Add _modify to @perceptible (#91)

* Add _modify to @perceptible

* Modernize syntax.

* Added CoW test

Sources/Perception/Macros.swift

@@ -16,7 +16,7 @@
 public macro Perceptible() =
   #externalMacro(module: "PerceptionMacros", type: "PerceptibleMacro")
 
-@attached(accessor, names: named(init), named(get), named(set))
+@attached(accessor, names: named(init), named(get), named(set), named(_modify))
 @attached(peer, names: prefixed(_))
 public macro PerceptionTracked() =
   #externalMacro(module: "PerceptionMacros", type: "PerceptionTrackedMacro")

Sources/PerceptionMacros/Extensions.swift

@@ -40,10 +40,7 @@ extension VariableDeclSyntax {
   }
 
   func accessorsMatching(_ predicate: (TokenKind) -> Bool) -> [AccessorDeclSyntax] {
-    let patternBindings = bindings.compactMap { binding in
-      binding
-    }
-    let accessors: [AccessorDeclListSyntax.Element] = patternBindings.compactMap { patternBinding in
+    let accessors: [AccessorDeclListSyntax.Element] = bindings.compactMap { patternBinding in
       switch patternBinding.accessorBlock?.accessors {
       case .accessors(let accessors):
         return accessors

Sources/PerceptionMacros/PerceptibleMacro.swift

@@ -367,7 +367,17 @@ public struct PerceptionTrackedMacro: AccessorMacro {
       }
       """
 
-    return [initAccessor, getAccessor, setAccessor]
+    let modifyAccessor: AccessorDeclSyntax =
+      """
+      _modify {
+      access(keyPath: \\.\(identifier))
+      \(raw: PerceptibleMacro.registrarVariableName).willSet(self, keyPath: \\.\(identifier))
+      defer { \(raw: PerceptibleMacro.registrarVariableName).didSet(self, keyPath: \\.\(identifier)) }
+      yield &_\(identifier)
+      }
+      """
+
+    return [initAccessor, getAccessor, setAccessor, modifyAccessor]
   }
 }
 

Tests/PerceptionMacrosTests/PerceptionMacrosTests.swift

@@ -42,6 +42,14 @@
                 _count = newValue
               }
             }
+            _modify {
+              access(keyPath: \.count)
+              _$perceptionRegistrar.willSet(self, keyPath: \.count)
+              defer {
+                _$perceptionRegistrar.didSet(self, keyPath: \.count)
+              }
+              yield &_count
+            }
           }
 
           private let _$perceptionRegistrar = Perception.PerceptionRegistrar()

Tests/PerceptionTests/ModifyTests.swift

@@ -0,0 +1,30 @@
+import Perception
+import XCTest
+
+final class ModifyTests: XCTestCase {
+  func testCOW() {
+    let subject = CowTest()
+    let startId = subject.container.id
+    XCTAssertEqual(subject.container.id, startId)
+    subject.container.mutate()
+    XCTAssertEqual(subject.container.id, startId)
+  }
+}
+
+struct CowContainer {
+  final class Contents { }
+  var contents = Contents()
+  mutating func mutate() {
+    if !isKnownUniquelyReferenced(&contents) {
+      contents = Contents()
+    }
+  }
+  var id: ObjectIdentifier {
+    ObjectIdentifier(contents)
+  }
+}
+
+@Perceptible
+final class CowTest {
+  var container = CowContainer()
+}