使用Alamofire进行NSURLSession并发请求

Question

我在测试应用中遇到了一些奇怪的行为。我发送到同一台服务器的同时发出大约50个GET请求。服务器是一块资源非常有限的小型硬件上的嵌入式服务器。为了优化每个请求的性能，我按如下方式配置Alamofire.Manager的一个实例：

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，它们会以2对的形式发送（正如预期的那样，使用Charles HTTP Proxy进行检查）。但奇怪的是，所有在第一次请求后30秒内没有完成的请求因为同时超时而被取消（即使它们还没有被发送）。以下是展示行为的插图：

这是一种预期的行为吗？如何确保请求在发送之前不会获得超时？

非常感谢！

Answer 1

是的，这是预期的行为。一种解决方案是将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在操作中包含了这些请求，这非常简单。但AFNetworking的NSURLSession实施从未这样做过，Alamofire也没有。

---

您可以轻松地将Request包装在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，我会做这样的事情：

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)
}

这样，这些请求将受到maxConcurrentOperationCount的约束，我们不必担心任何请求超时..

这是一个示例AsynchronousOperation基类，它负责与异步/并发NSOperation子类关联的KVN：

//
//  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`")
    }
}

/*
 Copyright (C) 2015 Apple Inc. All Rights Reserved.
 See LICENSE.txt for this sample’s licensing information

 Abstract:
 An extension to `NSLock` to simplify executing critical code.

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

import Foundation

extension NSLock {

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

    func withCriticalScope<T>( block: (Void) -> T) -> T {
        lock()
        let value = block()
        unlock()
        return value
    }
}

此模式还有其他可能的变体，但只需确保您（a）为true返回asynchronous; （b）按Concurrency Programming Guide: Operation Queues的配置并发执行操作部分所述，发布必要的isFinished和isExecuting KVN。