如何执行多个一个接一个完成的 Alamofire 请求？

Question

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开始。

我已经向question 询问了一个更一般的异步请求示例，该示例已通过OperationQueue 解决。但是，我没有成功使用 Alamofire 实现相同的目标。

public func performAlamofireRequest(_ number: Int, success: @escaping (Int) -> Void)->Void {
    Alamofire.request(String(format: "http://jsonplaceholder.typicode.com/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，如下所示：

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，输出是：

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

这显然是不正确的。

如何使用 Alamofire 实现这一目标？

Answer 1

问题与您提出的related question 中的问题相同：操作依赖于完成操作，如文档所述，但是您编写了代码，在异步调度未来执行请求后操作退出（操作您创建并添加到队列将按照其依赖项设置的顺序完成，但请求将由底层 Alamofire 的 NSURLSession 同时触发）。

如果您需要串行执行，例如可以执行以下操作：

// 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in connection to this question，可以说是重复的。还有Alamofire-Synchronous。