Flutter如何给控件实现微光特效
这篇文章主要介绍了Flutter如何给控件实现微光特效，具有一定借鉴价值，感兴趣的朋友可以参考下，希望大家阅读完这篇文章之后大有收获，下面让小编带着大家一起了解一下。
效果图
使用方法
Shimmer(
baseColor: const Color(0x08ffffff), // 背景颜色
highlightColor: Colors.white, // 高光的颜色
loop: 2, // 闪烁循环次数，不传默认一直循环
child: Image.asset('assets/images/watermelon.png',width: 325, height: 260, fit: BoxFit.contain),
)
实现原理
① 特效控件分为两层：底层显示调用方传入的控件；上层覆盖一层渐变着色器。
② 启动动画，根据动画的进度，对渐变着色器的区域进行绘制，当区域变大变小时，着色器高光的地方也在相应进行偏移。
③ 同时着色器不能超出底层控件的绘制范围，底层控件的形状是不规则的，渐变层不能超出底层控件的layer对象。这样才能实现完全贴合 底层控件形状 的微光闪烁。
控件分层显示
@override
Widget build(BuildContext context) {
return Stack(
children: [
// 底层控件
widget.child,
// 覆盖闪烁微光
AnimatedBuilder(
animation: _controller,
child: widget.child,
builder: (BuildContext context, Widget? child) => _Shimmer(
child: child,
percent: _controller.value,
direction: widget.direction,
gradient: widget.gradient,
),
)
],
);
开启动画
late AnimationController _controller;
int _count = 0;
@override
void initState() {
super.initState();
_controller = AnimationController(vsync: this, duration: widget.duration)
..addStatusListener((AnimationStatus status) {
if (status != AnimationStatus.completed) {
return;
}
_count++;
if (widget.loop != 0 && _count < widget.loop) {
_controller.forward(from: 0.0);
}
});
if (widget.loop == 0) {
_controller.repeat();
} else {
_controller.forward();
}
}
重点：着色器该如何绘制，又该如何通过AnimationController的进度进行偏移？由于着色器不能超出底层控件的绘制范围，所以必须拿到底层控件的绘制上下文【即 PaintingContext】，调用其pushLayer方法，让引擎把着色器绘制上去。
需要用到PaintingContext，自然就需要去管理RenderObject，所以着色器的编写使用RenderProxyBox进行计算并绘制出layer对象，计算的过程根据上面的AnimationController的进度进行计算。
class _ShimmerFilter extends RenderProxyBox {
ShimmerDirection _direction;
Gradient _gradient;
double _percent;
_ShimmerFilter(this._percent, this._direction, this._gradient);
@override
ShaderMaskLayer? get layer => super.layer as ShaderMaskLayer?;
set percent(double newValue) {
if (newValue != _percent) {
_percent = newValue;
markNeedsPaint();
}
}
set gradient(Gradient newValue) {
if (newValue != _gradient) {
_gradient = newValue;
markNeedsPaint();
}
}
set direction(ShimmerDirection newDirection) {
if (newDirection != _direction) {
_direction = newDirection;
markNeedsLayout();
}
}
@override
void paint(PaintingContext context, Offset offset) {
if (child != null) {
final double width = child!.size.width;
final double height = child!.size.height;
Rect rect;
double dx, dy;
if (_direction == ShimmerDirection.rtl) {
dx = _offset(width, -width, _percent);
dy = 0.0;
rect = Rect.fromLTWH(dx - width, dy, 3 * width, height);
} else if (_direction == ShimmerDirection.ttb) {
dx = 0.0;
dy = _offset(-height, height, _percent);
rect = Rect.fromLTWH(dx, dy - height, width, 3 * height);
} else if (_direction == ShimmerDirection.btt) {
dx = 0.0;
dy = _offset(height, -height, _percent);
rect = Rect.fromLTWH(dx, dy - height, width, 3 * height);
} else {
dx = _offset(-width, width, _percent);
dy = 0.0;
rect = Rect.fromLTWH(dx - width, dy, 3 * width, height);
}
layer ??= ShaderMaskLayer();
layer!
..shader = _gradient.createShader(rect)
..maskRect = offset & size
..blendMode = BlendMode.srcIn;
context.pushLayer(layer!, super.paint, offset);
} else {
layer = null;
}
}
double _offset(double start, double end, double percent) {
return start + (end - start) * percent;
}
}
Render对象绘制出来后，需要封装成widget使用，由于是单一组件，用SingleChildRenderObjectWidget即可。
class _Shimmer extends SingleChildRenderObjectWidget {
@override
_ShimmerFilter createRenderObject(BuildContext context) {
return _ShimmerFilter(percent, direction, gradient);
}
@override
void updateRenderObject(BuildContext context, _ShimmerFilter shimmer) {
shimmer.percent = percent;
shimmer.gradient = gradient;
shimmer.direction = direction;
}
}
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