Add PrintBuffer classes (SmingHub#2873)

Using ArduinoJson to serialize directly to a file stream performs poorly because writes are done byte-by-byte. This issue is mentioned under [performance](https://arduinojson.org/v6/api/json/serializejson/).

The mentioned [ArduinoStreamUtils](https://github.com/bblanchon/ArduinoStreamUtils) library is a bit hefty and doesn't particular fit with the existing Sming stream classes.

This PR adds a very simple `PrintBuffer` class which operates on a `Print` output and can be employed very easily if additional generic buffering is required.

Note that there is little benefit to using this with other memory-based streams so best left to the library or application to decide when its appropriate.

Sming/Core/Data/Buffer/PrintBuffer.cpp

@@ -0,0 +1,43 @@
+/****
+ * Sming Framework Project - Open Source framework for high efficiency native ESP8266 development.
+ * Created 2015 by Skurydin Alexey
+ * http://github.com/SmingHub/Sming
+ * All files of the Sming Core are provided under the LGPL v3 license.
+ *
+ * PrintBuffer.cpp
+ *
+ ****/
+
+#include "PrintBuffer.h"
+
+size_t BasePrintBuffer::write(uint8_t c)
+{
+	buffer[writeOffset++] = c;
+	if(writeOffset == bufferSize) {
+		flush();
+	}
+	return 1;
+}
+
+size_t BasePrintBuffer::write(const uint8_t* data, size_t size)
+{
+	size_t written{0};
+	while(size != 0) {
+		auto copySize = std::min(bufferSize - writeOffset, size);
+		memcpy(&buffer[writeOffset], data, copySize);
+		writeOffset += copySize;
+		written += copySize;
+		data += copySize;
+		size -= copySize;
+		if(writeOffset == bufferSize) {
+			flush();
+		}
+	}
+	return written;
+}
+
+void BasePrintBuffer::flush()
+{
+	output.write(buffer, writeOffset);
+	writeOffset = 0;
+}

Sming/Core/Data/Buffer/PrintBuffer.h

@@ -0,0 +1,155 @@
+/****
+ * Sming Framework Project - Open Source framework for high efficiency native ESP8266 development.
+ * Created 2015 by Skurydin Alexey
+ * http://github.com/SmingHub/Sming
+ * All files of the Sming Core are provided under the LGPL v3 license.
+ *
+ * PrintBuffer.h
+ *
+ ****/
+
+#pragma once
+
+#include <Data/Stream/ReadWriteStream.h>
+#include <memory>
+
+/**
+ * @brief Generic write-through buffer class
+ * @ingroup stream
+ * @note Call flush() at end of write operation to ensure all data is output
+ * This is done automatically when the buffer is destroyed.
+ */
+class BasePrintBuffer : public Print
+{
+public:
+	/**
+     * @brief Create buffer
+     * @param output Destination stream
+     * @param buffer buffer to use
+     * @param size Size of buffer
+     */
+	BasePrintBuffer(Print& output, uint8_t buffer[], size_t bufferSize)
+		: output(output), buffer(buffer), bufferSize(bufferSize)
+	{
+	}
+
+	~BasePrintBuffer()
+	{
+		flush();
+	}
+
+	size_t write(uint8_t c) override;
+
+	size_t write(const uint8_t* data, size_t size) override;
+
+	/**
+     * @brief Write any buffered content to output
+     */
+	void flush();
+
+private:
+	Print& output;
+	uint8_t* buffer;
+	size_t bufferSize;
+	size_t writeOffset{};
+};
+
+/**
+ * @brief Write-through buffer using stack only
+ * @tparam size Size of buffer
+ *
+ * Example usage:
+ *
+ *      FileStream stream("file.txt", File::ReadWrite);
+ * 	    {
+ *	        StaticPrintBuffer<256> buffer(stream);
+ * 	        writeSomeData(buffer);
+ * 	    } // Buffer flushed and destroyed when it goes out of scope
+ */
+template <size_t size> class StaticPrintBuffer : public BasePrintBuffer
+{
+public:
+	/**
+     * @brief Construct a stack-based buffer
+     * @param output Print destination
+     */
+	StaticPrintBuffer(Print& output) : BasePrintBuffer(output, buffer, size)
+	{
+	}
+
+private:
+	uint8_t buffer[size];
+};
+
+/**
+ * @brief Write-through buffer using heap storage
+ *
+ * Example usage:
+ *
+ *      FileStream stream("file.txt", File::ReadWrite);
+ * 	    {
+ *	        HeapPrintBuffer buffer(stream, 512);
+ * 	        writeSomeData(buffer);
+ * 	    } // Buffer flushed and destroyed when it goes out of scope
+ */
+class HeapPrintBuffer : public BasePrintBuffer
+{
+public:
+	/**
+     * @brief Construct a stack-based buffer
+     * @param output Print destination
+     * @param size Buffer size
+     */
+	HeapPrintBuffer(Print& output, size_t size) : HeapPrintBuffer(output, new uint8_t[size], size)
+	{
+	}
+
+private:
+	HeapPrintBuffer(Print& output, uint8_t* buffer, size_t size) : BasePrintBuffer(output, buffer, size), buffer(buffer)
+	{
+	}
+
+	std::unique_ptr<uint8_t[]> buffer;
+};
+
+/**
+ * @brief Write-through buffer using heap storage and owned stream pointer
+ *
+ * Example usage:
+ *
+ *      auto stream = std::make_unique<FileStream>("file.txt", File::ReadWrite);
+ *	    auto bufferedStream = new DynamicPrintBuffer(std::move(stream), 512);
+ *
+ *      // write to bufferedStream as required via callbacks, etc.
+ *      ...
+ *
+ *      // This destroys both buffer *and* the file stream
+ *      delete bufferedStream;
+ */
+class DynamicPrintBuffer : public BasePrintBuffer
+{
+public:
+	/**
+     * @brief Construct a stack-based buffer
+     * @param output Print destination, will take ownership of this
+     * @param size Buffer size
+     */
+	DynamicPrintBuffer(std::unique_ptr<Print>&& output, size_t size)
+		: DynamicPrintBuffer(output.release(), new uint8_t[size], size)
+	{
+	}
+
+	~DynamicPrintBuffer()
+	{
+		flush();
+	}
+
+private:
+	DynamicPrintBuffer(Print* output, uint8_t* buffer, size_t size)
+		: BasePrintBuffer(*output, buffer, size), output(output), buffer(buffer)
+	{
+	}
+
+	std::unique_ptr<Print> output;
+	std::unique_ptr<uint8_t[]> buffer;
+};

docs/source/framework/core/data/buffering.rst

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+Buffering
+=========
+
+In general, writing efficiently to files is best done in chunks, such as by building a line of data in a :cpp:class:`String` and writing it in one go.
+
+Sming offers a simple write-through buffering mechanism which can be used where necessary. The :library:`ArduinoJson` can benefit greatly from this.
+
+.. doxygenclass:: StaticPrintBuffer
+   :members:
+
+.. doxygenclass:: HeapPrintBuffer
+   :members:
+
+.. doxygenclass:: DynamicPrintBuffer
+   :members: