flutter - 如何在 flutter 中为路径设置动画？

Question

我想实现这里看到的路径动画效果:

This animation (I couldn't include it because the gif is too big)

我只想实现 map 上的路径动画，我知道我需要使用堆叠，放置我的 map ，然后使用Painter来绘制这样的路径，但是我怎样才能动画呢？

Answer 1

我知道这个问题有一个公认的答案，但我想展示这个问题的替代解决方案。

首先，从单个点创建自定义路径对于以下情况不是最佳的:

• 计算每个段的长度并非易事
• 以较小的增量均匀地制作步骤动画既困难又耗费资源
• 不适用于二次/贝塞尔线段

就像在旧的 Android 中一样 is this path tracing method , 非常相似的 PathMetrics 也是如此存在于 Flutter 中。

在这个问题的公认答案的基础上，这里有一种更通用的方法来为任何路径设置动画。

---

所以给定一个路径和一个动画百分比，我们需要提取从开始到那个百分比的路径:

Path createAnimatedPath(
  Path originalPath,
  double animationPercent,
) {
  // ComputeMetrics can only be iterated once!
  final totalLength = originalPath
      .computeMetrics()
      .fold(0.0, (double prev, PathMetric metric) => prev + metric.length);

  final currentLength = totalLength * animationPercent;

  return extractPathUntilLength(originalPath, currentLength);
}

所以现在我只需要提取一个路径直到给定长度(而不是百分比)。我们需要结合所有现有的路径，直到一定距离。然后将最后一个路径段的一部分添加到现有路径中。

这样做非常简单。

Path extractPathUntilLength(
  Path originalPath,
  double length,
) {
  var currentLength = 0.0;

  final path = new Path();

  var metricsIterator = originalPath.computeMetrics().iterator;

  while (metricsIterator.moveNext()) {
    var metric = metricsIterator.current;

    var nextLength = currentLength + metric.length;

    final isLastSegment = nextLength > length;
    if (isLastSegment) {
      final remainingLength = length - currentLength;
      final pathSegment = metric.extractPath(0.0, remainingLength);

      path.addPath(pathSegment, Offset.zero);
      break;
    } else {
      // There might be a more efficient way of extracting an entire path
      final pathSegment = metric.extractPath(0.0, metric.length);
      path.addPath(pathSegment, Offset.zero);
    }

    currentLength = nextLength;
  }

  return path;
}

---

整个示例所需的其余代码:

void main() => runApp(
  new MaterialApp(
    home: new AnimatedPathDemo(),
  ),
);

class AnimatedPathPainter extends CustomPainter {
  final Animation<double> _animation;

  AnimatedPathPainter(this._animation) : super(repaint: _animation);

  Path _createAnyPath(Size size) {
    return Path()
      ..moveTo(size.height / 4, size.height / 4)
      ..lineTo(size.height, size.width / 2)
      ..lineTo(size.height / 2, size.width)
      ..quadraticBezierTo(size.height / 2, 100, size.width, size.height);
  }

  @override
  void paint(Canvas canvas, Size size) {
    final animationPercent = this._animation.value;

    print("Painting + ${animationPercent} - ${size}");

    final path = createAnimatedPath(_createAnyPath(size), animationPercent);

    final Paint paint = Paint();
    paint.color = Colors.amberAccent;
    paint.style = PaintingStyle.stroke;
    paint.strokeWidth = 10.0;

    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}

class AnimatedPathDemo extends StatefulWidget {
  @override
  _AnimatedPathDemoState createState() => _AnimatedPathDemoState();
}

class _AnimatedPathDemoState extends State<AnimatedPathDemo>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;

  void _startAnimation() {
    _controller.stop();
    _controller.reset();
    _controller.repeat(
      period: Duration(seconds: 5),
    );
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: const Text('Animated Paint')),
      body: SizedBox(
        height: 300,
        width: 300,
        child: new CustomPaint(
          painter: new AnimatedPathPainter(_controller),
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _startAnimation,
        child: new Icon(Icons.play_arrow),
      ),
    );
  }

  @override
  void initState() {
    super.initState();
    _controller = new AnimationController(
      vsync: this,
    );
  }

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