iOS swift 如何在需要 return 值的函数内等待异步任务

Question

我正在使用 swift 3.0 并创建了一个 return 是整数数组的函数。整数数组非常具体，它们是从数据库中获取的，因此 HTTP 调用是异步的。这是一个函数，因为我在 3 个不同的控制器中使用它，所以写一次是有意义的。我的问题是异步代码是 returned 在底部的 return 语句之后，因此它是 returning nil 。我在这里尝试了示例，但是它不起作用，主要是因为我需要 return 值。这是我的代码

func ColorSwitch(label: [UILabel]) -> [Int] {

    for (index, _) in label.enumerated() {
       label[index].isHidden = true
    }

    // I need the value of this variable in the return
    // statement after the async is done
    var placeArea_id = [Int]()

    let urll:URL = URL(string:ConnectionString+"url")!

    let sessionn = URLSession.shared
    var requestt = URLRequest(url: urll)
    requestt.httpMethod = "POST"


    let group = DispatchGroup()
    group.enter()

    let parameterr = "http parameters"
    requestt.httpBody = parameterr.data(using: String.Encoding.utf8)
    let task =   sessionn.dataTask(with:requestt, completionHandler: {(data, response, error) in
        if error != nil {
            print("check check error")
        } else {
            do {

                let parsedData = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String:Any]
                DispatchQueue.main.async {

                    if let Profiles = parsedData?["Results"] as? [AnyObject] {
                        if placeArea_id.count >= 0 {
                            placeArea_id = [Int]()   
                        }

                        for Profiles in Profiles {

                            if let pictureS = Profiles["id"] as? Int {
                                placeArea_id.append(pictureS)
                            }
                        }
                    }
                    group.leave()
                }

            } catch let error as NSError {
                print(error)

            }
        }
    })
    task.resume()


    group.notify(queue: .main) {
   // This is getting the value however can't return it here since it 
   // expects type Void
    print(placeArea_id)

    }
   // this is nil
    return placeArea_id

}

我已经检查过，值在异步代码中 returning 现在只需要 return 任何建议都很好。

Answer 1

为此您需要使用闭包，或者将您的函数更改为同步。

func ColorSwitch(label: [UILabel], completion:@escaping ([Int])->Void) {
    completion([1,2,3,4]) // when you want to return
}

ColorSwitch(label: [UILabel()]) { (output) in
    // output is the array of ints
    print("output: \(output)")
}

这是一篇关于闭包的非常好的博客http://goshdarnclosuresyntax.com/

Answer 2

您的函数 return 不能真正从该函数中的异步操作中获取值。这将破坏异步性的目的。为了将该数据传回 ColorSwitch(label:) 函数之外，您还需要让它接受一个将在完成时调用的闭包，该闭包接受 [Int] 作为参数。您的方法声明需要如下所示：

func ColorSwitch(label: [UILabel], completion: @escaping ([Int]) -> Void) -> Void {

    for (index, _) in label.enumerated() {
        label[index].isHidden = true
    }

    var placeArea_id = [Int]()

    let urll:URL = URL(string:ConnectionString+"url")!

    let sessionn = URLSession.shared
    var requestt = URLRequest(url: urll)
    requestt.httpMethod = "POST"


    let group = DispatchGroup()
    group.enter()

    let parameterr = "http parameters"
    requestt.httpBody = parameterr.data(using: String.Encoding.utf8)
    let task =   sessionn.dataTask(with:requestt, completionHandler: {(data, response, error) in
        if error != nil {
            print("check check error")
        } else {
            do {

                let parsedData = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String:Any]
                DispatchQueue.main.async {

                    if let Profiles = parsedData?["Results"] as? [AnyObject] {
                        if placeArea_id.count >= 0 {
                            placeArea_id = [Int]()
                        }

                        for Profiles in Profiles {

                            if let pictureS = Profiles["id"] as? Int {
                                placeArea_id.append(pictureS)
                            }

                        }
                    }
                    group.leave()
                    completion(placeArea_id) // This is effectively your "return"
                }

            } catch let error as NSError {
                print(error)

            }

        }

    })
    task.resume()
}

以后可以这样称呼：

ColorSwitch(label: []) { (ids: [Int]) in
    print(ids)
}

iOS swift 如何在需要 return 值的函数内等待异步任务

iOS swift how can I await an async task inside a function that needs a return value

ios

swift

asynchronous

swift3

nsurlsessiondatatask