ButtonClick is called automatically when user presses mouse button with the mouse pointer over the particular button. Then it generates an OnButtonClick event. 

The AButton parameter indicates index of corresponding button. 

The Shift parameter indicates which shift keys (Shift, Ctrl, or Alt) were down when the user pressed the mouse button. 

Override this method to add class-specific processing when the button clicks.

See how descendant of TCustomBtnPanel overrides this method to assign current class type in DsnManager

TCustomBtnPanel Methods