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Android Flutter绘制有趣的 loading加载动画

作者：岛上码农

在网络速度较慢的场景，一个有趣的加载会提高用户的耐心和对 App 的好感。本篇我们利用Flutter 的 PathMetric来玩几个有趣的 loading 效果，感兴趣的可以动手尝试一下

前言

在网络速度较慢的场景，一个有趣的加载会提高用户的耐心和对 App 的好感，有些 loading 动效甚至会让用户有想弄清楚整个动效过程到底是怎么样的冲动。然而，大部分的 App的 loading 就是下面这种千篇一律的效果 —— 俗称“转圈”。

本篇我们利用Flutter 的 PathMetric来玩几个有趣的 loading 效果。

效果1：圆环内滚动的球

如上图所示，一个红色的小球在蓝色的圆环内滚动，而且在往上滚动的时候速度慢，往下滚动的时候有个明显的加速过程。这个效果实现的思路如下：

下面是实现代码：

// 动画控制设置
controller =
  AnimationController(duration: const Duration(seconds: 3), vsync: this);
animation = Tween<double>(begin: 0, end: 1.0).animate(CurvedAnimation(
  parent: controller,
  curve: Curves.slowMiddle,
))
..addListener(() {
  setState(() {});
});

// 绘制和动画控制方法
_drawLoadingCircle(Canvas canvas, Size size) {
  var paint = Paint()..style = PaintingStyle.stroke
    ..color = Colors.blue[400]!
    ..strokeWidth = 2.0;
  var path = Path();
  final radius = 40.0;
  var center = Offset(size.width / 2, size.height / 2);
  path.addOval(Rect.fromCircle(center: center, radius: radius));
  canvas.drawPath(path, paint);
  
  var innerPath = Path();
  final ballRadius = 4.0;
  innerPath.addOval(Rect.fromCircle(center: center, radius: radius - ballRadius));
  var metrics = innerPath.computeMetrics();
  paint.color = Colors.red;
  paint.style = PaintingStyle.fill;
  for (var pathMetric in metrics) {
    var tangent = pathMetric.getTangentForOffset(pathMetric.length * animationValue);
    canvas.drawCircle(tangent!.position, ballRadius, paint);
  }
}

效果2：双轨运动

上面的实现效果其实比较简单，就是绘制了一个圆和一个椭圆，然后让两个实心圆沿着路径运动。因为有了这个组合效果，趣味性增加不少，外面的椭圆看起来就像是一条卫星轨道一样。实现的逻辑如下：

具体实现的代码如下所示。

controller =
      AnimationController(duration: const Duration(seconds: 2), vsync: this);
  animation = Tween<double>(begin: 0, end: 1.0).animate(CurvedAnimation(
    parent: controller,
    curve: Curves.easeInOutSine,
  ))
    ..addListener(() {
      setState(() {});
    });

_drawTwinsCircle(Canvas canvas, Size size) {
  var paint = Paint()
    ..style = PaintingStyle.stroke
    ..color = Colors.blue[400]!
    ..strokeWidth = 2.0;

  final radius = 50.0;
  final ballRadius = 6.0;
  var center = Offset(size.width / 2, size.height / 2);
  var circlePath = Path()
    ..addOval(Rect.fromCircle(center: center, radius: radius));
  paint.style = PaintingStyle.stroke;
  paint.color = Colors.blue[400]!;
  canvas.drawPath(circlePath, paint);

  var circleMetrics = circlePath.computeMetrics();
  for (var pathMetric in circleMetrics) {
    var tangent = pathMetric
        .getTangentForOffset(pathMetric.length * animationValue);

    paint.style = PaintingStyle.fill;
    paint.color = Colors.blue;
    canvas.drawCircle(tangent!.position, ballRadius, paint);
  }

  paint.style = PaintingStyle.stroke;
  paint.color = Colors.green[600]!;
  var ovalPath = Path()
    ..addOval(Rect.fromCenter(center: center, width: 3 * radius, height: 40));
  canvas.drawPath(ovalPath, paint);
  var ovalMetrics = ovalPath.computeMetrics();

  for (var pathMetric in ovalMetrics) {
    var tangent =
        pathMetric.getTangentForOffset(pathMetric.length * animationValue);

    paint.style = PaintingStyle.fill;
    canvas.drawCircle(tangent!.position, ballRadius, paint);
  }
}

效果3：钟摆运动

钟摆运动的示意图如下所示，一条绳子系着一个球悬挂某处，把球拉起一定的角度释放后，球就会带动绳子沿着一条圆弧来回运动，这条圆弧的半径就是绳子的长度。

这个效果通过代码来实现的话，需要做下面的事情：

具体实现的代码如下，这里在绘制球的时候给 Paint 对象增加了一个 maskFilter 属性，以便让球看起来发光，更加好看点。

controller =
        AnimationController(duration: const Duration(seconds: 2), vsync: this);
animation = Tween<double>(begin: 0, end: 1.0).animate(CurvedAnimation(
  parent: controller,
  curve: Curves.easeInOutQuart,
))
  ..addListener(() {
    setState(() {});
  }
  ..addStatusListener((status) {
   if (status == AnimationStatus.completed) {
     controller.reverse();
   } else if (status == AnimationStatus.dismissed) {
     controller.forward();
   }
 });

_drawPendulum(Canvas canvas, Size size) {
  var paint = Paint()
    ..style = PaintingStyle.stroke
    ..color = Colors.blue[400]!
    ..strokeWidth = 2.0;

  final ceilWidth = 60.0;
  final pendulumHeight = 200.0;
  var ceilCenter =
      Offset(size.width / 2, size.height / 2 - pendulumHeight / 2);
  var ceilPath = Path()
    ..moveTo(ceilCenter.dx - ceilWidth / 2, ceilCenter.dy)
    ..lineTo(ceilCenter.dx + ceilWidth / 2, ceilCenter.dy);
  canvas.drawPath(ceilPath, paint);

  var pendulumArcPath = Path()
    ..addArc(Rect.fromCircle(center: ceilCenter, radius: pendulumHeight),
        3 * pi / 4, -pi / 2);

  paint.color = Colors.white70;
  var metrics = pendulumArcPath.computeMetrics();

  for (var pathMetric in metrics) {
    var tangent =
        pathMetric.getTangentForOffset(pathMetric.length * animationValue);

    canvas.drawLine(ceilCenter, tangent!.position, paint);
    paint.style = PaintingStyle.fill;
    paint.color = Colors.blue;
    paint.maskFilter = MaskFilter.blur(BlurStyle.solid, 4.0);
    canvas.drawCircle(tangent.position, 16.0, paint);
  }
}

总结

本篇介绍了三种 Loading 动效的绘制逻辑和实现代码，可以看到利用路径属性进行绘图以及动画控制可以实现很多有趣的动画效果。

以上就是Android Flutter绘制有趣的 loading加载动画的详细内容，更多关于Android Flutter加载动画的资料请关注脚本之家其它相关文章！

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