如何绘制 BackgroundImage 的三角形前景（Flutter Web）

Question

在 Flutter Web 中，我想为我目前尝试过的背景图像绘制一个三角形前景

                    CustomPaint(
                      //foregroundPainter: PaintTriangle(backgroundColor: backgroundColor),
                      //foregroundPainter didn't work either!
                      painter: PaintTriangle(backgroundColor: backgroundColor),
                      child: Container(
                        decoration: BoxDecoration(
                          image: DecorationImage(
                            fit: BoxFit.fill,
                            image: NetworkImage(cardBackgroundImageUrl),
                          ),
                        ),
                      ),
                    )

这是我的PaintTriangle 已经可以绘制三角形 我需要的只是如何在背景前面绘制这个三角形 我确实尝试将画家交给foregroundPainter 但它也不起作用，也试过了Stack 小部件也不起作用

import 'package:flutter/cupertino.dart';


class PaintTriangle extends CustomPainter {
  final Color backgroundColor;

  PaintTriangle({
    @required this.backgroundColor,
  });

  @override
  void paint(Canvas canvas, Size size) {
    final y = size.height;
    final x = size.width;

    final paint = Paint()
      ..color = backgroundColor;
    final path = Path();

    path
      ..moveTo(0, y)
      ..lineTo((x / 2), (y / 1.5))..lineTo(x, y);


    canvas.drawPath(path, paint);
  }

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

}

Answer 1

您可以在下面复制粘贴运行完整代码
使用Stack
代码 sn -p

Stack(
          children: <Widget>[
            Center(
              child: Image.network(
                'https://picsum.photos/250?image=9',
              ),
            ),
            Center(
              child: CustomPaint( //                       <-- CustomPaint widget
                size: Size(300, 300),
                painter: PaintTriangle(backgroundColor: Colors.blue),
              ),
            ),

工作演示

完整代码

import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';

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

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 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: Stack(
          children: <Widget>[
            Center(
              child: Image.network(
                'https://picsum.photos/250?image=9',
              ),
            ),
            Center(
              child: CustomPaint( //                       <-- CustomPaint widget
                size: Size(300, 300),
                painter: PaintTriangle(backgroundColor: Colors.blue),
              ),
            ),

          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}




class PaintTriangle extends CustomPainter {
  final Color backgroundColor;

  PaintTriangle({
    @required this.backgroundColor,
  });

  @override
  void paint(Canvas canvas, Size size) {
    final y = size.height;
    final x = size.width;

    final paint = Paint()
      ..color = backgroundColor;
    final path = Path();

    path
      ..moveTo(0, y)
      ..lineTo((x / 2), (y / 1.5))..lineTo(x, y);


    canvas.drawPath(path, paint);
  }

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

}