这些源模型和目标模型会得到筛选；只有那些选中元素的父类和所有子类才可以看见。
The source and target models are filtered; only the parents and all children of the selected element are visible.
removeAttr() 以一个属性名作为参数，它将从所有当前选中的元素中移除具有该名称的属性。
removeAttr(), which takes an attribute name as an argument, will remove the named attribute from all of the currently selected elements.
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.
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