this.workerMessages.delete(workerId);
        }
    }
}

Phase 2: Multi-Threading Blocks Extension

Create src/blocks/scratch3_multithread.js:

class Scratch3MultiThreadBlocks {
    constructor(runtime) {
        this.runtime = runtime;
        if (!this.runtime.workerManager) {
            const WorkerManager = require('../engine/worker-manager');
            this.runtime.workerManager = new WorkerManager(runtime);
        }
    }

    getPrimitives() {
        return {
            multithread_createworker: this.createWorker,
            multithread_sendmessage: this.sendMessage,
            multithread_sendmessageandwait: this.sendMessageAndWait,
            multithread_terminateworker: this.terminateWorker,
            multithread_getdata: this.getData
        };
    }

    getHats() {
        return {
            multithread_whenreceived: {
                restartExistingThreads: true,
                edgeActivated: false
            }
        };
    }

    createWorker(args, util) {
        // Simple worker template that listens and responds
        const workerCode = `
            self.onmessage = function(e) {
                const { message, data } = e.data;
                // Process message (customizable by user via args.CODE)
                ${args.CODE || ''}
                // Send response back
                self.postMessage({ message: message + '_response', data: data });
            };
        `;
        return this.runtime.workerManager.createWorker(workerCode);
    }

    sendMessage(args, util) {
        const workerId = Cast.toNumber(args.WORKER_ID);
        const message = Cast.toString(args.MESSAGE);
        const data = args.DATA;
        this.runtime.workerManager.sendMessage(workerId, message, data);
    }

    sendMessageAndWait(args, util) {
        const workerId = Cast.toNumber(args.WORKER_ID);
        const message = Cast.toString(args.MESSAGE);
        
        if (!util.stackFrame.sentMessage) {
            // Send message
            this.runtime.workerManager.sendMessage(workerId, message, args.DATA);
            util.stackFrame.sentMessage = true;
            util.stackFrame.responseMessage = message + '_response';
            
            // Start listening for response
            util.stackFrame.startedThreads = util.startHats(
                'multithread_whenreceived', {
                    WORKER_ID: workerId,
                    MESSAGE: util.stackFrame.responseMessage
                }
            );
            
            if (util.stackFrame.startedThreads.length === 0) {
                return;
            }
        }
        
        // Wait for triggered threads (same pattern as broadcastAndWait)
        const waiting = util.stackFrame.startedThreads
            .some(thread => this.runtime.threads.indexOf(thread) !== -1);
        
        if (waiting) {
            if (util.stackFrame.startedThreads
                .every(thread => this.runtime.isWaitingThread(thread))
            ) {
                util.yieldTick();
            } else {
                util.yield();
            }
        }
    }

    terminateWorker(args, util) {
        const workerId = Cast.toNumber(args.WORKER_ID);
        this.runtime.workerManager.terminateWorker(workerId);
    }

    getData(args, util) {
        // Get data from worker message (stored in hat block context)
        return util.stackFrame.workerData || '';
    }

    getInfo() {
        return {
            id: 'multithread',
            name: 'Multi-Threading',
            color1: '#FF6680',
            color2: '#FF4D6A',
            color3: '#FF3355',
            blocks: [
                {
                    opcode: 'createworker',
                    blockType: BlockType.REPORTER,
                    text: 'create worker with code [CODE]',
                    arguments: {
                        CODE: {
                            type: ArgumentType.STRING,
                            defaultValue: '// Worker code here'
                        }
                    }
                },
                {
                    opcode: 'sendmessage',
                    blockType: BlockType.COMMAND,
                    text: 'send [MESSAGE] to worker [WORKER_ID] with data [DATA]',
                    arguments: {
                        MESSAGE: { type: ArgumentType.STRING, defaultValue: 'calculate' },
                        WORKER_ID: { type: ArgumentType.NUMBER, defaultValue: 1 },
                        DATA: { type: ArgumentType.STRING, defaultValue: '{}' }
                    }
                },
                {
                    opcode: 'sendmessageandwait',
                    blockType: BlockType.COMMAND,
                    text: 'send [MESSAGE] to worker [WORKER_ID] and wait',
                    arguments: {
                        MESSAGE: { type: ArgumentType.STRING, defaultValue: 'calculate' },
                        WORKER_ID: { type: ArgumentType.NUMBER, defaultValue: 1 },
                        DATA: { type: ArgumentType.STRING, defaultValue: '{}' }
                    }
                },
                {
                    opcode: 'whenreceived',
                    blockType: BlockType.HAT,
                    text: 'when I receive [MESSAGE] from worker [WORKER_ID]',
                    arguments: {
                        MESSAGE: { type: ArgumentType.STRING, defaultValue: 'result' },
                        WORKER_ID: { type: ArgumentType.NUMBER, defaultValue: 1 }
                    }
                },
                {
                    opcode: 'getdata',
                    blockType: BlockType.REPORTER,
                    text: 'worker message data'
                },
                {
                    opcode: 'terminateworker',
                    blockType: BlockType.COMMAND,
                    text: 'terminate worker [WORKER_ID]',
                    arguments: {
                        WORKER_ID: { type: ArgumentType.NUMBER, defaultValue: 1 }
                    }
                }
            ]
        };
    }
}

Phase 3: Compiler Support

Add IR opcodes in src/compiler/enums.js:

StackOpcode.MULTITHREAD_SEND = 'multithread.send';
StackOpcode.MULTITHREAD_TERMINATE = 'multithread.terminate';
InputOpcode.MULTITHREAD_CREATE = 'multithread.create';

Add IR generation in src/compiler/irgen.js:

case 'multithread_sendmessage':
    return new IntermediateStackBlock(StackOpcode.MULTITHREAD_SEND, {
        workerId: this.descendInputOfBlock(block, 'WORKER_ID'),
        message: this.descendInputOfBlock(block, 'MESSAGE'),
        data: this.descendInputOfBlock(block, 'DATA')
    });

Add JS generation in src/compiler/jsgen.js:

case StackOpcode.MULTITHREAD_SEND:
    this.source += `runtime.workerManager.sendMessage(`;
    this.source += `${this.descendInput(node.workerId)}, `;
    this.source += `${this.descendInput(node.message)}, `;
    this.source += `${this.descendInput(node.data)});\n`;
    break;

Phase 4: Register Blocks

In src/engine/runtime.js, add to defaultBlockPackages:

const defaultBlockPackages = {
    // ... existing packages
    scratch3_multithread: require('../blocks/scratch3_multithread')
};

Key Design Decisions

1. Broadcast-like Abstraction: Kids don't see postMessage - they see "send message to worker" just like broadcast
2. Worker IDs: Simple numeric IDs make it easy to manage multiple workers
3. Message Matching: Hat blocks filter by both worker ID and message name
4. Synchronization: sendMessageAndWait uses the same pattern as broadcastAndWait
5. No Breaking Changes: This is purely additive - existing projects unaffected
6. Compiler Compatible: IR opcodes ensure compiled mode works seamlessly

Next Steps

1. Implement WorkerManager class
2. Create scratch3_multithread blocks
3. Add compiler support (IR + JS generation)
4. Test with both interpreter and compiler modes
5. Add safety limits (max workers, message size limits)
6. Consider adding a visual worker monitor UI

This approach leverages existing patterns and infrastructure while providing a kid-friendly abstraction over Web Workers. The broadcast-like interface makes it intuitive while the underlying implementation uses proper worker threads for actual parallelism.

Would you like me to start implementing any specific part of this plan, or would you prefer to discuss the architecture further?

[supervoidcoder]

@coderabbitai can you post here the FULL implementation plan start to finish completely?
also let's do it for like clones, it has 300 clone limit but you can disable the cap at the risk of crashing....

so same for this! but maybe the initial limit should be lower

[coderabbitai]

Tip

For best results, initiate chat on the files or code changes.

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@supervoidcoder Here's the COMPLETE IMPLEMENTATION PLAN from start to finish for multi-threading blocks with configurable worker limits (following the clone pattern):

🎯 Complete Implementation Plan

Overview

• Worker Limit: 10 workers by default (lower than clones' 300)
• Configurable: Can be changed via runtime.runtimeOptions.maxWorkers
• Safety: Prevents crashes like clone limit prevents memory issues
• Pattern: Follows broadcast-like abstraction for kid-friendliness

---

📁 File Structure

New Files to Create:

1. src/engine/worker-manager.js - Worker lifecycle management
2. src/blocks/scratch3_multithread.js - Block implementations
3. Updates to existing files for compiler support

---

🔧 Step 1: Create Worker Manager (src/engine/worker-manager.js)

/**
 * @fileoverview
 * Manager for Web Worker threads in the Scratch VM.
 * Handles worker creation, message passing, and lifecycle management.
 */

class WorkerManager {
    constructor (runtime) {
        /**
         * Reference to the runtime.
         * @type {Runtime}
         */
        this.runtime = runtime;

        /**
         * Map of worker ID to Worker instance.
         * @type {Map<number, Worker>}
         */
        this.workers = new Map();

        /**
         * Map of worker ID to pending messages queue.
         * Used for "when I receive" hat block filtering.
         * @type {Map<number, Array<{message: string, data: *}>>}
         */
        this.workerMessages = new Map();

        /**
         * Counter for generating unique worker IDs.
         * @type {number}
         */
        this.nextWorkerId = 1;

        /**
         * Currently known number of workers, used to enforce worker limit.
         * @type {number}
         */
        this._workerCounter = 0;
    }

    /**
     * Update the worker counter to track how many workers are created.
     * @param {number} changeAmount How many workers have been created/destroyed.
     */
    changeWorkerCounter (changeAmount) {
        this._workerCounter += changeAmount;
    }

    /**
     * Return whether there are workers available to create.
     * @return {boolean} True until the number of workers hits runtimeOptions.maxWorkers
     */
    workersAvailable () {
        return this._workerCounter < this.runtime.runtimeOptions.maxWorkers;
    }

    /**
     * Create a new Web Worker with the given code.
     * Returns null if worker limit is reached.
     * @param {string} code JavaScript code to run in the worker.
     * @return {number|null} Worker ID, or null if limit reached.
     */
    createWorker (code) {
        if (!this.workersAvailable()) {
            return null; // Hit max worker limit
        }

        const workerId = this.nextWorkerId++;
        
        // Wrap user code in a worker template that handles message protocol
        const workerCode = `
// Worker ${workerId}
self.addEventListener('message', function(e) {
    const { message, data, _internal } = e.data;
    
    // Handle internal control messages
    if (_internal === 'ping') {
        self.postMessage({ _internal: 'pong' });
        return;
    }
    
    // User code execution context
    const sendMessage = function(responseMessage, responseData) {
        self.postMessage({
            message: responseMessage,
            data: responseData
        });
    };
    
    // Make received message and data available to user code
    const receivedMessage = message;
    const receivedData = data;
    
    try {
        // User code
        ${code}
    } catch (error) {
        self.postMessage({
            message: 'error',
            data: String(error)
        });
    }
});

// Signal that worker is ready
self.postMessage({ _internal: 'ready', workerId: ${workerId} });
`;

        try {
            const blob = new Blob([workerCode], {type: 'application/javascript'});
            const workerUrl = URL.createObjectURL(blob);
            const worker = new Worker(workerUrl);

            // Listen for messages from worker
            worker.onmessage = (e) => {
                this.handleWorkerMessage(workerId, e.data);
            };

            worker.onerror = (error) => {
                // Trigger error hat blocks
                this.runtime.startHats('multithread_whenreceived', {
                    WORKER_ID: workerId,
                    MESSAGE: 'error'
                });
            };

            this.workers.set(workerId, worker);
            this.workerMessages.set(workerId, []);
            this.changeWorkerCounter(1);

            // Clean up blob URL after worker is created
            URL.revokeObjectURL(workerUrl);

            return workerId;
        } catch (error) {
            return null;
        }
    }

    /**
     * Send a message to a worker.
     * @param {number} workerId ID of the worker.
     * @param {string} message Message name/type.
     * @param {*} data Data to send with the message.
     */
    sendMessage (workerId, message, data) {
        const worker = this.workers.get(workerId);
        if (worker) {
            worker.postMessage({message, data});
        }
    }

    /**
     * Handle a message received from a worker.
     * @param {number} workerId ID of the worker that sent the message.
     * @param {object} data Message data from the worker.
     */
    handleWorkerMessage (workerId, data) {
        // Skip internal protocol messages
        if (data._internal) {
            return;
        }

        // Store the message for potential retrieval
        const messageQueue = this.workerMessages.get(workerId);
        if (messageQueue) {
            messageQueue.push(data);
            // Keep queue size limited
            if (messageQueue.length > 100) {
                messageQueue.shift();
            }
        }

        // Trigger hat blocks with this message
        this.runtime.startHats('multithread_whenreceived', {
            WORKER_ID: workerId,
            MESSAGE: data.message,
            DATA: data.data
        });
    }

    /**
     * Get the most recent message data from a worker.
     * @param {number} workerId ID of the worker.
     * @return {*} The most recent message data, or empty string if none.
     */
    getLastMessageData (workerId) {
        const messageQueue = this.workerMessages.get(workerId);
        if (messageQueue && messageQueue.length > 0) {
            return messageQueue[messageQueue.length - 1].data;
        }
        return '';
    }

    /**
     * Terminate a worker and clean up resources.
     * @param {number} workerId ID of the worker to terminate.
     */
    terminateWorker (workerId) {
        const worker = this.workers.get(workerId);
        if (worker) {
            worker.terminate();
            this.workers.delete(workerId);
            this.workerMessages.delete(workerId);
            this.changeWorkerCounter(-1);
        }
    }

    /**
     * Terminate all workers. Called when runtime is disposed.
     */
    terminateAll () {
        for (const workerId of this.workers.keys()) {
            this.terminateWorker(workerId);
        }
        this._workerCounter = 0;
    }

    /**
     * Check if a worker exists.
     * @param {number} workerId ID of the worker to check.
     * @return {boolean} True if worker exists.
     */
    workerExists (workerId) {
        return this.workers.has(workerId);
    }
}

module.exports = WorkerManager;

---

🧩 Step 2: Create Multi-Threading Blocks (src/blocks/scratch3_multithread.js)

const Cast = require('../util/cast');
const BlockType = require('../extension-support/block-type');
const ArgumentType = require('../extension-support/argument-type');

/**
 * Icon for multi-threading blocks (svg data uri)
 */
const blockIconURI = 'data:image/svg+xml;base64,PHN2ZyB3aWR0aD0iNDAiIGhlaWdodD0iNDAiIHZpZXdCb3g9IjAgMCA0MCA0MCIgeG1sbnM9Imh0dHA6Ly93d3cudzMub3JnLzIwMDAvc3ZnIj48ZyBmaWxsPSIjRkY2NjgwIj48cmVjdCB4PSI1IiB5PSI1IiB3aWR0aD0iMTAiIGhlaWdodD0iMTAiLz48cmVjdCB4PSIyNSIgeT0iNSIgd2lkdGg9IjEwIiBoZWlnaHQ9IjEwIi8+PHJlY3QgeD0iNSIgeT0iMjUiIHdpZHRoPSIxMCIgaGVpZ2h0PSIxMCIvPjxyZWN0IHg9IjI1IiB5PSIyNSIgd2lkdGg9IjEwIiBoZWlnaHQ9IjEwIi8+PC9nPjwvc3ZnPg==';

/**
 * Class for multi-threading blocks.
 * @constructor
 */
class Scratch3MultiThreadBlocks {
    constructor (runtime) {
        /**
         * The runtime instantiating this block package.
         * @type {Runtime}
         */
        this.runtime = runtime;

        // Initialize worker manager if it doesn't exist
        if (!this.runtime.workerManager) {
            const WorkerManager = require('../engine/worker-manager');
            this.runtime.workerManager = new WorkerManager(runtime);
        }
    }

    /**
     * @returns {object} metadata for this extension and its blocks.
     */
    getInfo () {
        return {
            id: 'multithread',
            name: 'Multi-Threading',
            blockIconURI: blockIconURI,
            color1: '#FF6680',
            color2: '#FF4D6A',
            color3: '#FF3355',
            blocks: [
                {
                    opcode: 'createworker',
                    blockType: BlockType.REPORTER,
                    text: 'create worker with code [CODE]',
                    arguments: {
                        CODE: {
                            type: ArgumentType.STRING,
                            defaultValue: '// receivedMessage and receivedData are available\n// Use sendMessage(name, data) to reply\nsendMessage("result", receivedData * 2);'
                        }
                    }
                },
                {
                    opcode: 'sendmessage',
                    blockType: BlockType.COMMAND,
                    text: 'send [MESSAGE] with data [DATA] to worker [WORKER_ID]',
                    arguments: {
                        MESSAGE: {
                            type: ArgumentType.STRING,
                            defaultValue: 'calculate'
                        },
                        DATA: {
                            type: ArgumentType.STRING,
                            defaultValue: '42'
                        },
                        WORKER_ID: {
                            type: ArgumentType.NUMBER,
                            defaultValue: 1
                        }
                    }
                },
                {
                    opcode: 'sendmessageandwait',
                    blockType: BlockType.COMMAND,
                    text: 'send [MESSAGE] with data [DATA] to worker [WORKER_ID] and wait',
                    arguments: {
                        MESSAGE: {
                            type: ArgumentType.STRING,
                            defaultValue: 'calculate'
                        },
                        DATA: {
                            type: ArgumentType.STRING,
                            defaultValue: '42'
                        },
                        WORKER_ID: {
                            type: ArgumentType.NUMBER,
                            defaultValue: 1
                        }
                    }
                },
                {
                    opcode: 'whenreceived',
                    blockType: BlockType.HAT,
                    text: 'when I receive [MESSAGE] from worker [WORKER_ID]',
                    arguments: {
                        MESSAGE: {
                            type: ArgumentType.STRING,
                            defaultValue: 'result'
                        },
                        WORKER_ID: {
                            type: ArgumentType.NUMBER,
                            defaultValue: 1
                        }
                    }
                },
                {
                    opcode: 'getmessagedata',
                    blockType: BlockType.REPORTER,
                    text: 'message data from worker [WORKER_ID]',
                    arguments: {
                        WORKER_ID: {
                            type: ArgumentType.NUMBER,
                            defaultValue: 1
                        }
                    }
                },
                {
                    opcode: 'terminateworker',
                    blockType: BlockType.COMMAND,
                    text: 'terminate worker [WORKER_ID]',
                    arguments: {
                        WORKER_ID: {
                            type: ArgumentType.NUMBER,
                            defaultValue: 1
                        }
                    }
                },
                {
                    opcode: 'workerexists',
                    blockType: BlockType.BOOLEAN,
                    text: 'worker [WORKER_ID] exists?',
                    arguments: {
                        WORKER_ID: {
                            type: ArgumentType.NUMBER,
                            defaultValue: 1
                        }
                    }
                },
                {
                    opcode: 'workercount',
                    blockType: BlockType.REPORTER,
                    text: 'worker count'
                },
                {
                    opcode: 'workerlimit',
                    blockType: BlockType.REPORTER,
                    text: 'worker limit'
                }
            ]
        };
    }

    /**
     * Get the hat blocks for this extension.
     * @returns {object} Hat block metadata.
     */
    getHats () {
        return {
            multithread_whenreceived: {
                restartExistingThreads: true,
                edgeActivated: false
            }
        };
    }

    /**
     * Get primitives implemented by this package.
     * @return {object} Primitives implemented by this package.
     */
    getPrimitives () {
        return {
            multithread_createworker: this.createWorker,
            multithread_sendmessage: this.sendMessage,
            multithread_sendmessageandwait: this.sendMessageAndWait,
            multithread_getmessagedata: this.getMessageData,
            multithread_terminateworker: this.terminateWorker,
            multithread_workerexists: this.workerExists,
            multithread_workercount: this.workerCount,
            multithread_workerlimit: this.workerLimit
        };
    }

    /**
     * Create a new worker.
     * @param {object} args - the block arguments.
     * @param {string} args.CODE - JavaScript code to run in the worker.
     * @return {number|string} Worker ID or empty string if limit reached.
     */
    createWorker (args) {
        const code = Cast.toString(args.CODE);
        const workerId = this.runtime.workerManager.createWorker(code);
        return workerId !== null ? workerId : '';
    }

    /**
     * Send a message to a worker (fire and forget).
     * @param {object} args - the block arguments.
     * @param {string} args.MESSAGE - Message name.
     * @param {*} args.DATA - Data to send.
     * @param {number} args.WORKER_ID - Worker ID.
     */
    sendMessage (args) {
        this._sendMessage(
            Cast.toNumber(args.WORKER_ID),
            Cast.toString(args.MESSAGE),
            args.DATA
        );
    }

    _sendMessage (workerId, message, data) {
        this.runtime.workerManager.sendMessage(workerId, message, data);
    }

    /**
     * Send a message to a worker and wait for a response.
     * @param {object} args - the block arguments.
     * @param {BlockUtility} util - utility object provided by the runtime.
     * @return {Promise|undefined} Promise that resolves when response received, or undefined.
     */
    sendMessageAndWait (args, util) {
        const workerId = Cast.toNumber(args.WORKER_ID);
        const message = Cast.toString(args.MESSAGE);

        if (!util.stackFrame.sentMessage) {
            // First time: send the message
            this._sendMessage(workerId, message, args.DATA);
            util.stackFrame.sentMessage = true;
            util.stackFrame.waitingForWorker = workerId;
            util.stackFrame.waitingForMessage = message;

            // Start any hat blocks that match this worker/message
            util.stackFrame.startedThreads = util.startHats(
                'multithread_whenreceived', {
                    WORKER_ID: workerId,
                    MESSAGE: message
                }
            );

            if (util.stackFrame.startedThreads.length === 0) {
                // No hat blocks to wait for
                return;
            }
        }

        // Wait for all started threads to complete
        // This is the same pattern as broadcast_and_wait
        const waiting = util.stackFrame.startedThreads
            .some(thread => this.runtime.threads.indexOf(thread) !== -1);

        if (waiting) {
            // Check if all threads are waiting - if so, we can tick
            if (util.stackFrame.startedThreads
                .every(thread => this.runtime.isWaitingThread(thread))
            ) {
                util.yieldTick();
            } else {
                util.yield();
            }
        }
    }

    /**
     * Get the most recent message data from a worker.
     * @param {object} args - the block arguments.
     * @param {number} args.WORKER_ID - Worker ID.
     * @return {*} Message data.
     */
    getMessageData (args) {
        const workerId = Cast.toNumber(args.WORKER_ID);
        return this.runtime.workerManager.getLastMessageData(workerId);
    }

    /**
     * Terminate a worker.
     * @param {object} args - the block arguments.
     * @param {number} args.WORKER_ID - Worker ID.
     */
    terminateWorker (args) {
        const workerId = Cast.toNumber(args.WORKER_ID);
        this.runtime.workerManager.terminateWorker(workerId);
    }

    /**
     * Check if a worker exists.
     * @param {object} args - the block arguments.
     * @param {number} args.WORKER_ID - Worker ID.
     * @return {boolean} True if worker exists.
     */
    workerExists (args) {
        const workerId = Cast.toNumber(args.WORKER_ID);
        return this.runtime.workerManager.workerExists(workerId);
    }

    /**
     * Get the current worker count.
     * @return {number} Number of active workers.
     */
    workerCount () {
        return this.runtime.workerManager._workerCounter;
    }

    /**
     * Get the worker limit.
     * @return {number} Maximum number of workers allowed.
     */
    workerLimit () {
        return this.runtime.runtimeOptions.maxWorkers;
    }
}

module.exports = Scratch3MultiThreadBlocks;

---

🔄 Step 3: Update Runtime (src/engine/runtime.js)

Add to constants section (around line 960):

/**
 * How many workers can be created at a time.
 * @const {number}
 */
static get MAX_WORKERS () {
    // tw: worker limit is set per-runtime in runtimeOptions, this is only the initial value
    return 10;
}

Add to constructor initialization (around line 468):

this.runtimeOptions = {
    maxClones: Runtime.MAX_CLONES,
    maxWorkers: Runtime.MAX_WORKERS, // ADD THIS LINE
    miscLimits: true,
    fencing: true
};

Add to constructor (around line 305, after _cloneCounter):

/**
 * Worker manager for multi-threading support.
 * Initialized lazily when multithread blocks are used.
 * @type {WorkerManager}
 */
this.workerManager = null;

Add to dispose method (find dispose() method and add):

dispose () {
    // ... existing dispose code ...
    
    // Clean up workers
    if (this.workerManager) {
        this.workerManager.terminateAll();
    }
}

Register the blocks (in defaultBlockPackages around line 235):

const defaultBlockPackages = {
    scratch3_motion: require('../blocks/scratch3_motion'),
    scratch3_looks: require('../blocks/scratch3_looks'),
    scratch3_sound: require('../blocks/scratch3_sound'),
    scratch3_event: require('../blocks/scratch3_event'),
    scratch3_control: require('../blocks/scratch3_control'),
    scratch3_sensing: require('../blocks/scratch3_sensing'),
    scratch3_operators: require('../blocks/scratch3_operators'),
    scratch3_data: require('../blocks/scratch3_data'),
    scratch3_procedures: require('../blocks/scratch3_procedures'),
    scratch3_multithread: require('../blocks/scratch3_multithread') // ADD THIS LINE
};

---

⚙️ Step 4: Compiler Support

4a. Add Opcodes (src/compiler/jsexecute.js - if enums are there, or create new constants)

Add these constants near the top of the compiler files:

// In the appropriate compiler file that defines opcodes
const MultithreadOpcodes = {
    CREATE: 'multithread.create',
    SEND: 'multithread.send',
    SEND_AND_WAIT: 'multithread.sendAndWait',
    TERMINATE: 'multithread.terminate',
    GET_DATA: 'multithread.getData',
    EXISTS: 'multithread.exists',
    COUNT: 'multithread.count',
    LIMIT: 'multithread.limit'
};

4b. Update IR Generator (src/compiler/irgen.js)

Find the section where block cases are handled and add:

// In the descendStackBlock method
case 'multithread_createworker':
    return intermediateStackBlock('multithread.create', {
        code: this.descendInput(block.inputs.CODE)
    });

case 'multithread_sendmessage':
    return intermediateStackBlock('multithread.send', {
        workerId: this.descendInput(block.inputs.WORKER_ID),
        message: this.descendInput(block.inputs.MESSAGE),
        data: this.descendInput(block.inputs.DATA)
    });

case 'multithread_terminateworker':
    return intermediateStackBlock('multithread.terminate', {
        workerId: this.descendInput(block.inputs.WORKER_ID)
    });

// In the descendInputOfBlock method
case 'multithread_getmessagedata':
    return intermediateInput('multithread.getData', {
        workerId: this.descendInput(block.inputs.WORKER_ID)
    });

case 'multithread_workerexists':
    return intermediateInput('multithread.exists', {
        workerId: this.descendInput(block.inputs.WORKER_ID)
    });

case 'multithread_workercount':
    return intermediateInput('multithread.count', {});

case 'multithread_workerlimit':
    return intermediateInput('multithread.limit', {});

4c. Update JS Generator (src/compiler/jsgen.js)

Find the section where opcodes are generated to JavaScript and add:

// In the main generation switch for stack blocks
case 'multithread.create':
    this.source += `runtime.workerManager.createWorker(${this.descendInput(node.code)});\n`;
    break;

case 'multithread.send':
    this.source += `runtime.workerManager.sendMessage(`;
    this.source += `${this.descendInput(node.workerId)}, `;
    this.source += `${this.descendInput(node.message)}, `;
    this.source += `${this.descendInput(node.data)});\n`;
    break;

case 'multithread.terminate':
    this.source += `runtime.workerManager.terminateWorker(${this.descendInput(node.workerId)});\n`;
    break;

// In the input expression generation
case 'multithread.getData':
    this.source += `runtime.workerManager.getLastMessageData(${this.descendInput(node.workerId)})`;
    break;

case 'multithread.exists':
    this.source += `runtime.workerManager.workerExists(${this.descendInput(node.workerId)})`;
    break;

case 'multithread.count':
    this.source += `runtime.workerManager._workerCounter`;
    break;

case 'multithread.limit':
    this.source += `runtime.runtimeOptions.maxWorkers`;
    break;

---

🧪 Step 5: Testing

Test Script 1: Basic Worker Creation

// In Scratch blocks:
// 1. Create worker: set [workerId v] to (create worker with code [sendMessage("hello", "world");])
// 2. Check: if <worker (workerId) exists?> then
// 3.   say (join [Worker ] (workerId) [ created!])
// 4. Terminate: terminate worker (workerId)

Test Script 2: Message Passing

// Script 1 (Main):
// when green flag clicked
// set [workerId v] to (create worker with code [sendMessage("doubled", receivedData * 2);])
// send [double] with data [21] to worker (workerId) and wait
// say (message data from worker (workerId))

// Script 2 (Hat):
// when I receive [doubled] from worker (workerId)
// say (join [Got result: ] (message data from worker (workerId)))

Test Script 3: Worker Limit

// when green flag clicked
// repeat (15)
//   set [id v] to (create worker with code [// empty])
//   if <(id) = []> then
//     say [Hit worker limit!]
//   end
// end
// say (join [Created ] (worker count) [ workers])

---

📋 Step 6: Configuration API

Users can change the worker limit programmatically:

// In the embedding application (e.g., scratch-gui)
vm.runtime.setRuntimeOptions({
    maxWorkers: 50 // Increase to 50 workers
});

// Or disable the limit (use at your own risk!)
vm.runtime.setRuntimeOptions({
    maxWorkers: Infinity
});

---

🎨 Step 7: UI Considerations (Optional)

In scratch-gui, you could add a settings option:

// In GUI settings
<Setting
    label="Max Workers"
    type="number"
    value={vm.runtime.runtimeOptions.maxWorkers}
    onChange={(value) => vm.runtime.setRuntimeOptions({maxWorkers: value})}
    min={1}
    max={100}
/>

---

🔒 Step 8: Safety Features

The implementation includes:

1. Worker Limit: Default 10, prevents browser crashes
2. Message Queue Limit: Max 100 messages per worker
3. Error Handling: Workers that error trigger error hat blocks
4. Resource Cleanup: Workers terminated on project stop/dispose
5. Null Checks: Returns empty string when worker doesn't exist

---

📊 Comparison: Clones vs Workers

Feature	Clones	Workers
Default Limit	300	10
Counter Variable	_cloneCounter	_workerCounter
Availability Check	clonesAvailable()	workersAvailable()
Counter Update	changeCloneCounter()	changeWorkerCounter()
Runtime Option	maxClones	maxWorkers
Creation Method	makeClone()	createWorker()
Hat Block	control_start_as_clone	multithread_whenreceived

---

✅ Implementation Checklist

• Create src/engine/worker-manager.js
• Create src/blocks/scratch3_multithread.js
• Update src/engine/runtime.js:
  • Add MAX_WORKERS constant
  • Add maxWorkers to runtimeOptions
  • Add workerManager property
  • Add worker cleanup to dispose()
  • Register multithread blocks in defaultBlockPackages
• Update Compiler (if using compiler):
  • Add IR opcodes
  • Update irgen.js to generate IR for multithread blocks
  • Update jsgen.js to generate JS from IR
• Test basic functionality
• Test worker limit enforcement
• Test message passing
• Test "and wait" synchronization
• Test cleanup on project stop

---

🚀 Usage Examples for Kids

Example 1: Simple Math Worker

when green flag clicked
set [worker v] to (create worker with code [sendMessage("answer", receivedData + 10)])
send [add] with data [32] to worker (worker) and wait
say (message data from worker (worker)) // Says "42"

Example 2: Background Counter

when green flag clicked
set [counter v] to (create worker with code [
  let count = 0;
  setInterval(() => {
    count++;
    sendMessage("count", count);
  }, 1000);
])

when I receive [count] from worker (counter)
say (join [Count: ] (message data from worker (counter)))

---

This complete implementation mirrors the clone system's safety and configurability while providing a kid-friendly abstraction over Web Workers! 🎉