Flutter自定义CustomPainter中获取ImageShader参数ui.Image对象的方法

ImageShader需要传入dart:ui包里的Image对象，ui.Image需要异步请求获取：

  Future<ui.Image> loadImage() async {
    ByteData byteData = await rootBundle.load('assets/images/target_image.jpg');
    ui.Codec codec =
        await ui.instantiateImageCodec(byteData.buffer.asUint8List());
    ui.FrameInfo frameInfo = await codec.getNextFrame();
    return frameInfo.image;
  }

建议在State初始化的时候获取异步资源，再传入到自定义的CustomPainter中，否则会报错：

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  ui.Image _image;

  @override
  void initState() {
    super.initState();
    loadImage().then((value) {
      setState(() {
        _image = value;
      });
    });
  }

  Future<ui.Image> loadImage() async {
    ByteData byteData = await rootBundle.load('assets/images/target_image.jpg');
    ui.Codec codec =
        await ui.instantiateImageCodec(byteData.buffer.asUint8List());
    ui.FrameInfo frameInfo = await codec.getNextFrame();
    return frameInfo.image;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        color: Colors.white,
        child: _image == null
            ? Center(
                child: Text("loading..."),
              )
            : CustomPaint(
                painter: PaperPainter(_image),
              ),
      ),
    );
  }
}

class PaperPainter extends CustomPainter {
  ui.Image _image;
  Paint _paint = Paint();

  PaperPainter(this._image);

  @override
  void paint(Canvas canvas, Size size) async {
    print("image: $_image");

    _paint.shader = ImageShader(_image, TileMode.repeated, TileMode.repeated,
        Float64List.fromList([1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1]));

    canvas.drawCircle(Offset(100, 100), 50, _paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return true;
  }
}

网上另一种获取ui.Image对象的方法：

  Future<ui.Image> loadImage() {
    ImageProvider provider = AssetImage('assets/images/target_image.jpg');
    Completer<ui.Image> completer = Completer<ui.Image>();
    ImageStreamListener listener;
    ImageStream stream = provider.resolve(ImageConfiguration());
    listener = ImageStreamListener((info, syno) {
      final ui.Image image = info.image;
      completer.complete(image);
      stream.removeListener(listener);
    });
    stream.addListener(listener);
    return completer.future;
  }