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 children nodes are set to the same state, which allows for quick selection of entire categories or even the entire tree.
当一个元素被选中或者去掉的时候，它的所有子结点都将被设置为同样的状态，从而我们能够快速地完成整个类别甚至整棵树的全选操作。
Selected style: This style is applied when the widget is set to its selected state.
选择风格：这种风格应用部件时，设置为它的选中状态。
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