最佳实践是定义一个可多次访问的可重用函数。

可重用的功能:

例如，AppDelegate.swift是一个全局函数。

func backgroundThread(_ delay: Double = 0.0, background: (() -> Void)? = nil, completion: (() -> Void)? = nil) {
    dispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)) {
        background?()

        let popTime = dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC)))
        dispatch_after(popTime, dispatch_get_main_queue()) {
            completion?()
        }
    }
}

注意:在Swift 2.0中替换QOS_CLASS_USER_INITIATED。使用QOS_CLASS_USER_INITIATED。rawValue相反

用法:

a .在后台运行一个进程，延迟3秒:

    backgroundThread(3.0, background: {
            // Your background function here
    })

B.在后台运行一个进程，然后在前台运行一个补全:

    backgroundThread(background: {
            // Your function here to run in the background
    },
    completion: {
            // A function to run in the foreground when the background thread is complete
    })

C.延迟3秒-注意使用completion参数而不使用background参数:

    backgroundThread(3.0, completion: {
            // Your delayed function here to be run in the foreground
    })

多用途螺纹功能

public enum QueueType {
        case Main
        case Background
        case LowPriority
        case HighPriority

        var queue: DispatchQueue {
            switch self {
            case .Main:
                return DispatchQueue.main
            case .Background:
                return DispatchQueue(label: "com.app.queue",
                                     qos: .background,
                                     target: nil)
            case .LowPriority:
                return DispatchQueue.global(qos: .userInitiated)
            case .HighPriority:
                return DispatchQueue.global(qos: .userInitiated)
            }
        }
    }

    func performOn(_ queueType: QueueType, closure: @escaping () -> Void) {
        queueType.queue.async(execute: closure)
    }

像这样使用它:

performOn(.Background) {
    //Code
}

你必须将你想要在后台运行的更改与你想要在UI上运行的更新分开:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
    // do your task

    dispatch_async(dispatch_get_main_queue()) {
        // update some UI
    }
}

下面的代码是否有缺点(当需要启动前台屏幕之后)?

import Foundation
import UIKit

class TestTimeDelay {

    static var connected:Bool = false
    
    static var counter:Int = 0

    static func showAfterDelayControl(uiViewController:UIViewController) {
        NSLog("TestTimeDelay", "showAfterDelayControl")
    }
    
    static func tryReconnect() -> Bool {
        counter += 1
        NSLog("TestTimeDelay", "Counter:\(counter)")
        return counter > 4
    }

    static func waitOnConnectWithDelay(milliseconds:Int, uiViewController: UIViewController) {
        
        DispatchQueue.global(qos: .background).async {
            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + DispatchTimeInterval.milliseconds(milliseconds), execute: {
                waitOnConnect(uiViewController: uiViewController)
            })
        }
    }

    static func waitOnConnect(uiViewController:UIViewController) {

        connected = tryReconnect()
        if connected {
            showAfterDelayControl(uiViewController: uiViewController)
        }
        else {
            waitOnConnectWithDelay(milliseconds: 200, uiViewController:uiViewController)
        }
     }
}   

dispatch_async(dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0), {
    // Conversion into base64 string
    self.uploadImageString =  uploadPhotoDataJPEG.base64EncodedStringWithOptions(NSDataBase64EncodingOptions.EncodingEndLineWithCarriageReturn)
})

我真的很喜欢Dan Beaulieu的答案，但是它在Swift 2.2中不起作用，我认为我们可以避免那些讨厌的强制打开!

func backgroundThread(delay: Double = 0.0, background: (() -> Void)? = nil, completion: (() -> Void)? = nil) {

    dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)) {

        background?()

        if let completion = completion{
            let popTime = dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC)))
            dispatch_after(popTime, dispatch_get_main_queue()) {
                completion()
            }
        }
    }
}