Flutter Expanded 源码和使用

源码

class Expanded extends Flexible {
  /// Creates a widget that expands a child of a [Row], [Column], or [Flex]
  /// so that the child fills the available space along the flex widget's
  /// main axis.
  const Expanded({
    Key? key,
    int flex = 1,
    required Widget child,
  }) : super(key: key, flex: flex, fit: FlexFit.tight, child: child);
}

从源码和注释可以看到 Expand 只能作为 Row Column Flex的 Children ; 单独使用会报错

使用

class FlexLayoutTestRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        //Flex的两个子widget按1：2来占据水平空间  
        Flex(
          direction: Axis.horizontal,
          children: <Widget>[
            Expanded(
              flex: 1,
              child: Container(
                height: 30.0,
                color: Colors.red,
              ),
            ),
            Expanded(
              flex: 2,
              child: Container(
                height: 30.0,
                color: Colors.green,
              ),
            ),
          ],
        ),
        Padding(
          padding: const EdgeInsets.only(top: 20.0),
          child: SizedBox(
            height: 100.0,
            //Flex的三个子widget，在垂直方向按2：1：1来占用100像素的空间  
            child: Flex(
              direction: Axis.vertical,
              children: <Widget>[
                Expanded(
                  flex: 2,
                  child: Container(
                    height: 30.0,
                    color: Colors.red,
                  ),
                ),
                Spacer(
                  flex: 1,
                ),
                Expanded(
                  flex: 1,
                  child: Container(
                    height: 30.0,
                    color: Colors.green,
                  ),
                ),
              ],
            ),
          ),
        ),
      ],
    );
  }
}

Spacer 源码

class Spacer extends StatelessWidget {
  const Spacer({Key? key, this.flex = 1})
    : assert(flex != null),
      assert(flex > 0),
      super(key: key);
  
  final int flex;

  @override
  Widget build(BuildContext context) {
    return Expanded(
      flex: flex,
      child: const SizedBox.shrink(),
    );
  }
}