What's behavior for SetCurrentAction?

HI, I am new to learn LibreCAD.
I see the code in SetCurrentAction, just Suspend PreAction.
So, if I use drawLineAction and click one point on canvas,then chose other draw action likes drawArcAction,
it seems not to finish the drawLineAction.
I means where to finish drawLineAction and clean it from EventHandler before close windows which will call killAllAction.

Sorry for my poor English.

BTW, it seems the code in QG_graphicsview.keypressevent() Key_up/down/right/left has no effect to scroll.
Because the event msg is captured by command widget.