Swift 使用Alamofire的NSURLSession并发请求

我的测试应用程序出现了一些奇怪的行为。我有大约50个同时发送到同一服务器的GET请求。服务器是一个嵌入式服务器，位于一小块硬件上，资源非常有限。为了优化每个请求的性能，我将

Alamofire.Manager

的一个实例配置如下：

let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
configuration.HTTPMaximumConnectionsPerHost = 2
configuration.timeoutIntervalForRequest = 30
let manager = Alamofire.Manager(configuration: configuration)

当我使用

manager.request（…）

发送请求时，它们将以2对进行调度（正如预期的那样，通过Charles HTTP Proxy进行检查）。但奇怪的是，所有在第一次请求后30秒内未完成的请求都会因为超时而被取消（即使它们尚未发送）。下面是一个演示行为的示例：

这是一种预期的行为吗？我如何确保请求在发送之前不会超时

---

非常感谢

是的，这是预期的行为。一种解决方案是将您的请求包装在自定义的异步

NSOperation

子类中，然后使用操作队列的

maxConcurrentOperationCount

来控制并发请求的数量，而不是

HTTPMaximumConnectionsPerHost

参数

最初的AFNetworking在将请求包装到操作中方面做得很好，这使得它变得微不足道。但是AFNetworking的

NSURLSession

实现从来没有做到这一点，Alamofire也没有做到

---

您可以轻松地将

请求

包装到

NSOperation

子类中。例如：

class NetworkOperation: AsynchronousOperation {

    // define properties to hold everything that you'll supply when you instantiate
    // this object and will be used when the request finally starts
    //
    // in this example, I'll keep track of (a) URL; and (b) closure to call when request is done

    private let urlString: String
    private var networkOperationCompletionHandler: ((_ responseObject: Any?, _ error: Error?) -> Void)?

    // we'll also keep track of the resulting request operation in case we need to cancel it later

    weak var request: Alamofire.Request?

    // define init method that captures all of the properties to be used when issuing the request

    init(urlString: String, networkOperationCompletionHandler: ((_ responseObject: Any?, _ error: Error?) -> Void)? = nil) {
        self.urlString = urlString
        self.networkOperationCompletionHandler = networkOperationCompletionHandler
        super.init()
    }

    // when the operation actually starts, this is the method that will be called

    override func main() {
        request = Alamofire.request(urlString, method: .get, parameters: ["foo" : "bar"])
            .responseJSON { response in
                // do whatever you want here; personally, I'll just all the completion handler that was passed to me in `init`

                self.networkOperationCompletionHandler?(response.result.value, response.result.error)
                self.networkOperationCompletionHandler = nil

                // now that I'm done, complete this operation

                self.completeOperation()
        }
    }

    // we'll also support canceling the request, in case we need it

    override func cancel() {
        request?.cancel()
        super.cancel()
    }
}

然后，当我想发起我的50个请求时，我会这样做：

let queue = OperationQueue()
queue.maxConcurrentOperationCount = 2

for i in 0 ..< 50 {
    let operation = NetworkOperation(urlString: "http://example.com/request.php?value=\(i)") { responseObject, error in
        guard let responseObject = responseObject else {
            // handle error here

            print("failed: \(error?.localizedDescription ?? "Unknown error")")
            return
        }

        // update UI to reflect the `responseObject` finished successfully

        print("responseObject=\(responseObject)")
    }
    queue.addOperation(operation)
}

---

此模式还有其他可能的变体，但请确保（a）为

异步
返回
true
；（b）您发布必要的
isFinished
和
isExecuting
KVN，如中的并发执行配置操作部分所述。
也许您实际想要设置的是'TimeoutiterValforResource
，而不是
TimeoutiterValforRequest`？谢谢，但是我尝试了这两种方法，同样的事情不断发生。你的方法在Alamofire 4中不再有效，请更新它。你使用了什么程序来制作这张图？@NikKov我使用了photoshopWow，非常感谢Rob，得到这么好的答案并不经常发生！工作起来很有魅力。堆栈溢出上的所有用户贡献都是使用。请参阅此网页底部页脚中的链接。总之，作者保留Stack Overflow贡献的版权，但我们也授予永久许可证，允许将这些特定贡献用于任何目的，包括商业目的，唯一要求是（a）要求归属，以及（b）您将同样分享。简言之，是的，它可以自由使用。@JAHelia-不，我没有，因为我的
异步操作
在执行后将闭包设置为
nil
，从而解决了任何强引用循环。@fam-在这种异步操作模式中，在完成此操作后，您可以创建一个新的操作来执行所需的操作，并使其依赖于单个网络请求的所有单个操作。@FAM“我看到您设置了
maxConcurrentOperationCount=2
，但您确实调用了50次请求…”这就是问题的关键：OP想要将50个请求排队，但决不能同时运行两个以上。
maxConcurrentOperationCount
简单地指示在任何给定时间可以运行多少。（您不希望同时运行太多的请求，因为（a）
URLSession
一次只能运行这么多个请求，因此您可能会面临后一个请求超时的风险；以及（b）内存问题。）在排队处理多个请求时，上述方法可以实现可控的并发度。
//
//  AsynchronousOperation.swift
//
//  Created by Robert Ryan on 9/20/14.
//  Copyright (c) 2014 Robert Ryan. All rights reserved.
//

import Foundation

/// Asynchronous Operation base class
///
/// This class performs all of the necessary KVN of `isFinished` and
/// `isExecuting` for a concurrent `NSOperation` subclass. So, to developer
/// a concurrent NSOperation subclass, you instead subclass this class which:
///
/// - must override `main()` with the tasks that initiate the asynchronous task;
///
/// - must call `completeOperation()` function when the asynchronous task is done;
///
/// - optionally, periodically check `self.cancelled` status, performing any clean-up
///   necessary and then ensuring that `completeOperation()` is called; or
///   override `cancel` method, calling `super.cancel()` and then cleaning-up
///   and ensuring `completeOperation()` is called.

public class AsynchronousOperation : Operation {

    private let stateLock = NSLock()

    private var _executing: Bool = false
    override private(set) public var isExecuting: Bool {
        get {
            return stateLock.withCriticalScope { _executing }
        }
        set {
            willChangeValue(forKey: "isExecuting")
            stateLock.withCriticalScope { _executing = newValue }
            didChangeValue(forKey: "isExecuting")
        }
    }

    private var _finished: Bool = false
    override private(set) public var isFinished: Bool {
        get {
            return stateLock.withCriticalScope { _finished }
        }
        set {
            willChangeValue(forKey: "isFinished")
            stateLock.withCriticalScope { _finished = newValue }
            didChangeValue(forKey: "isFinished")
        }
    }

    /// Complete the operation
    ///
    /// This will result in the appropriate KVN of isFinished and isExecuting

    public func completeOperation() {
        if isExecuting {
            isExecuting = false
        }

        if !isFinished {
            isFinished = true
        }
    }

    override public func start() {
        if isCancelled {
            isFinished = true
            return
        }

        isExecuting = true

        main()
    }

    override public func main() {
        fatalError("subclasses must override `main`")
    }
}

/*
 Abstract:
 An extension to `NSLocking` to simplify executing critical code.

 Adapted from Advanced NSOperations sample code in WWDC 2015 https://developer.apple.com/videos/play/wwdc2015/226/
 Adapted from https://developer.apple.com/sample-code/wwdc/2015/downloads/Advanced-NSOperations.zip
 */

import Foundation

extension NSLocking {

    /// Perform closure within lock.
    ///
    /// An extension to `NSLocking` to simplify executing critical code.
    ///
    /// - parameter block: The closure to be performed.

    func withCriticalScope<T>(block: () throws -> T) rethrows -> T {
        lock()
        defer { unlock() }
        return try block()
    }
}