How do I check clicking and dragging in the Editor?

I have an editor script where I need to check if the developer clicks in one area, and then holds it until it reaches another area. I tried to make something like this with the Unity Handles, but it does not activate Handle buttons when the mouse is held down.

 function OnSceneGUI(){
     if(Handles.Button(somePosition,someDirection,
             size,pickSize, Handles.DrawRectangle)){
         //action when handle is clicked
     }
 }

I'm trying to get something like this:

 if mouse down:
     if handle1 touched:
         nextHandle = 1;
     if handle2 touched:
         nextHandle = 2;
     if handle3 touched:
        nextHandle = 3;

     if(lastHandle != nextHandle):
        SomeFunction(lastHandle, nextHandle);

     lastHandle = nextHandle;
 else:
     nextHandle = -1;
     lastHandle = -2;

When the player holds down the mouse and drags from handle1 to handle2, for example, the script would be able to pick that up.