dart/flutter：使用 ui.Gradient.linear 时出现语法错误

Question

我坚持用我扑的项目，在那里我可以不实例化一个Android Studio中有语法错误ui.Gradient使用其ui.Gradient.linear构造。

代码

这是在 CustomPainter 子类中。 我已将编译器错误发布为下面的评论。

  import 'dart:ui';

  ...

  @override
  void paint(Canvas canvas, Size size) {
    final tracePaint = Paint()
      ..strokeJoin = StrokeJoin.round
      ..strokeWidth = 2.0
      ..color = traceColor
      ..style = PaintingStyle.stroke;

    ...

    // HERE: I've tried several ways as below and I simply cannot use this constructor.

    tracePaint.shader = Gradient.linear(
            from: Offset(size.width-currentX.toDouble(), size.height),
            to: Offset(size.width-currentX.toDouble(), 0),
            colors: colors,
            colorStops: colorStops,).createShader(...); // The method 'linear' isn't defined for the class Gradient.
    tracePaint.shader = ui.Gradient.linear(...).createShader(...); // Undefined name 'ui'.
    tracePaint.shader = dart.ui.Gradient.linear(...).createShader(...); // Undefined name 'dart'.

做出的努力

我检查了文档：

• https://api.flutter.dev/flutter/dart-ui/Gradient-class.html
• https://api.flutter.dev/flutter/dart-ui/Gradient/Gradient.linear.html

，但没找到我错的地方。

提示将不胜感激。

Answer 1

您可以在下面复制粘贴运行完整代码
第一步： import 'dart:ui' as ui;
第 2 步：请删除from: to: colors: colorStops:

工作演示

完整代码

import 'dart:math';
import 'dart:ui' as ui;

import 'package:flutter/material.dart';

class X1Painter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final tracePaint = Paint()
      ..strokeJoin = StrokeJoin.round
      ..strokeWidth = 2.0
      ..color = Colors.blue
      ..style = PaintingStyle.stroke;

    // create a bounding square, based on the centre and radius of the arc
    Rect rect = new Rect.fromCircle(
      center: new Offset(165.0, 55.0),
      radius: 180.0,
    );

    tracePaint.shader = ui.Gradient.linear(
      Offset(0, 0),
      Offset(200, 200),
      <Color>[
        Colors.green.withOpacity(1.0),
        Colors.green.withOpacity(0.3),
        Colors.yellow.withOpacity(0.2),
        Colors.red.withOpacity(0.1),
        Colors.red.withOpacity(0.0),
      ],
      [
        0.0,
        0.5,
        0.7,
        0.9,
        1.0,
      ],
    );

    // and draw an arc
    canvas.drawArc(rect, pi / 4, pi * 3 / 4, true, tracePaint);
  }

  @override
  bool shouldRepaint(X1Painter oldDelegate) {
    return true;
  }
}

class X1Demo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: const Text('Arcs etc')),
      body: new CustomPaint(
        painter: new X1Painter(),
      ),
    );
  }
}

void main() {
  runApp(
    new MaterialApp(
      theme: new ThemeData.dark(),
      home: new X1Demo(),
    ),
  );
}