While a gadget is active, Intuition sends it a gm_handleinput message for
every timer pulse, mouse move, mouse click, and key press that takes
place.  A timer event pulse arrives about every tenth of a second.  Mouse
move events can arrive at a much higher rate than the timer pulses.
Without even considering the keyboard, a gadget can get a lot of
GM_HANDLEINPUT messages in a short amount of time. Because the active
gadget has to handle a large volume of GM_HANDLEINPUT messages, the
overhead of this method should be kept to a minimum.

Because the gadget will always receive a gm_goactive message before it is
active and a gm_goinactive message after it is no longer active, the
gadget can use these methods to allocate, initialize, and deallocate
temporary resources it needs for the gm_handleinput method.  this can
significantly reduce the overhead of GM_HANDLEINPUT because it eliminates
the need to allocate, initialize, and deallocate resources for every
GM_HANDLEINPUT message.

Note that the rastport from obtaingirport() is not cachable using this
method.  If the gm_handleinput method needs to use a rastport, it has to
obtain and release the RastPort for every GM_HANDLEINPUT message using
ObtainGIRPort() and releasegirport().

 rkmbuttonclass.c