swift - NSURLSession 与 Alamofire 的并发请求

Question

我的测试应用程序出现了一些奇怪的行为。我有大约 50 个同时发送到同一台服务器的 GET 请求。该服务器是资源非常有限的一小块硬件上的嵌入式服务器。为了优化每个请求的性能，我配置了一个 Alamofire.Manager 实例，如下所示:

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，它们会成对发送 2 个(正如预期的那样，已使用 Charles HTTP 代理进行检查)。奇怪的是，所有在第一个请求后 30 秒内未完成的请求都会同时因为超时而被取消(即使它们尚未发送)。这是展示行为的插图:

这是预期的行为吗？我如何确保请求在发送之前不会超时？

非常感谢!

Answer 1

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 来控制并发请求数而不是 HTTPMaximumConnectionsPerHost 参数。

最初的 AFNetworking 在将请求包装在操作中做得非常出色，这使得这变得微不足道。但是 AFNetworking 的 NSURLSession 实现从未这样做过，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 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)
}

这样，这些请求将受到 maxConcurrentOperationCount 的限制，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`")
    }
}

/*
 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()
    }
}

此模式还有其他可能的变体，但只要确保您 (a) 为 asynchronous 返回 true； (b) 您按照 Concurrency Programming Guide: Operation Queues 的为并发执行配置操作 部分所述发布必要的isFinished 和isExecuting KVN| .