# From https://expobrain.net/2010/07/31/simple-event-dispatcher-in-python
class EventDispatcher(object):
    """
    Generic event dispatcher which listen and dispatch events
    """

    def __init__(self):
        self._events = dict()

    def __del__(self):
        """
        Remove all listener references at destruction time
        """
        self._events = None

    def has_listener(self, event_type, listener):
        """
        Return true if listener is register to event_type
        """
        # Check for event type and for the listener
        if event_type in self._events.keys():
            return listener in self._events[event_type]
        else:
            return False

    def dispatch_event(self, event):
        """
        Dispatch an instance of Event class
        """
        # Dispatch the event to all the associated listeners
        if event.type in self._events.keys():
            listeners = self._events[event.type]

            for listener in listeners:
                listener(event)

    def add_event_listener(self, event_type, listener):
        """
        Add an event listener for an event type
        """
        # Add listener to the event type
        if not self.has_listener(event_type, listener):
            listeners = self._events.get(event_type, [])

            listeners.append(listener)

            self._events[event_type] = listeners

    def remove_event_listener(self, event_type, listener):
        """
        Remove event listener.
        """
        # Remove the listener from the event type
        if self.has_listener(event_type, listener):
            listeners = self._events[event_type]

            if len(listeners) == 1:
                # Only this listener remains so remove the key
                del self._events[event_type]

            else:
                # Update listeners chain
                listeners.remove(listener)

                self._events[event_type] = listeners