c++ - flutter :即使 shouldRepaint() 返回 true，自定义画家也不会重新绘制

Question

我正在通过从 C/C++ 端获取颜色数据并使用 CustomPainter 绘制屏幕来测试 flutter FFI .
但令我惊讶的是，即使我将画家设置为始终重画，paint()函数只被调用两次。
代码

class _ColorViewPainter extends CustomPainter {
  Color clrBackground;

  _ColorViewPainter({
    this.clrBackground = Colors.black
  });

  @override
  bool shouldRepaint(_ColorViewPainter old) => true;

  @override
  void paint(Canvas canvas, Size size) {
    print("paint: start");

    final color = ffiGetColor().ref;

    final r = color.r;
    final g = color.g;
    final b = color.b;

    print("color: $r, $g, $b");

    final paint = Paint()
        ..strokeJoin = StrokeJoin.round
        ..strokeWidth = 1.0
        ..color = Color.fromARGB(255, r, g, b)
        ..style = PaintingStyle.fill;

    final width = size.width;
    final height = size.height;
    final content = Offset(0.0, 0.0) & Size(width, height);
    canvas.drawRect(content, paint);

    print("paint: end");
  }
}

ffiGetColor()函数只是从 C/C++ 端检索颜色 RGB 结构。我可以看到屏幕被更新了两次，日志显示:

I/flutter (12096): paint: start
I/flutter (12096): color: 255, 0, 0
I/flutter (12096): paint: end
I/flutter (12096): paint: start
I/flutter (12096): color: 0, 255, 0
I/flutter (12096): paint: end
I/Surface (12096): opservice is null false

但就是这样。尽管我显然希望它用 shouldRepaint 重新绘制. flutter 没有这样做。
怎么了？
这是我的环境

$ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel dev, v1.18.0-dev.5.0, on Mac OS X 10.15.5 19F101, locale en-CA)
 
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 11.5)
[✓] Android Studio (version 4.0)
[✗] Cannot determine if IntelliJ is installed
    ✗ Directory listing failed
[✓] VS Code (version 1.44.2)
[✓] Connected device (1 available)

! Doctor found issues in 1 category.

Answer 1

感谢@pskink 的提示，我设法让我的 CustomPainter 不断地重新绘制，尽管它仍然不完美。
见 dart/flutter: CustomPaint updates at a lower rate than ValueNotifier's value update
因为当前的解决方案仍然存在更新率问题，所以我只是简单地报告一下我所得到的:
我基本上必须构造一个通知器并通过轮询在适当的位置为其分配 ffi 检索到的值。

// in initState() of State class
_notifier = ValueNotifier<NativeColor>(ffiGetColor().ref);

...

// in a timer method
_notifier.value = ffiGetColor().ref;

然后用 CustomPaint , 绑定(bind)通知器以在其构造函数中重绘

  @override
  Widget build(BuildContext context) {
    return Container(

      ...


        child: CustomPaint(
          painter: _ColorViewPainter(
              context: context,
              notifier: _notifier,
              ...
          )
        )
    );
  }

class _ColorViewPainter extends CustomPainter {
  ValueNotifier<NativeColor> notifier;
  BuildContext context;
  Color clrBackground;

  _ColorViewPainter({this.context, this.notifier, this.clrBackground})
    : super(repaint: notifier) {
  }

  @override
  bool shouldRepaint(_ColorViewPainter old) {
    print('should repaint');
    return true;
  }

  @override
  void paint(Canvas canvas, Size size) {
    print("paint: start");
    final r = notifier.value.r;
    final g = notifier.value.g;
    final b = notifier.value.b;
    print("color: $r, $g, $b");
    final paint = Paint()
        ..strokeJoin = StrokeJoin.round
        ..strokeWidth = 1.0
        ..color = Color.fromARGB(255, r, g, b)
        ..style = PaintingStyle.fill;

    final width = size.width;
    final height = size.height;
    final content = Offset(0.0, 0.0) & Size(width, height);
    canvas.drawRect(content, paint);
    print("paint: end");
  }

}

问题 ValueNotifier依赖于以下条件才能向其监听器发出通知

它绑定(bind)的值类型定义了 operator ==

ValueNotifier的值字段，您选择的数据对象绑定(bind)到该字段，被明确重新分配 带有关于更改的新数据对象。

当我们将通知程序绑定(bind)到自定义类时，这一点至关重要。使用自定义类，我们必须

重载其operator == .

将新对象分配给 ValueNotifier<MyClass>.value而不是通过调用自己的常规方法来修改数据对象的值。

否则 CustomPaint的 paint()不会在所需的更改上调用。
举个例子，这个自定义类没有资格与 ValueNotifier 绑定(bind)。 , 因为没有重载 operator== :

class MyClass {
  int prop = 0;

  void changeValue(newValue) {
    prop = newValue;
  }

}

再举一个例子，假设我们有一个自定义类:

class MyClass {
  int prop = 0;

  @override
  bool operator ==(covariant MyClass other) {
    return other is MyClass && prop != other. prop;
  }

  void changeValue(newValue) {
    prop = newValue;
  }

}

这将起作用:

class _MyViewState extends State<MyView> {
  ValueNotifier<MyClass> notifier;
  Timer _timer;

  ....

  @override
  initState() {
    super.initState();

    notifier = ValueNotifier<MyClass>(MyClass());
    _timer = Timer.periodic(Duration(milliseconds: 10), _updateData);
  }

  _updateData(Timer t) {
     var myObj = MyClass(newValue);
     notifier.value = myObj;
  }

}

但这行不通。

class _MyViewState extends State<MyView> {
  ValueNotifier<MyClass> notifier;
  Timer _timer;

  ....

  @override
  initState() {
    super.initState();

    notifier = ValueNotifier<MyClass>(MyClass());
    _timer = Timer.periodic(Duration(milliseconds: 10), _updateData);
  }


  _updateData(Timer t) {
     var newValue = getNewValueSomewhere();
     notifier.value.changeValue(newValue);
  }