DragLayerMonitor是传入到DragLayer的collect方法的一个参数，通过这个对象的方法可以获取到全局的拖拽的状态。
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isDragging() Returns true if a drag operation is in progress. Returns false otherwise.
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getItemType() Returns a string or an ES6 symbol identifying the type of the current dragged item. Returns null if no item is being dragged.
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getItem() Returns a plain object representing the currently dragged item. Every drag source must specify it by returning an object from its beginDrag() method. Returns null if no item is being dragged.
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getInitialClientOffset() Returns the { x, y } client offset of the pointer at the time when the current drag operation has started. Returns null if no item is being dragged.
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getInitialSourceClientOffset() Returns the { x, y } client offset of the drag source component's root DOM node at the time when the current drag operation has started. Returns null if no item is being dragged.
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getClientOffset() Returns the last recorded { x, y } client offset of the pointer while a drag operation is in progress. Returns null if no item is being dragged.
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getDifferenceFromInitialOffset() Returns the { x, y } difference between the last recorded client offset of the pointer and the client offset when current the drag operation has started. Returns null if no item is being dragged.
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getSourceClientOffset() Returns the projected { x, y } client offset of the drag source component's root DOM node, based on its position at the time when the current drag operation has started, and the movement difference. Returns null if no item is being dragged.