gazebo SystemPlugin to handle key event and Mouse event won't response key-press event and mouse event

edit

I have written a Gazebo SystemPlugin to handle key-pressed event and mouse-button-pressed event. The plug work fine in gzclient (I am using gazebo-9) but no response in gzweb when click on a keyboard key or a mouse button in gzweb scene. Any suggestion for this issue? Thanks a lot!