The callback that is invoked when the app receives an event.
#include <glview/glview.h>
typedef void(* event_callback)(bps_event_t *event, int domain, int code, void *callback_data);
This callback is invoked for every event read from the application's event queue. Processing events occurs during each iteration of the glview_loop() execution loop. Each event popped from the queue is passed into the event_callback.