如何在 Swift 3 中同时发出 https 请求

Question

我在执行 https 请求时遇到问题，如果请求没有任何错误，我永远不会收到消息，这是一个命令行工具应用程序，我有一个允许 http 请求的 plist，我总是看到完成块。

typealias escHandler = ( URLResponse?, Data? ) -> Void

func getRequest(url : URL, _ handler : @escaping escHandler){    
let session = URLSession.shared
var request = URLRequest(url:url)
request.cachePolicy = .reloadIgnoringLocalCacheData
request.httpMethod = "GET"
let task = session.dataTask(with: url ){ (data,response,error) in
        handler(response,data)
}

task.resume()
}


func startOp(action : @escaping () -> Void) -> BlockOperation{

let exOp = BlockOperation(block: action)    
exOp.completionBlock = {

print("Finished")

}
return exOp
}

     for sUrl in textFile.components(separatedBy: "\n"){
     let url = URL(string: sUrl)!

        let queu = startOp {
            getRequest(url: url){  response, data  in

                print("REACHED")



            }

        }
      operationQueue.addOperation(queu)
      operationQueue.waitUntilAllOperationsAreFinished()

Answer 1

一个问题是您的操作只是启动请求，但由于请求是异步执行的，因此操作会立即完成，而不是真正等待请求完成。在异步请求完成之前，您不想完成操作。

如果您想使用操作队列执行此操作，技巧是您必须子类化

Operation

并为

isExecuting

和

isFinished

执行必要的 KVO。然后，您可以在开始请求时更改

isExecuting

，在完成请求时更改

isFinished

，并使用两者关联的 KVO。这些都在并发编程指南：定义自定义操作对象中进行了概述，特别是在配置并发执行的操作部分。 （请注意，本指南有点过时（它指的是

isConcurrent

属性，已被替换为

isAsynchronous

；它重点关注 Objective-C；等等），但它向您介绍了这些问题。

无论如何，这是一个抽象类，我用它来封装所有这些愚蠢的异步操作：

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

public class AsynchronousOperation: Operation {

    override public var isAsynchronous: Bool { true }

    private let lock = NSLock()

    private var _executing: Bool = false
    override private(set) public var isExecuting: Bool {
        get {
            lock.withLock { _executing }
        }
        set {
            willChangeValue(forKey: #keyPath(isExecuting))
            lock.withLock { _executing = newValue }
            didChangeValue(forKey: #keyPath(isExecuting))
        }
    }

    private var _finished: Bool = false
    override private(set) public var isFinished: Bool {
        get {
            lock.withLock { _finished }
        }
        set {
            willChangeValue(forKey: #keyPath(isFinished))
            lock.withLock { _finished = newValue }
            didChangeValue(forKey: #keyPath(isFinished))
        }
    }

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

    public func finish() {
        if isExecuting {
            isExecuting = false
            isFinished = true
        }
    }

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

        isExecuting = true

        main()
    }
}

然后，我可以创建一个使用它的

NetworkOperation

：

class NetworkOperation: AsynchronousOperation {
    var task: URLSessionTask!

    init(session: URLSession, url: URL, requestCompletionHandler: @escaping (Data?, URLResponse?, Error?) -> ()) {
        super.init()

        task = session.dataTask(with: url) { data, response, error in
            requestCompletionHandler(data, response, error)
            self.finish()
        }
    }

    override func main() {
        task.resume()
    }

    override func cancel() {
        task.cancel()
        super.cancel()
    }
}

无论如何，完成此操作后，我现在可以为网络请求创建操作，例如：

let queue = OperationQueue()
queue.name = "com.domain.app.network"

let url = URL(string: "http://...")!
let operation = NetworkOperation(session: .shared, url: url) { data, response, error in
    guard let data = data, error == nil else {
        print("\(error)")
        return
    }
    
    let string = String(data: data, encoding: .utf8)
    print("\(string)")
    // do something with `data` here
}

let operation2 = BlockOperation {
    print("done")
}

operation2.addDependency(operation)

queue.addOperations([operation, operation2], waitUntilFinished: false) // if you're using command line app, you'd might use `true` for `waitUntilFinished`, but with standard Cocoa apps, you generally would not

注意，在上面的示例中，我添加了第二个操作，仅打印一些内容，使其依赖于第一个操作，以说明第一个操作在网络请求完成后才完成。

显然，您通常不会使用原始示例中的

waitUntilAllOperationsAreFinished

，也不会使用我示例中的

waitUntilFinished

的

addOperations

选项。但因为您正在处理一个命令行应用程序，并且在这些请求完成之前您不想退出，所以这种模式是可以接受的。 （我提到这一点只是为了让未来的读者对随意使用

waitUntilFinished

感到惊讶，这通常是不可取的。）