在d3.js的拖放动作期间，鼠标悬停事件没有被触发

I'm trying to drag a circle over other circle to connect two path. The problem is when I drag a circle (c2) over another (c1), if the circle c1 is created before of the circle c2, the c1 mouse over event is not fired.I'm trying to drag a circle over other circle t