Throttle new tasks down when older tasks are still executing.

Throttler.swift

//
//  Throttler.swift
//  3D Viewer
//
//  Created by Damiaan on 20/02/2019.
//  Copyright © 2019 Devian. All rights reserved.
//

import Dispatch

public class Throttler {
	private let group = DispatchGroup()
	private let worker  = DispatchQueue(label: "Throttle worker",  attributes: [])
	private let manager = DispatchQueue(label: "Throttle manager", attributes: [])

	private var todo: ( () -> Void )?

	var notExecuting: Bool { return group.wait(timeout: .now()) == .success }

	public func execute(work: @escaping () -> Void) {
		manager.sync { todo = work }
		if notExecuting {
			executeRemainingWork()
		}
	}

	private func executeRemainingWork() {
		manager.sync {
			group.enter()
			if let work = todo {
				todo = nil
				worker.async {
					work()
					self.group.leave()
					self.executeRemainingWork()
				}
			} else {
				group.leave()
			}
		}
	}
}