为什么 Flutter 像 Column() 这样的小部件不需要`.children`

Question

您可以在编译器没有任何错误的情况下返回Column()似乎真的不对。 什么是没有子级的列？

在内部，它默认为children: const [] 。

有谁知道这里的理由是什么？ 似乎它阻止编译器指出一些非常明显的错误，并且还会吞下您不打算成为 null 的 null 值。

Answer 1

一列在Flutter代码中有一个默认大小，即0。如果添加了一个小部件，则大小会发生变化，以便小部件适合。此外，children：[]只是一个列表，如果它为空，则没有小部件将显示。 ¯_(ツ)_/¯

这是顺便说一句。 class 列：

class Column extends Flex {
  /// Creates a vertical array of children.
  ///
  /// The [mainAxisAlignment], [mainAxisSize], [crossAxisAlignment], and
  /// [verticalDirection] arguments must not be null.
  /// If [crossAxisAlignment] is [CrossAxisAlignment.baseline], then
  /// [textBaseline] must not be null.
  ///
  /// The [textDirection] argument defaults to the ambient [Directionality], if
  /// any. If there is no ambient directionality, and a text direction is going
  /// to be necessary to disambiguate `start` or `end` values for the
  /// [crossAxisAlignment], the [textDirection] must not be null.
  Column({
    Key? key,
    MainAxisAlignment mainAxisAlignment = MainAxisAlignment.start,
    MainAxisSize mainAxisSize = MainAxisSize.max,
    CrossAxisAlignment crossAxisAlignment = CrossAxisAlignment.center,
    TextDirection? textDirection,
    VerticalDirection verticalDirection = VerticalDirection.down,
    TextBaseline? textBaseline,
    List<Widget> children = const <Widget>[],
  }) : super(
    children: children,
    key: key,
    direction: Axis.vertical,
    mainAxisAlignment: mainAxisAlignment,
    mainAxisSize: mainAxisSize,
    crossAxisAlignment: crossAxisAlignment,
    textDirection: textDirection,
    verticalDirection: verticalDirection,
    textBaseline: textBaseline,
  );
}