Advanced-NSOperations/Earthquakes/Operations/DelayOperation.swift

/*
Copyright (C) 2015 Apple Inc. All Rights Reserved.
See LICENSE.txt for this sample’s licensing information

Abstract:
This file shows how to make an operation that efficiently waits.
*/

import Foundation

/**
    `DelayOperation` is an `EarthquakeOperation` that will simply wait for a
    given time interval, or until a specific `Date`.

    It is important to note that this operation does **not** use the `sleep()`
    function, since that is inefficient and blocks the thread on which it is called.
    Instead, this operation uses `DispatchQueue.asyncAfter` to know
    when the appropriate amount of time has passed.

    If the interval is negative, or the `Date` is in the past, then this operation
    immediately finishes.
*/
class DelayOperation: EarthquakeOperation {
    // MARK: Types

    private enum Delay {
        case Interval(Int)
        case Date(Date)
    }
    
    // MARK: Properties
    
    private let delay: Delay
    
    // MARK: Initialization
    
    init(interval: TimeInterval) {
        delay = .Interval(Int(interval))
        super.init()
    }
    
    init(until date: NSDate) {
        delay = .Date(date as Date)
        super.init()
    }
    
    override func execute() {
        let interval: Int
        
        // Figure out how long we should wait for.
        switch delay {
            case .Interval(let theInterval):
                interval = theInterval

            case .Date(let date):
            interval = Int(date.timeIntervalSinceNow)
        }
        
        guard interval > 0 else {
            finish()
            return
        }

        DispatchQueue.global(qos: .default).asyncAfter(deadline: .now() + .seconds(interval)) {
            // If we were cancelled, then finish() has already been called.
            if !self.isCancelled {
                self.finish()
            }
        }
    }
    
    override func cancel() {
        super.cancel()
        // Cancelling the operation means we don't want to wait anymore.
        self.finish()
    }
}