Swift 3.0 +

在Swift 3.0中，很多东西都被现代化了。在后台队列上运行一些东西看起来像这样:

DispatchQueue.global(qos: .userInitiated).async {
    print("This is run on a background queue")

    DispatchQueue.main.async {
        print("This is run on the main queue, after the previous code in outer block")
    }
}

Swift 1.2到2.3

let qualityOfServiceClass = QOS_CLASS_USER_INITIATED
let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
dispatch_async(backgroundQueue, {
    print("This is run on a background queue")

    dispatch_async(dispatch_get_main_queue(), { () -> Void in
        print("This is run on the main queue, after the previous code in outer block")
    })
})

Pre Swift 1.2 -已知问题

从Swift 1.1开始，苹果在没有做一些修改的情况下不支持上述语法。传递qos_class_user_initialized实际上不起作用，而是使用Int(qos_class_user_initialized .value)。

有关更多信息，请参阅apple文档

Swift 3.0 +

在Swift 3.0中，很多东西都被现代化了。在后台队列上运行一些东西看起来像这样:

DispatchQueue.global(qos: .userInitiated).async {
    print("This is run on a background queue")

    DispatchQueue.main.async {
        print("This is run on the main queue, after the previous code in outer block")
    }
}

Swift 1.2到2.3

let qualityOfServiceClass = QOS_CLASS_USER_INITIATED
let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
dispatch_async(backgroundQueue, {
    print("This is run on a background queue")

    dispatch_async(dispatch_get_main_queue(), { () -> Void in
        print("This is run on the main queue, after the previous code in outer block")
    })
})

Pre Swift 1.2 -已知问题

从Swift 1.1开始，苹果在没有做一些修改的情况下不支持上述语法。传递qos_class_user_initialized实际上不起作用，而是使用Int(qos_class_user_initialized .value)。

有关更多信息，请参阅apple文档

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)
        }
     }
}   

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()
            }
        }
    }
}

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
}

来自Jameson Quave的教程

斯威夫特2

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
    //All stuff here
})