Framework Foundation提供了Flutter应用需要的最基础功能

使用Foundatation实现的Demo是这样的。代码共50行，大家可以先跳过直接看下面的解释，跟着解释看代码。

bool tapped = false;
runUiImp() {
  window.onBeginFrame = render;
  window.onMetricsChanged = window.scheduleFrame;
  window.onPointerDataPacket = handlePointerPacket;
  window.scheduleFrame();
}
void render(Duration duration) {
  final PictureRecorder recorder = new PictureRecorder();

  Size size = window.physicalSize;
  final Rect bounds = Rect.fromLTWH(0, 0, size.width / 2, size.height / 2);
  final Canvas c = Canvas(recorder, bounds);
  Paint background = Paint()..color = Color(0xffffffff);
  c.drawPaint(background);

  Paint centerPaint = Paint()..color = colorWith(tapped);
  centerPaint.style = PaintingStyle.fill;
  c.drawRect(centerRect(), centerPaint);

  Picture picture = recorder.endRecording();
  SceneBuilder builder = new SceneBuilder();
  builder.pushTransform(Float64List.fromList(Matrix4.identity().storage));
  builder.addPicture(Offset.zero, picture);
  Scene scene = builder.build();
  window.render(scene);
}
Rect centerRect() {
  Size size = window.physicalSize;
  Size centerSize = Size(100, 100) * window.devicePixelRatio;
  return Rect.fromLTWH(
      (size.width - centerSize.width) / 2,
      (size.height - centerSize.height) / 2,
      centerSize.width,
      centerSize.height);
}
bool inRect(double x, double y) {
  Rect rect = centerRect();
  return x < rect.right && x > rect.left && y < rect.bottom && y > rect.top;
}
void handlePointerPacket(PointerDataPacket dataPacket) {
  PointerData data = dataPacket.data.first;
  if (inRect(data.physicalX, data.physicalY)) {
    tapped = !tapped;
    window.scheduleFrame();
  }
}

整个实现的核心是两个方法render和handlePointerPacket.

render方法给出绘制内容的逻辑，其中调用Canvas的一系列draw方法，分别绘制了背景和方块。

布局需要手动维护，在Demo的实现中，centerRect方法提供了居中布局的实现。

handlePointerPacket实现了点击事件的处理，实现进行hit test, 然后修改tapped属性，并调用window.scheduleFrame触发页面的刷新。

从这个Demo中可以看出： Foundation提供了Flutter应用需要的最基础功能，但是实现的逻辑相对复杂。

开发者需要手动计算所有的布局，实现绘制逻辑，对于事件处理，需要实现完整的hit test。

在这样简单的Demo中是可行的，但是Foundation提供的接口来实现一个正常规模的APP，对开发者来说简直是噩梦。