昨天有同学在QQ群里问一个动画效果怎么实现

image.png

想了一下，步骤如下：

1. 画一条曲线
2. 取到曲线上的点
3. 有所有点信息了，想怎么画动画都可以

其实难点就是取到曲线上的点信息。还好Flutter的Path提供了一个computeMetrics的函数，通过这个函数我们可以拿到PathMetric，再通过PathMetric的getTangentForOffset函数拿到相关点的信息

核心的paint函数如下：

void paint(Canvas canvas, Size size) {

    if (_path == null) {
      return;
    }
    PathMetrics pms = _path.computeMetrics();
    PathMetric pm = pms.elementAt(0);
    double len = pm.length;
    double linelen = len / 20;

    double tmpStart = _fraction;
    double end = (_fraction + linelen) > len ? len : (_fraction + linelen);
    if (end >= len) {
      tmpStart = len - linelen;
    }
    for (; tmpStart < end; tmpStart++) {
      Tangent t = pm.getTangentForOffset(tmpStart);
      _points.add(t.position);
    }

    canvas.drawPath(_path, _paint);
    canvas.drawPoints(PointMode.points, _points, _paint2);
  }

剩下的就是普通的Flutter动画控制了

上一个完成的效果gif

path_animation.gif

没有其他说的了，全部代码如下：

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

class PathAnimation extends StatefulWidget {
  @override
  _PathAnimationState createState() => _PathAnimationState();
}

class _PathAnimationState extends State<PathAnimation>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation _animation;
  double _fraction = 0.0;
  int _seconds = 6;
  Path _path;
  GlobalKey _canvasKey = GlobalKey();

  @override
  void initState() {
    _controller =
        AnimationController(vsync: this, duration: Duration(seconds: _seconds));
    
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Path Animation'),
      ),
      body: Container(
        color: Colors.grey,
        child: Column(
          children: <Widget>[
            CustomPaint(
            painter: PathPainter(_path, _fraction),
            child: Container(
              key: _canvasKey,
              height: 500.0,
            ),
          ),
          RaisedButton(
            onPressed: _startAnimation,
            child: Text('Go'),
          )
          ],
        ),
      ),
    );
  }

  Path _getPath(Size size) {
    Path path = Path();
    path.moveTo(0, size.height / 2);
    // path.quadraticBezierTo(
    //     size.width / 2, size.height, size.width, size.height / 2);
    path.cubicTo(size.width / 4, 3 * size.height / 4, 3 * size.width / 4, size.height / 4, size.width, size.height);

    return path;
  }

  _startAnimation() {
    RenderBox renderBox = _canvasKey.currentContext.findRenderObject();
    Size s = renderBox.size;

    _path = _getPath(s);
    PathMetrics pms = _path.computeMetrics();
    PathMetric pm = pms.elementAt(0);
    double len = pm.length;
    
    _animation = Tween(begin: 0.0, end: len).animate(_controller)
      ..addListener(() {
        setState(() {
          _fraction = _animation.value;
        });
      })
      ..addStatusListener((status){
        if (status == AnimationStatus.completed) {
          _controller.stop();
        }
      });

    _controller.forward();
    
  }
}

class PathPainter extends CustomPainter {
  double _fraction;
  Path _path;
  List<Offset> _points = List<Offset>();

  PathPainter(this._path, this._fraction);

  Paint _paint = Paint()
    ..color = Colors.blue
    ..style = PaintingStyle.stroke
    ..strokeWidth = 4.0;

  Paint _paint2 = Paint()
    ..color = Colors.red
    ..style = PaintingStyle.stroke
    ..strokeWidth = 4.0;

  @override
  void paint(Canvas canvas, Size size) {

    if (_path == null) {
      return;
    }
    PathMetrics pms = _path.computeMetrics();
    PathMetric pm = pms.elementAt(0);
    double len = pm.length;
    double linelen = len / 20;

    double tmpStart = _fraction;
    double end = (_fraction + linelen) > len ? len : (_fraction + linelen);
    if (end >= len) {
      tmpStart = len - linelen;
    }
    for (; tmpStart < end; tmpStart++) {
      Tangent t = pm.getTangentForOffset(tmpStart);
      _points.add(t.position);
    }

    canvas.drawPath(_path, _paint);
    canvas.drawPoints(PointMode.points, _points, _paint2);
  }

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