4.2 弹性布局 - Flex

  Flex({
    super.key,
    required this.direction, // 弹性布局的方向
    this.mainAxisAlignment = MainAxisAlignment.start, // 主轴的对齐方式
    this.mainAxisSize = MainAxisSize.max,
    this.crossAxisAlignment = CrossAxisAlignment.center,
    this.textDirection,
    this.verticalDirection = VerticalDirection.down,
    this.textBaseline, // NO DEFAULT: we don't know what the text's baseline should be
    this.clipBehavior = Clip.none,
    super.children,
  })

    Expanded({
      super.key,
      super.flex, // 弹性系数，0或null，没有弹性
      required super.child,
    })

• 1. Row 和 Column都继承自Flex
• 2. Expanded只能作为Flex,Row,Column的孩子