如何在Swift中将completionHandler Closure与Return结合使用？ [英] How to use completionHandler Closure with return in Swift?

问题描述

我正在尝试为我们提供一个RESTful API，该API返回一些json数据。我想封装创建HTTP请求并在其自己的方法中设置标头的代码，以便可以通过输入url String来调用它，然后让该方法返回JSON对象。

I am trying to us a RESTful API that returns some json data. I want to encapsulate the code that creates the HTTP Request and sets the headers in its own method so I can call it by entering a url String and then have the method return a JSON object.

在以下代码段中，我已经创建了请求对象并设置了标头，并将该变量称为 req。我尚未声明任何名为data，response或error的对象。我有以下代码可以正确打印出JSON对象

In the following snippet of code, I have already created the request object and set the headers, and I call that variable "req". I have not declared any objects named data, response, or error. I have the following code that correctly prints out a JSON object

let sesh = NSURLSession.sharedSession()
    let dataTask = sesh.dataTaskWithRequest(req, completionHandler: {(data, response, error) in
        var jsonError : NSError?
        let jsonBlob = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableLeaves, error: &jsonError)
        println(jsonBlob)
        });

    dataTask.resume()

这是我的问题。我如何做到这一点，以便该completeHandler块能够返回类型为 AnyObject！的jsonBlob？如果我将代码稍微修改为以下内容：

So here's my question. How do I make it so that this completionHandler block is able to return the jsonBlob, which is of type "AnyObject!"? If I modify the code slightly to be the following:

let sesh = NSURLSession.sharedSession()
    let dataTask = sesh.dataTaskWithRequest(req, completionHandler: {(data, response, error) -> AnyObject! in
        var jsonError : NSError?
        let jsonBlob : AnyObject! = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableLeaves, error: &jsonError)
        return jsonBlob
        });

    dataTask.resume()

然后程序将不会编译为调用dataTaskWithRequest：completionHandler会给出编译器警告，提示：

then the program will not compile as the call to dataTaskWithRequest:completionHandler gives a compiler warning saying:

 Could not find an overload for 'dataTaskWithRequest' that accepts the supplied arguments

我不明白。我正在使用正确的语法来返回闭包，如Swift Docs的本页中所述：

I don't understand this. I'm using the correct syntax for returning closures, as is given in this page of the Swift Docs:

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html

推荐答案

func getSomething(callback: (Array<AnyObject>) -> ()) {
    var dataTask = NSURLSessionDataTask()
    dataTask = session.dataTaskWithRequest(request) { (data, response, error) in
        if (error == nil) {
            var callbackArray = Array<MyObject>()
            let responseDict = NSJSONSerialization.JSONObjectWithData(data, options: .MutableContainers, error: nil) as NSDictionary
            let response = responseDict.objectForKey("response_key") as NSDictionary
            let array = response.objectForKey("array_key") as NSArray

            for item: AnyObject in array {
                var arrayItem = MyObject(dict: item as NSDictionary)
                callbackArray.append(arrayItem)
            }

            callback(callbackArray)
        } else {
            // handle an error
        }
    }
    dataTask.resume()
}

然后您可以执行以下操作：

Then you could do something like:

getSomething() { (response) in
    if let responseArray = response as? Array<MyObject> {
        self.somethings = responseArray
    }
}

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