/**
 * Internal function to unobserve the state of a `Stateful`.  It emits a `statecomplete` event which can be
 * cancelled.
 *
 * @param stateful The `Stateful` object to unobserve
 */
function completeStatefulState(stateful: Stateful<State>): void {
	const observedState = observedStateMap.get(stateful);
	if (observedState) {
		observedState.handle.destroy();
		const statecomplete = createCancelableEvent({
			type: 'state:completed',
			target: stateful
		});
		stateful.emit(statecomplete);
		if (!statecomplete.defaultPrevented) {
			stateful.destroy();
		}
	}
}

/**
 * Internal function that actually applies the state to the Stateful's state and
 * emits the `statechange` event.
 *
 * @param stateful The Stateful instance
 * @param state The State to be set
 */
function setStatefulState(target: Stateful<State>, state: State): void {
	const previousState = stateWeakMap.get(target);
	if (!previousState) {
		throw new Error('Unable to set destroyed state');
	}
	const type = 'state:changed';
	state = deepAssign(previousState, state);
	const eventObject = {
		type,
		state,
		target
	};
	target.emit(eventObject);
}