[framework] use Uint8List for SMC (#100582)

packages/flutter/lib/src/foundation/serialization.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
 import 'dart:typed_data';
 
 /// Write-only buffer for incrementally building a [ByteData] instance.
@@ -13,30 +14,59 @@ import 'dart:typed_data';
 class WriteBuffer {
   /// Creates an interface for incrementally building a [ByteData] instance.
   factory WriteBuffer() {
-    final List<int> buffer = <int>[];
     final ByteData eightBytes = ByteData(8);
     final Uint8List eightBytesAsList = eightBytes.buffer.asUint8List();
-    return WriteBuffer._(buffer, eightBytes, eightBytesAsList);
+    return WriteBuffer._(Uint8List(8), eightBytes, eightBytesAsList);
   }
 
   WriteBuffer._(this._buffer, this._eightBytes, this._eightBytesAsList);
 
-  List<int> _buffer;
+  Uint8List _buffer;
+  int _currentSize = 0;
   bool _isDone = false;
   final ByteData _eightBytes;
   final Uint8List _eightBytesAsList;
   static final Uint8List _zeroBuffer = Uint8List(8);
 
+  void _add(int byte) {
+    if (_currentSize == _buffer.length) {
+      _resize();
+    }
+    _buffer[_currentSize] = byte;
+    _currentSize += 1;
+  }
+
+  void _append(Uint8List other) {
+    final int newSize = _currentSize + other.length;
+    if (newSize >= _buffer.length) {
+      _resize(newSize);
+    }
+    _buffer.setRange(_currentSize, newSize, other);
+    _currentSize += other.length;
+  }
+
   void _addAll(Uint8List data, [int start = 0, int? end]) {
-    for (int i = start; i < (end ?? _eightBytesAsList.length); i++) {
-      _buffer.add(data[i]);
+    final int newEnd = end ?? _eightBytesAsList.length;
+    final int newSize = _currentSize + (newEnd - start);
+    if (newSize >= _buffer.length) {
+      _resize(newSize);
     }
+    _buffer.setRange(_currentSize, newSize, data);
+    _currentSize = newSize;
+  }
+
+  void _resize([int? requiredLength]) {
+    final int doubleLength = _buffer.length * 2;
+    final int newLength = math.max(requiredLength ?? 0, doubleLength);
+    final Uint8List newBuffer = Uint8List(newLength);
+    newBuffer.setRange(0, _buffer.length, _buffer);
+    _buffer = newBuffer;
   }
 
   /// Write a Uint8 into the buffer.
   void putUint8(int byte) {
     assert(!_isDone);
-    _buffer.add(byte);
+    _add(byte);
   }
 
   /// Write a Uint16 into the buffer.
@@ -78,40 +108,40 @@ class WriteBuffer {
   /// Write all the values from a [Uint8List] into the buffer.
   void putUint8List(Uint8List list) {
     assert(!_isDone);
-    _buffer.addAll(list);
+    _append(list);
   }
 
   /// Write all the values from an [Int32List] into the buffer.
   void putInt32List(Int32List list) {
     assert(!_isDone);
     _alignTo(4);
-    _buffer.addAll(list.buffer.asUint8List(list.offsetInBytes, 4 * list.length));
+    _append(list.buffer.asUint8List(list.offsetInBytes, 4 * list.length));
   }
 
   /// Write all the values from an [Int64List] into the buffer.
   void putInt64List(Int64List list) {
     assert(!_isDone);
     _alignTo(8);
-    _buffer.addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
+    _append(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
   }
 
   /// Write all the values from a [Float32List] into the buffer.
   void putFloat32List(Float32List list) {
     assert(!_isDone);
     _alignTo(4);
-    _buffer.addAll(list.buffer.asUint8List(list.offsetInBytes, 4 * list.length));
+    _append(list.buffer.asUint8List(list.offsetInBytes, 4 * list.length));
   }
 
   /// Write all the values from a [Float64List] into the buffer.
   void putFloat64List(Float64List list) {
     assert(!_isDone);
     _alignTo(8);
-    _buffer.addAll(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
+    _append(list.buffer.asUint8List(list.offsetInBytes, 8 * list.length));
   }
 
   void _alignTo(int alignment) {
     assert(!_isDone);
-    final int mod = _buffer.length % alignment;
+    final int mod = _currentSize % alignment;
     if (mod != 0) {
       _addAll(_zeroBuffer, 0, alignment - mod);
     }
@@ -122,8 +152,8 @@ class WriteBuffer {
     if (_isDone) {
       throw StateError('done() must not be called more than once on the same $runtimeType.');
     }
-    final ByteData result = Uint8List.fromList(_buffer).buffer.asByteData();
-    _buffer = <int>[];
+    final ByteData result = _buffer.buffer.asByteData(0, _currentSize);
+    _buffer = Uint8List(0);
     _isDone = true;
     return result;
   }

packages/flutter/test/foundation/serialization_test.dart

@@ -9,6 +9,12 @@ import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   group('Write and read buffer round-trip', () {
+    test('of empty buffer', () {
+      final WriteBuffer write = WriteBuffer();
+      final ByteData written = write.done();
+
+      expect(written.lengthInBytes, 0);
+    });
     test('of single byte', () {
       final WriteBuffer write = WriteBuffer();
       write.putUint8(201);