swift - DispatchGroup 和可能的竞争条件？

Question

以下是否会产生可能的竞争条件？

let syncGroup = DispatchGroup()
var tempData: [Int]
for index in 1...10 {
      syncGroup.enter()
      // Alamofire is being called in the following. 
      // I am leaving out implementation, just to simplify.
      asyncTaskUsingAlamofire(completionHandler: { (data: [Data]) in
         // Possible race condition?
         // Should I use something like `sync()` and how?
         tempData.append(data)
         syncGroup.leave()
      })
}
syncGroup.notify(queue: .main) {
// We can go on, all data is added to tempData
}

我可以看到 swift 有一个 sync() 如果你会如何解决这个问题？

Answer 1

串行队列

非常类似于Sulthan's answer ，我只是通常在这种情况下使用自定义串行队列

let group = DispatchGroup()
var results = [Int]()
let serialQueue = DispatchQueue(label: "serialQueue")

for _ in 1...10 {
    group.enter()
    asyncTaskUsingAlamofire { data in
        serialQueue.async {
            // Multiple instances of this block will be executed into a serial queue
            // So max 1 block at a time
            // This means results is accessed safely
            results.append(contentsOf: data)
            group.leave()
        }
    }
}

group.notify(queue: .main) {
    print(results)
}

results 数组是安全访问的，因为 block 的每个实例都会改变它

results.append(contentsOf: data)
group.leave()

在串行队列中执行。

serialQueue.async {
    results.append(contentsOf: data)
    group.leave()
}

换句话说， block 的 10 个实例被排入 serialQueue 并一次执行一个。

这确实保证了结果的安全访问。