Why InputPreProcessors.HandleMouseMoveEvent in OnMouseMove Function?SlateApplication.cpp

OnMouseDown  -> ProcessMouseButtonDownEvent -> InputPreProcessors.HandleMouseButtonDownEvent
OnMouseUp  -> ProcessMouseButtonUpEvent  -> InputPreProcessors.HandleMouseButtonUpEvent

but 
In OnMosueMove, it has InputPreProcessors.HandleMouseMoveEvent

because I want use PreProcessor to hook mouse event, on android platform. I can get Mouse Down/Up Event, but I cann’t hook mouse move event ! So why HandleMouseMoveEvent did’t called in ProcessMouseMoveEvent ?