When you initiate a drag-and-drop sequence and invoke this handler, it creates onmousemove and onmouseup handlers and installs them in the target element.

在初始化拖放序列并调用此处理程序时，它会创建onmousemove和onmouseup处理程序并能在目标元素内安装它们。

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First, a DragSource object is required to initiate a drag-and-drop operation.

首先，需要一个DragSource对象来初始化拖放操作。

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