throttle.swift

/** Throttle wraps a block with throttling logic, guarantueeing that the block will never be called (by enquueing 
asynchronously on `queue`) more than once each `interval` seconds. If the wrapper callback is called more than once
in an interval, it will use the most recent call's parameters when eventually calling the wrapped block (after `interval`
has elapsed since the last call to the wrapped function) - i.e. calls are not queued and may get 'lost' by being superseded
by a newer call. */
public func throttle<P>(interval: TimeInterval, queue: DispatchQueue, _ block: ((P) -> ())) -> ((P) -> ()) {
	var lastExecutionTime: TimeInterval? = nil
	var scheduledExecutionParameters: P? = nil
	let mutex = Mutex()

	return { (p: P) -> () in
		return mutex.locked {
			let currentTime = NSDate.timeIntervalSinceReferenceDate

			if let lastExecuted = lastExecutionTime {
				if (currentTime - lastExecuted) > interval {
					// Not the first execution, but the last execution was more than interval ago. Execute now.
					lastExecutionTime = currentTime
					queue.async {
						block(p)
					}
				}
				else {
					// Last execution was less than interval ago
					if scheduledExecutionParameters != nil {
						// Another execution was already planned, just mae sure it will use latest parameters
						scheduledExecutionParameters = p
					}
					else {
						// Schedule execution
						scheduledExecutionParameters = p
						let scheduleDelay = lastExecuted + interval - currentTime

						queue.asyncAfter(deadline: DispatchTime.now() + scheduleDelay) {
							// Delayed execution
							let p = mutex.locked { () -> P in
								let params = scheduledExecutionParameters!
								scheduledExecutionParameters = nil
								return params
							}
							block(p)
						}
					}
				}
			}
			else {
				// First time this function is executed
				lastExecutionTime = currentTime
				queue.async {
					block(p)
				}
			}
		}
	}
}