Flutter——实现flow布局，自定义绘制子控件

在Flutter中，可以实现流式布局的的组件有两个：Wrap和Flow。横向排列子组件，自动换行。

实现效果：

下面是Flow布局实现的部分代码：

/**
 * 自定义流式布局
 */
class FlowLayout extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return new Flow(delegate: TestFlowDelegate(margin: EdgeInsets.all(10.0)),
      children: [
        new Container(width: 30.0, height: 50.0, color: Colors.red,),
        new Container(width: 80.0, height: 80.0, color: Colors.green,),
        new Container(width: 80.0, height: 70.0, color: Colors.blue,),
        new Container(width: 80.0, height: 160.0, color: Colors.yellow,),
        new Container(width: 130.0, height: 30.0, color: Colors.brown,),
        new Container(width: 80.0, height: 80.0, color: Colors.purple,),
      ],);
  }

}

class TestFlowDelegate extends FlowDelegate {
  EdgeInsets margin = EdgeInsets.zero; //上下左右皆为0
  TestFlowDelegate({this.margin}); //是每一个子控件的margin

  @override
  void paintChildren(FlowPaintingContext context) {
    var x = margin.left;
    var y = margin.top;
    var lineH = 0.0;

    for (int i = 0; i < context.childCount; i++) {
      var w = x + margin.right + context
          .getChildSize(i)
          .width;
      var ch = context
          .getChildSize(i)
          .height + margin.top + margin.bottom;
      if (w < context.size.width) {
        //小于屏幕宽度
        context.paintChild(i, transform: Matrix4.translationValues(x, y, 0.0));
        x = w + margin.left;
        //对比其他整数，0:相同、1:大于、-1:小于
        if (lineH.compareTo(ch) < 0) {
          //取行高和子控件高度的较大值为新的行高
          lineH = ch;
        }
      } else {
        //需要换行
        x = margin.left;
        y = y + lineH;
        context.paintChild(i, transform: Matrix4.translationValues(x, y, 0.0));
        x = x + context
            .getChildSize(i)
            .width + margin.right + margin.left;
      }
      
    }
  }

  @override
  bool shouldRepaint(FlowDelegate oldDelegate) {
    return oldDelegate != this;
  }

  getSize(BoxConstraints constraints) {
    //指定Flow的大小，高度必须是确定的值，这个大小在绘制之前就会确定，所以不能用子控件的高度和来计算
    return Size(double.infinity, 280.0);
  }

}