abstract-state-router/lib/state-transition-manager.js

module.exports = emitter => {
	let currentTransitionAttempt = null
	let nextTransition = null

	function doneTransitioning() {
		currentTransitionAttempt = null
		if (nextTransition) {
			beginNextTransitionAttempt()
		}
	}

	const isTransitioning = () => !!currentTransitionAttempt

	function beginNextTransitionAttempt() {
		currentTransitionAttempt = nextTransition
		nextTransition = null
		currentTransitionAttempt.beginStateChange()
	}

	function cancelCurrentTransition() {
		currentTransitionAttempt.transition.cancelled = true
		const err = new Error('State transition cancelled by the state transition manager')
		err.wasCancelledBySomeoneElse = true
		emitter.emit('stateChangeCancelled', err)
	}

	emitter.on('stateChangeAttempt', beginStateChange => {
		nextTransition = createStateTransitionAttempt(beginStateChange)

		if (isTransitioning() && currentTransitionAttempt.transition.cancellable) {
			cancelCurrentTransition()
		} else if (!isTransitioning()) {
			beginNextTransitionAttempt()
		}
	})

	emitter.on('stateChangeError', doneTransitioning)
	emitter.on('stateChangeCancelled', doneTransitioning)
	emitter.on('stateChangeEnd', doneTransitioning)

	function createStateTransitionAttempt(beginStateChange) {
		const transition = {
			cancelled: false,
			cancellable: true,
		}
		return {
			transition,
			beginStateChange: (...args) => beginStateChange(transition, ...args),
		}
	}
}