Flutter Canvas绘制详解

Flutter开发

发布日期: 2020-03-07

文章字数: 939

阅读时长: 4 分

阅读次数:

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前言

Flutter是一个UI框架,一切皆Widget.Flutter可以说和Android是无缝对接,毕竟都是出自Google,Material风格更是轻车熟路,对Android同学来说,上手较快.本文要讲的Canvas也和Android中Canvas也是极其相似,一脉相承.Canvas一般可以用来绘制比较难以实现的特殊图形,控件等,比如一些多边形,弧形等等吧.Canvas就是画布,内部封装了用于绘制绘制基本图形的接口,比如矩形,圆形,点,线等.
Flutter中为我们提供了封装好的组件CustomerPaint Widget,这个组件与画笔Paint Widget组合使用就可以绘制出需要的图形.

1.CustomerPainter

CustomerPainter是真实绘制的基础类,需要绘制的图形和画笔都是在此类中实现,一般会自定义一个类继承此基类,然后重写两个方法:

1.1 paint方法

@override
void paint(Canvas canvas, Size size) {
  // TODO: implement paint
}

这个方法就是实现具体绘制的方法

1.2 shouldRepaint方法

@override
bool shouldRepaint(CustomPainter oldDelegate) {
  // TODO: implement shouldRepaint
  return null;
}

这个方法控制的是是否需要重绘

2. Paint

Paint是画笔,画笔有多个属性,可以设置颜色,线宽,填充等样式.

3. CustomerPaint

CustomerPaint可以理解为绘制的封装类,其内部成员变量就包含CustomerPainter,其构造函数:

const CustomPaint({
  Key key,
  this.painter,
  this.foregroundPainter,
  this.size = Size.zero,
  this.isComplex = false,
  this.willChange = false,
  Widget child,
})

painter: 自定义绘制的基础类绘制效果会显示在child的下面,效果如下:

foregroundPainter:自定义绘制的基础类绘制效果会显示在child的上面,效果如下:

child:子控件,也就是上图中的绿色区域
size:绘制区域大小,其取决于child
1)如果child == null,则绘制区域为size大小;
2)如果child != null,则绘制区域为child大小;
3)如果想child != null && 指定size大小,可以用SizeBox包裹一下CustomPaint;
isComplex:是否复杂绘制,默认false,如果true,则会开启缓存,避免一些不必要的绘制提高性能.
willChange:这个和isComplex一起使用,默认false,如果isComplex == true,willChange表示绘制下一帧是否会改变.

Flutter绘制的基本步骤

1. 创建继承于基类的绘制类

///
/// @ClassName FlutterPainter
/// @Description 绘制类
/// @Author waitwalker
/// @Date 2020-03-07
///
class FlutterPainter extends CustomPainter {

}

2. 在绘制类中重写paint()和shouldRepaint()方法

///
/// @ClassName FlutterPainter
/// @Description 绘制类
/// @Author waitwalker
/// @Date 2020-03-07
///
class FlutterPainter extends CustomPainter {

  @override
  void paint(Canvas canvas, Size size) {
    // TODO: implement paint
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    // TODO: implement shouldRepaint
    return null;
  }
}

3. 初始化画笔Paint&调用Canvas绘制图形

///
/// @ClassName FlutterPainter
/// @Description 绘制类
/// @Author waitwalker
/// @Date 2020-03-07
///
class FlutterPainter extends CustomPainter {

  @override
  void paint(Canvas canvas, Size size) {
    /// 初始化画笔
    var paint = Paint()
        ..strokeWidth = 25.0
        ..color = Colors.red;

    /// 通过canvas画一条直线
    /// 这里可以根据需要绘制圆形,矩形等等
    canvas.drawLine(Offset(95, 0), Offset(95, 300), paint);
  }

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

class _HomeState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: CustomPaint(
          foregroundPainter: FlutterPainter(),
          child: Container(
            width: 200,
            height: 200,
            color: Color(0x5a00C800),
          ),
        ),
      ),
    );
  }
}