当选择或取消选择一个元素时，它所有的子节点都设置为相同的状态，这可以用来快速选择整个类别甚至整个树。
When an element is selected or deselected, all of its child nodes are set to the same state, which allows for quick selection of entire categories or even the entire tree. For the exercise.
当选择或取消选择一个元素时，它所有的子节点都设置为相同的状态，这可以用来快速选择整个类别甚至整个树。
When an element is selected or deselected, all of its child nodes are set to the same state, which allows for quick selection of entire categories or even the entire tree. For the exercise.
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