如何在Swift中使用completionHandler Closure返回？

Question

我正在尝试使用RESTful API来返回一些json数据.我想封装创建HTTP请求的代码并在其自己的方法中设置标头,以便我可以通过输入url String来调用它,然后让方法返回一个JSON对象.

在下面的代码片段中,我已经创建了请求对象并设置了标题,我将该变量称为"req".我没有声明任何名为data,response或error的对象.我有以下代码正确打印出一个JSON对象

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()

所以这是我的问题.如何使这个completionHandler块能够返回类型为"AnyObject!"的jsonBlob？如果我稍微修改代码如下:

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对dataTaskWithRequest的调用:completionHandler给出编译器警告说:

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

我不明白这一点.我正在使用正确的语法来返回闭包,如Swift Docs的这一页所示:

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

Answer 1

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()
}

然后你可以这样做:

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