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.
通常这都是并发行为，因为每个结点都会在其本地的缓存状态上接收并执行查询，最后返回结果。
This would typically happen in parallel, as each node receives and performs the query on its locally cached state. And returns results.
定义：与特定表单相联系的全部实例数据结点的值或状态的内部树型表示。
Definition: an internal tree representation of the values and state of all the instance data nodes associated with a particular form.
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