如何检查两个异步任务是否成功完成 [英] How to check if two asynchronous tasks are done with success

问题描述

在函数中实现此流程图的最佳和最简便的方法是什么? 现在，我正在使用两个调度组，但是我需要检查它们是否都已完成，而不仅是在完成时.

What is the best and easiest way to implement this flowchart in a function? Right now I'm using two dispatch groups but I need to check if they're both done, and not only when they finish.

如果完成，那么:

• friends数组将包含元素
• 昵称数组将包含元素

注意:FB是Facebook，FIR是Firebase数据库

note: FB is Facebook and FIR is Firebase database

推荐答案

您可以使用DispatchGroup进行此操作.尝试以下游乐场；

You could do this using DispatchGroup. Try the following playground;

import UIKit
import XCPlayground

let dispatchGroup = DispatchGroup.init()

for index in 0...4 {
    dispatchGroup.enter()
    let random = drand48()
    let deadline = DispatchTime.now() + random/1000
    print("entered \(index)")
    DispatchQueue.global(qos: .background).asyncAfter(deadline: deadline, execute: {
        print("leaving \(index)")
        dispatchGroup.leave()
    })
}

dispatchGroup.notify(queue: .global()) {
    print("finished all")
}

应该输出类似于

entered 0
leaving 0
entered 1
entered 2
leaving 1
leaving 2
entered 3
leaving 3
entered 4
leaving 4
finished all

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