Each time a mouse drag event occurs, a (possibly quite short) black line is drawn between the saved point and the location of the event.
每次发生鼠标拖动事件时，在保存的点和该事件位置之间就会画出一条黑线(可能很短)。
2
Click on the center point (inside the circle) and drag the light into the position where you want it.
点击中间的那个点，(聚光灯范围圆圈里)并将其拖曳到你想要的地方。
3
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.