Note that these scroll bars appear only for those containers that extend the Container class.

注意，这些滚动条仅出现在扩展 Container 类的容器上。

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In a container, Flex needs to be able to add many things in addition to the container, such as scroll bars, borders, overlays, and so on.

在容器中，Flex需要能够添加除了容器之外的更多其他东西，比如滚动条、边框和覆盖图等。

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You need the measurement phase because it helps LayoutManager decide whether there should be scroll bars on the parent container.

您需要使用度量阶段，因为它帮助 LayoutManager 确定父容器是否包含滚动条。

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