ColorFilter example

main.dart

import 'dart:ui' as ui;
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

/// Applies a color filter before painting its child.
class ColorFiltered extends SingleChildRenderObjectWidget {
  /// Creates a widget that color filters its child.
  ///
  /// The [colorFilter] argument must not be null.
  const ColorFiltered({
    Key key,
    @required this.colorFilter,
    Widget child,
  }) : assert(colorFilter != null),
        super(key: key, child: child);

  /// The color filter to apply to the existing painted content when painting the child.
  final ColorFilter colorFilter;

  @override
  RenderColorFiltered createRenderObject(BuildContext context) {
    return RenderColorFiltered(colorFilter: colorFilter);
  }

  @override
  void updateRenderObject(BuildContext context, RenderColorFiltered renderObject) {
    renderObject.colorFilter = colorFilter;
  }
}

/// A composited layer that applies a color filter to its children.
class ColorFilterLayer extends ContainerLayer {
  /// Creates a color filter layer.
  ColorFilterLayer({
    @required ColorFilter colorFilter,
  }) : _colorFilter = colorFilter;

  /// The color filter to apply to the children.
  ///
  /// The scene must be explicitly recomposited after this property is changed
  /// (as described at [Layer]).
  ColorFilter get colorFilter => _colorFilter;
  ColorFilter _colorFilter;
  set colorFilter(ColorFilter value) {
    if (value != _colorFilter) {
      _colorFilter = value;
      markNeedsAddToScene();
    }
  }

  @override
  ui.EngineLayer addToScene(ui.SceneBuilder builder, [ Offset layerOffset = Offset.zero ]) {
    // TODO(jackson): We should use our colorFilter here.
    // Unfortunately SceneBuilder doesn't support ColorFilter.matrix, only
    // ColorFilters with a color and blend mode are supported. An engine
    // update is required to support arbitrary color filters.
    builder.pushColorFilter(Colors.red.withOpacity(0.5), BlendMode.difference);
    addChildrenToScene(builder, layerOffset);
    builder.pop();
    return null; // this does not return an engine layer yet.
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<ColorFilter>('colorFilter', colorFilter));
  }
}

/// Applies a color filter to the existing painted content and then paints [child].
class RenderColorFiltered extends RenderProxyBox {
  /// Creates a color filter.
  ///
  /// The [colorFilter] argument must not be null.
  RenderColorFiltered({ RenderBox child, @required ColorFilter colorFilter })
      : assert(colorFilter != null),
        _colorFilter = colorFilter,
        super(child);

  /// The color filter to apply to the existing painted content before painting
  /// the child.
  ColorFilter get colorFilter => _colorFilter;
  ColorFilter _colorFilter;
  set colorFilter(ColorFilter value) {
    assert(value != null);
    if (_colorFilter == value)
      return;
    _colorFilter = value;
    markNeedsPaint();
  }

  @override
  bool get alwaysNeedsCompositing => child != null;

  @override
  void paint(PaintingContext context, Offset offset) {
    if (child != null) {
      assert(needsCompositing);
      context.pushLayer(ColorFilterLayer(colorFilter: _colorFilter), super.paint, offset);
    }
  }
}

void main() => runApp(MyApp());

// This is the standard "flutter create" sample app with a ColorFiltered added
class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return ColorFiltered(
      colorFilter: ColorFilter.mode(
        Colors.red.withOpacity(0.5),
        BlendMode.values[_counter % BlendMode.values.length],
      ),
      child:
      Scaffold(
        appBar: AppBar(
          // Here we take the value from the MyHomePage object that was created by
          // the App.build method, and use it to set our appbar title.
          title: Text(widget.title),
        ),
        body: Center(
          // Center is a layout widget. It takes a single child and positions it
          // in the middle of the parent.
          child: Column(
            // Column is also layout widget. It takes a list of children and
            // arranges them vertically. By default, it sizes itself to fit its
            // children horizontally, and tries to be as tall as its parent.
            //
            // Invoke "debug painting" (press "p" in the console, choose the
            // "Toggle Debug Paint" action from the Flutter Inspector in Android
            // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
            // to see the wireframe for each widget.
            //
            // Column has various properties to control how it sizes itself and
            // how it positions its children. Here we use mainAxisAlignment to
            // center the children vertically; the main axis here is the vertical
            // axis because Columns are vertical (the cross axis would be
            // horizontal).
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text(
                'You have pushed the button this many times:',
              ),
              Text(
                '$_counter',
                style: Theme.of(context).textTheme.display1,
              ),
            ],
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: _incrementCounter,
          tooltip: 'Increment',
          child: Icon(Icons.add),
        ), // This trailing comma makes auto-formatting nicer for build methods.
      ),
    );
  }
}