A node can only be expanded if it has children and its navigational state is set to expanded.
一个节点只是在它拥有子代，而且其导航状态被设置成展开时，该节点才可以展开。
A node can only be collapsed if it has children and its navigational state is set to collapsed.
一个节点只是在它拥有子代，而且其分解状态被设置成展开时，该节点才可以分解。
A node can only be collapsed if it has children and its navigational state is set to collapsed.
一个节点只是在它拥有子代，而且其分解状态被设置成展开时，该节点才可以分解。
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