叠加布局组件

叠加布局组件包含 Stack 和 IndexedStack，Stack 组件将子组件叠加显示，根据子组件的顺序依次向上叠加，用法如下：

Stack(
  children: <Widget>[
    Container(
      height: 200,
      width: 200,
      color: Colors.red,
    ),
    Container(
      height: 170,
      width: 170,
      color: Colors.blue,
    ),
    Container(
      height: 140,
      width: 140,
      color: Colors.yellow,
    )
  ],
)

Stack 对未定位（不被 Positioned 包裹）子组件的大小由 fit 参数决定，默认值是 StackFit.loose ，表示子组件自己决定，StackFit.expand 表示尽可能的大，用法如下：

Stack(
  fit: StackFit.expand,
  children: <Widget>[
    Container(
      height: 200,
      width: 200,
      color: Colors.red,
    ),
    Container(
      height: 170,
      width: 170,
      color: Colors.blue,
    ),
    Container(
      height: 140,
      width: 140,
      color: Colors.yellow,
    )
  ],
)

效果只有黄色（最后一个组件的颜色），并不是其他组件没有绘制，而是另外两个组件被黄色组件覆盖。

Stack 对未定位（不被 Positioned 包裹）子组件的对齐方式由 alignment 控制，默认左上角对齐，用法如下：

Stack(
  alignment: AlignmentDirectional.center,
  children: <Widget>[
    Container(
      height: 200,
      width: 200,
      color: Colors.red,
    ),
    Container(
      height: 170,
      width: 170,
      color: Colors.blue,
    ),
    Container(
      height: 140,
      width: 140,
      color: Colors.yellow,
    )
  ],
)

通过 Positioned 定位的子组件：

Stack(
  alignment: AlignmentDirectional.center,
  children: <Widget>[
    Container(
      height: 200,
      width: 200,
      color: Colors.red,
    ),
    Container(
      height: 170,
      width: 170,
      color: Colors.blue,
    ),
    Positioned(
      left: 30,
      right: 40,
      bottom: 50,
      top: 60,
      child: Container(
        color: Colors.yellow,
      ),
    )
  ],
)

top 、bottom 、left 、right 四种定位属性，分别表示距离上下左右的距离。

如果子组件超过 Stack 边界由 overflow 控制，默认是裁剪，下面设置总是显示的用法：

Stack(
  overflow: Overflow.visible,
  children: <Widget>[
    Container(
      height: 200,
      width: 200,
      color: Colors.red,
    ),
    Positioned(
      left: 100,
      top: 100,
      height: 150,
      width: 150,
      child: Container(
        color: Colors.green,
      ),
    )
  ],
)

IndexedStack 是 Stack 的子类，Stack 是将所有的子组件叠加显示，而 IndexedStack 通过 index 只显示指定索引的子组件，用法如下：

class IndexedStackDemo extends StatefulWidget {
  @override
  _IndexedStackDemoState createState() => _IndexedStackDemoState();
}

class _IndexedStackDemoState extends State<IndexedStackDemo> {
  int _index = 0;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        SizedBox(height: 50,),
        _buildIndexedStack(),
        SizedBox(height: 30,),
        _buildRow(),
      ],
    );
  }

  _buildIndexedStack() {
    return IndexedStack(
      index: _index,
      children: <Widget>[
        Center(
          child: Container(
            height: 300,
            width: 300,
            color: Colors.red,
            alignment: Alignment.center,
            child: Icon(
              Icons.fastfood,
              size: 60,
              color: Colors.blue,
            ),
          ),
        ),
        Center(
          child: Container(
            height: 300,
            width: 300,
            color: Colors.green,
            alignment: Alignment.center,
            child: Icon(
              Icons.cake,
              size: 60,
              color: Colors.blue,
            ),
          ),
        ),
        Center(
          child: Container(
            height: 300,
            width: 300,
            color: Colors.yellow,
            alignment: Alignment.center,
            child: Icon(
              Icons.local_cafe,
              size: 60,
              color: Colors.blue,
            ),
          ),
        ),
      ],
    );
  }

  _buildRow() {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        IconButton(
          icon: Icon(Icons.fastfood),
          onPressed: () {
            setState(() {
              _index = 0;
            });
          },
        ),
        IconButton(
          icon: Icon(Icons.cake),
          onPressed: () {
            setState(() {
              _index = 1;
            });
          },
        ),
        IconButton(
          icon: Icon(Icons.local_cafe),
          onPressed: () {
            setState(() {
              _index = 2;
            });
          },
        ),
      ],
    );
  }
}