如何在Swift中优化链接异步请求

Question

我的代码执行请求链接像A-> B-> C我正在使用URLSession所有请求以正确的顺序和预期的行为完成.但我想知道如何优化这种链接,因为它看起来非常复杂而且不可重复使用.我正在寻找如何以更灵活的方式进行链接的建议.

我的代码:

URLSession.shared.dataTask(with: URLRequest(url: URL(string: "first")!)){ data , res , err in
    let second = URLRequest(url: URL(string: "second")!)
    URLSession.shared.dataTask(with: second){ data , res , err in
        let third = URLRequest(url: URL(string: "second")!)
        URLSession.shared.dataTask(with:third){ data , res , err in
         }.resume()
    }.resume()
}.resume()

Answer 1

实际上你可以使用如下的OperationQueues来使用依赖项:

func operationQueueWithBlockandCancel(){

    let mainQueue = OperationQueue.main

    let operationBlock1 = BlockOperation()
    let operationBlock2 = BlockOperation()
    let operationBlock3 = BlockOperation()


    operationBlock1.addExecutionBlock {
       //Any task
    }

    operationBlock2.addExecutionBlock {
       //Any task
    }


    operationBlock3.addExecutionBlock {
        //Any task
    }

    //Add dependency as required
    operationBlock3.addDependency(operationBlock2)
    operationBlock2.addDependency(operationBlock1)

    opQueue.addOperations([operationBlock2,operationBlock1,operationBlock3,], waitUntilFinished: false)

}

Answer 2

正如@Paulw11 建议的：

PromiseKit + PMKFoundation

import PromiseKit
import PMKFoundation

let session = URLSession.shared
firstly {
    session.dataTask(with: URLRequest(url: URL(string: "first")!))
} .then { data in
    session.dataTask(with: URLRequest(url: URL(string: "second")!))
} .then { data in
    session.dataTask(with: URLRequest(url: URL(string: "third")!))
} .then { data -> () in
    // The data here is the data fro the third URL
} .catch { error in
    // Any error in any step can be handled here
}

通过 1 次（且仅 1 次）重试，您可以使用recover. recover与此类似，catch只是预计then可以重试前一个。但是，这不是循环并且仅执行一次。

func retry(url: URL, on error: Error) -> Promise<Data> {
    guard error == MyError.retryError else { throw error }

    // Retry the task if a retry-able error occurred.
    return session.dataTask(with: URLRequest(url: url))
}

let url1 = URL(string: "first")!
let url2 = URL(string: "second")!
let url3 = URL(string: "third")!

let session = URLSession.shared
firstly {
    session.dataTask(with: URLRequest(url: url1))
} .then { data in
    session.dataTask(with: URLRequest(url: url2))
} .recover { error in
    retry(url: url2, on: error)
} .then { data in
    session.dataTask(with: URLRequest(url: url3))
} .recover { error in
    retry(url: url3, on: error)
} .then { data -> () in
    // The data here is the data fro the third URL
} .catch { error in
    // Any error in any step can be handled here
}

注意：为了在不指定返回类型和需要return语句的情况下完成这项工作，我需要thenandrecover恰好为 1 行。所以我创建了方法来进行处理。