swift - 如何执行一个接一个完成的多个 Alamofire 请求？

Question

我想执行多个 Alamofire 请求。但是，由于数据依赖性，新请求只能在前一个请求完成后才开始。

我已经问了 question一个更通用的异步请求示例，该示例使用 OperationQueue 解决。但是，我没有成功地使用 Alamofire 实现同样的目标。

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
    }
}

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

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 related question 中的问题相同你提出:操作依赖于完成一个操作，如记录的那样，但是你已经编写了代码，在异步调度请求以供将来执行后操作退出(你创建并添加到队列的操作将按照他们设置的顺序完成依赖项，但请求将由底层 Alamofire 的 NSURLSession 同时触发)。

如果您需要串行执行，您可以执行以下操作:

// 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 .