斯威夫特5

为了简化，创建一个“DispatchQueue+Extensions.swift”文件，内容如下:

import Foundation

typealias Dispatch = DispatchQueue

extension Dispatch {

    static func background(_ task: @escaping () -> ()) {
        Dispatch.global(qos: .background).async {
            task()
        }
    }

    static func main(_ task: @escaping () -> ()) {
        Dispatch.main.async {
            task()
        }
    }
}

用法:

Dispatch.background {
    // do stuff

    Dispatch.main { 
        // update UI
    }
}

Swift 3版本

Swift 3利用新的DispatchQueue类来管理队列和线程。要在后台线程上运行一些东西，你可以使用:

let backgroundQueue = DispatchQueue(label: "com.app.queue", qos: .background)
backgroundQueue.async {
    print("Run on background thread")
}

或者如果你想在两行代码中实现:

DispatchQueue.global(qos: .background).async {
    print("Run on background thread")

    DispatchQueue.main.async {
        print("We finished that.")
        // only back on the main thread, may you access UI:
        label.text = "Done."
    }
}

你也可以在本教程中获得一些关于Swift 3中GDC的深入信息。

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)
        }
     }
}   

Dan Beaulieu在swift5中给出了答案(从swift 3.0.1开始也可以工作)。

斯威夫特5.0.1

extension DispatchQueue {

    static func background(delay: Double = 0.0, background: (()->Void)? = nil, completion: (() -> Void)? = nil) {
        DispatchQueue.global(qos: .background).async {
            background?()
            if let completion = completion {
                DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: {
                    completion()
                })
            }
        }
    }

}

使用

DispatchQueue.background(delay: 3.0, background: {
    // do something in background
}, completion: {
    // when background job finishes, wait 3 seconds and do something in main thread
})

DispatchQueue.background(background: {
    // do something in background
}, completion:{
    // when background job finished, do something in main thread
})

DispatchQueue.background(delay: 3.0, completion:{
    // do something in main thread after 3 seconds
})

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
})

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