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。
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.
除了结果之外，history元素还保持同被选中的提供者、类别和规则的联系。
In addition to results, the history element also keeps track of which providers, categories, and rules were selected.
除了结果之外，history元素还保持同被选中的提供者、类别和规则的联系。
In addition to results, the history element also keeps track of which providers, categories, and rules were selected.
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