Flutter教学目录持续更新中

Github源代码持续更新中

1.CustomMultiChildLayout介绍

使用一个委托来对多个孩子进行设置大小和定位的小部件

2.CustomMultiChildLayout属性

• delegate：MultiChildLayoutDelegate
• children = const <Widget>[]：LayoutId

3.LayoutId属性

• id：子组件id
• child：

4.使用

class CustomMultiChildLayoutPage extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('CustomMultiChildLayout'),
      ),
      body: _MyMultiChildLayout(
        children: [
          LayoutId(
            id: 'A',
            child: Container(
              width: 50,
              height: 50,
              color: Colors.blue,
            ),
          ),
          LayoutId(
            id: 'B',
            child: Container(
              width: 100,
              height: 100,
              color: Colors.green,
            ),
          ),
          LayoutId(
            id: 'C',
            child: Container(
              width: 50,
              height: 120,
              color: Colors.amber,
            ),
          ),
          LayoutId(
            id: 'D',
            child: Container(
              width: 120,
              height: 50,
              color: Colors.red,
            ),
          ),
        ],
      ),
    );
  }
}

class _MyMultiChildLayout extends CustomMultiChildLayout {
  _MyMultiChildLayout({
    Key key,
    List<Widget> children = const <Widget>[],
  }) : super(
            key: key,
            delegate: _MyMultiChildLayoutDelegate(children),
            children: children);
}

class _MyMultiChildLayoutDelegate extends MultiChildLayoutDelegate {
  List<String> layoutIds;
  Map<String, Size> sizeMap = {};

  _MyMultiChildLayoutDelegate(List<Widget> children) {
    layoutIds = _getLayoutIds(children);
  }

  _getLayoutIds(List<Widget> children) {
    return children.map((e) {
      return (e as LayoutId).id as String;
    }).toList();
  }

  @override
  void performLayout(Size size) {
    //布局子组件
    print('performLayout size = $size');
    for (String id in layoutIds) {
      if (hasChild(id)) {
        sizeMap[id] = layoutChild(id, BoxConstraints());
        switch (id) {
          case 'A':
            positionChild(id, Offset.zero);
            break;
          case 'B':
            positionChild(id, Offset(sizeMap['A'].width, 0));
            break;
          case 'C':
            positionChild(id, Offset(0, sizeMap['A'].height));
            break;
          case 'D':
            positionChild(id, Offset(sizeMap['C'].width, sizeMap['B'].height));
            break;
        }
      }
    }
  }

  @override
  bool shouldRelayout(covariant MultiChildLayoutDelegate oldDelegate) {
    print('shouldRelayout');
    sizeMap.clear();
    return true;
  }

  @override
  void positionChild(Object childId, Offset offset) {
    //定位子组件
    super.positionChild(childId, offset);
  }

  @override
  Size layoutChild(Object childId, BoxConstraints constraints) {
    //确定child的约束，用于确定child的大小
    print('layoutChild childId = $childId constraints = $constraints');
    return super.layoutChild(childId, constraints);
  }


  @override
  Size getSize(BoxConstraints constraints) {
    //获取父容器约束条件确定CustomSingleChildLayout大小
    print('getSize constraints = $constraints');
    return super.getSize(constraints);
  }

  @override
  String toString() {
    return super.toString();
  }

  @override
  bool hasChild(Object childId) {
    //子组件是否存在
    return super.hasChild(childId);
  }
}

• CustomMultiChildLayout可以对多个子组件进行约束、定位。这里是个简单的使用例子，这个组件的强大就是你可以通过它去非常方便的创建自己需求的布局组件，比如自己写一个相对布局，约束布局等。
• Flow跟CustomMultiChildLayout都是支持多个子组件，但是Flow使用的的都是矩阵变换，不是真正在改变子组件的尺寸，位置
• CustomSingleChildLayout只能支持单个子组件

这三种给可以对比了解一下：
Flutter（61）：Layout组件之CustomSingleChildLayout
Flutter（64）：Layout组件之Flow

下一节：Builder、LayoutBuilder、StatefulBuilder、SliverLayoutBuilder

Flutter（69）：Builder、LayoutBuilder、StatefulBuilder、SliverLayoutBuilder

Flutter教学目录持续更新中

Github源代码持续更新中