浅谈Flutter 中渐变的高级用法(3种)

砸漏

发布于 2020-10-16 10:44:23

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发布于 2020-10-16 10:44:23

文章被收录于专栏：恩蓝脚本

Flutter 中渐变有三种：

LinearGradient：线性渐变
RadialGradient：放射状渐变
SweepGradient：扇形渐变

看下原图，下面的渐变都是在此图基础上完成。

LinearGradient

给一张图片添加从上到下的线性渐变：

ShaderMask(
 shaderCallback: (Rect bounds) {
  return LinearGradient(
   begin: Alignment.topCenter,
   end: Alignment.bottomCenter,
   colors: [Colors.red,Colors.blue,Colors.green],
  ).createShader(bounds);
 },
 blendMode: BlendMode.color,
 child: Image.asset(
  'assets/images/b.jpg',
  fit: BoxFit.cover,
 ),
)

begin 和 end 表示渐变的方向，上面设置的方向是从顶部中间到底部中间。

color 表示渐变的颜色。

设置各个渐变色的结束点：

Color color = Colors.orange;
return ShaderMask(
 shaderCallback: (Rect bounds) {
  return LinearGradient(
    begin: Alignment.topCenter,
    end: Alignment.bottomCenter,
    colors: [color,color,Colors.transparent,Colors.transparent,color,color],
    stops: [0,.4,.41,.6,.61,1]
  ).createShader(bounds);
 },
 blendMode: BlendMode.color,
 child: Image.asset(
  'assets/images/b.jpg',
  fit: BoxFit.cover,
 ),
);

stops 的个数要对应 color 。

由于中间设置的渐变色为透明，所以中间是原图。

RadialGradient

RadialGradient 是放射状渐变。

ShaderMask(
   shaderCallback: (Rect bounds) {
    return RadialGradient(
     radius: .5,
     colors: <Color [Colors.red, Colors.blue],
    ).createShader(bounds);
   },
   blendMode: BlendMode.color,
   child: Image.asset(
    'assets/images/b.jpg',
    fit: BoxFit.cover,
   ),
  )

实现中间显示圆形原图，其他地方有灰色蒙板：

ShaderMask(
 shaderCallback: (Rect bounds) {
  return RadialGradient(
   radius: .6,
   colors: <Color [
    Colors.transparent,
    Colors.transparent,
    Colors.grey.withOpacity(.7),
    Colors.grey.withOpacity(.7)
   ],
   stops: [0, .5, .5, 1],
  ).createShader(bounds);
 },
 blendMode: BlendMode.srcATop,
 child: Image.asset(
  'assets/images/b.jpg',
  fit: BoxFit.cover,
 ),
)

SweepGradient

SweepGradient 扇形渐变效果。

ShaderMask(
 shaderCallback: (Rect bounds) {
  return SweepGradient(
   colors: <Color [
    Colors.red,
    Colors.blue
   ],
  ).createShader(bounds);
 },
 child: Image.asset(
  'assets/images/b.jpg',
  fit: BoxFit.cover,
 ),
)

startAngle 和 endAngle 表示开始和结束角度。

绘制渐变圆环：

Container(
    width: 200,
    height: 200,
    child: CustomPaint(
     painter: _CircleProgressPaint(.5),
    ),
   )

class _CircleProgressPaint extends CustomPainter {
 final double progress;

 _CircleProgressPaint(this.progress);

 Paint _paint = Paint()
  ..style = PaintingStyle.stroke
  ..strokeWidth = 20;

 @override
 void paint(Canvas canvas, Size size) {
  _paint.shader = ui.Gradient.sweep(
    Offset(size.width / 2, size.height / 2), [Colors.red, Colors.yellow]);
  canvas.drawArc(
    Rect.fromLTWH(0, 0, size.width, size.height), 0, pi*2, false, _paint);
 }

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

除了图片，可以给任何组件加入渐变效果，比如文字：

ShaderMask(
 shaderCallback: (Rect bounds) {
  return LinearGradient(
   colors: <Color [Colors.blue, Colors.red],
   tileMode: TileMode.mirror,
  ).createShader(bounds);
 },
 blendMode: BlendMode.srcATop,
 child: Center(
  child: Text(
   '老孟，一枚有态度的程序员',
   style: TextStyle(fontSize: 24),
  ),
 ),
)