ios - 使用闭包时如何防止时序问题？

我试图找到从不同的转义闭包返回的数字的和。在主线程中返回的总和。

import Foundation

var randomTime: Int {
  return Int.random(in: 0...1000)
}

func first(completion: @escaping (Int) -> Void) {
  DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(randomTime)) {
    completion(1)
  }
}

func second(completion: @escaping (Int) -> Void) {
  DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(randomTime)) {
    completion(2)
  }
}

func third(completion: @escaping (Int) -> Void) {
  DispatchQueue(label: "anotherThread").async {
    completion(3)
  }
}

func fourth(completion: @escaping (Int) -> Void) {
  DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(randomTime)) {
    completion(4)
}
}

最佳答案

如果我把你的问题弄清楚了，你想对数字求和，但它们的值根据服务器的响应或某种延迟而在不同的时间出现如果是这样，则必须使用DispatchGroup
这里有一个helper函数，它调用您的方法first(completion: @escaping (Int) -> Void)。。。。。fourth(completion: @escaping (Int) -> Void)并仅在收到最后一个值时通知主队列。我在代码上加了一些注释以帮助理解。如果有什么不清楚的地方请告诉我。

    func computeOutPutAfterReceivingAllValues(completion: @escaping(_ sum: Int) -> Void) {
        // Make a dispatch group which will notify main queue after making sure that all requests have been proceed.
        let computeGroup = DispatchGroup()
        var allNumbers: [Int] = []
        computeGroup.enter()
        first { (firstNumber) in
            allNumbers.append(firstNumber)
            self.second(completion: { (secondNumber) in
                allNumbers.append(secondNumber)
                self.third(completion: { (thirdNumber) in
                    allNumbers.append(thirdNumber)
                    self.fourth(completion: { (fourthNumber) in
                        allNumbers.append(fourthNumber)
                         // IMPORTANT: Leave a group after the last call.
                        computeGroup.leave()
                    })
                })
            })

        }


        // Notify Main queue and sum all your numbers
        computeGroup.notify(queue: .main) {
            /// Sum all your numbers in main queue
            let sum = allNumbers.reduce(0, +)
            completion(sum)
        }
    }

用法：
你可以在视图didLoad中测试这个。

 override func viewDidLoad() {
        super.viewDidLoad()
        computeOutPutAfterReceivingAllValues { (sum) in
            print("Here is the sum of all numbers: \(sum)")
        }
    }

// Output on console
    Here is the sum of all numbers: 10