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Kivy is mostly event-based, meaning the flow of the program is determined by events.

Clock events


The Clock object allows you to schedule a function call in the future as a one-time event with schedule_once(), or as a repetitive event with schedule_interval().

You can also create Triggered events with create_trigger(). Triggers have the advantage of being called only once per frame, even if you have scheduled multiple triggers for the same callback.

Input events


All the mouse click, touch and scroll wheel events are part of the MotionEvent, extended by Input Postprocessing and dispatched through the on_motion event in the Window class. This event then generates the on_touch_down(), on_touch_move() and on_touch_up() events in the Widget.

For an in-depth explanation, have a look at Input management.

Class events


Our base class EventDispatcher, used by Widget, uses the power of our Properties for dispatching changes. This means when a widget changes its position or size, the corresponding event is automatically fired.

In addition, you have the ability to create your own events using register_event_type(), as the on_press and on_release events in the Button widget demonstrate.

Another thing to note is that if you override an event, you become responsible for implementing all its behaviour previously handled by the base class. The easiest way to do this is to call super():

def on_touch_down(self, touch):
    if super().on_touch_down(touch):
        return True
    if not self.collide_point(touch.x, touch.y):
        return False
    print('you touched me!')
    return True

Get more familiar with events by reading the Events and Properties documentation.