Each click-and drag operation ends with the release of the mouse button, at which point you want to remove all the handlers you installed at the beginning of the click-and-drag operation.
每个点击-拖动操作最后都是释放鼠标按钮，此时您希望移除在点击-拖放操作开始时安装的所有处理程序。
This way, the mouse handlers don't have to worry about cleaning up after themselves—they just need to take care of the click-and-drag operation itself.
这样一来，鼠标处理程序就不必在事后负责清理工作 — 它们只需负责点击-拖动操作本身。
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