Kivy is mostly event-based, meaning the flow of the program is determined by events.
Clock events
The :doc:`/api-kivy.clock` allows you to schedule a function call in the future as a one-time event with :meth:`~kivy.clock.ClockBase.schedule_once`, or as a repetitive event with :meth:`~kivy.clock.ClockBase.schedule_interval`.
You can also create Triggered events with :meth:`~kivy.clock.ClockBase.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 :class:`~kivy.input.motionevent.MotionEvent`, extended by :doc:`/api-kivy.input.postproc` and dispatched through the :attr:`~kivy.core.window.WindowBase.on_motion` event in the :class:`Window <kivy.core.window.WindowBase>` class. This event then generates the :meth:`~kivy.uix.widget.Widget.on_touch_down`, :meth:`~kivy.uix.widget.Widget.on_touch_move` and :meth:`~kivy.uix.widget.Widget.on_touch_up` events in the :class:`~kivy.uix.widget.Widget`.
For an in-depth explanation, have a look at :doc:`/api-kivy.input`.
Class events
Our base class :class:`~kivy.event.EventDispatcher`, used by :class:`~kivy.uix.widget.Widget`, uses the power of our :doc:`/api-kivy.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 :meth:`~kivy.event.EventDispatcher.register_event_type`, as the on_press and on_release events in the :class:`~kivy.uix.button.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 :doc:`/guide/events` documentation.