如何执行接连完成的多个Alamofire请求？

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我想执行多个Alamofire请求。但是，由于数据依赖性，新请求仅应在前一个请求完成时才开始。

I would like to perform multiple Alamofire requests. However, because of data dependency a new request should only start when the previous is finished.

我已经问过，其中有一个更一般的异步请求示例，该示例由 OperationQueue 解决。但是，我无法通过Alamofire实现相同的功能。

I already asked a question with a more general example of an asynchronous request which was solved with OperationQueue. However, I do not succeed to achieve the same with Alamofire.

public func performAlamofireRequest(_ number: Int, success: @escaping (Int) -> Void)->Void { Alamofire.request(String(format: "jsonplaceholder.typicode/posts/%i", number+1)) // NSURLSession dispatch queue .responseString { response in // Completion handler at main dispatch queue? if response.result.isSuccess { // print("data") } else if response.result.isFailure { // print("error") } success(number) // Always leave closure in this example } }

为了确保在下一个请求开始之前完成请求，我使用 OperationQueue 如下：

To assure that requests are finished before a next request is started, I use OperationQueue as follows:

let operationQueue = OperationQueue.main for operationNumber in 0..<4 { // Create some operations let operation = BlockOperation(block: { performAlamofireRequest(operationNumber) { number in print("Operation #\(number) finished") } }) operation.name = "Operation #\(operationNumber)" if operationNumber > 0 { operation.addDependency(operationQueue.operations.last!) } operationQueue.addOperation(operation) }

但是，输出为：

Operation #0 finished Operation #3 finished Operation #2 finished Operation #1 finished

这显然是不正确的。

如何用Alamofire做到这一点？

How would it be possible to achieve this with Alamofire?

推荐答案

问题与您提出的相关问题：如文档所述，操作依赖项是对操作的完成，但是您已经编写了代码，其中该操作在异步调度将来执行的请求后退出操作（您创建并添加到的操作队列将按照其依赖关系设置的顺序完成，但请求将由Alamofire底层的NSURLSession并发触发。

The issue is just the same as in the related question you posed: the operation dependencies are on finishing an operation, as documented, but you have written code where the operation exits after asynchronously dispatching a request for future execution (the operations you created and added to a queue will finish in the order set by their dependencies, but the requests will be fired concurrently by the NSURLSession underlying Alamofire).

如果需要串行执行，则可以执行以下操作：

If you need serial execution, you can for instance do the following:

// you should create an operation queue, not use OperationQueue.main here – // synchronous network IO that would end up waiting on main queue is a real bad idea. let operationQueue = OperationQueue() let timeout:TimeInterval = 30.0 for operationNumber in 0..<4 { let operation = BlockOperation { let s = DispatchSemaphore(value: 0) self.performAlamofireRequest(operationNumber) { number in // do stuff with the response. s.signal() } // the timeout here is really an extra safety measure – the request itself should time out and end up firing the completion handler. s.wait(timeout: DispatchTime(DispatchTime.now, Int64(timeout * Double(NSEC_PER_SEC)))) } operationQueue.addOperation(operation) }

讨论了各种其他解决方案 ，可以说是重复的。还有 Alamofire-Synchronous 。

Various other solutions are discussed in connection to this question, arguably a duplicate. There's also Alamofire-Synchronous.