如何在Swift中从闭包返回值？

Question

我使用fabric插件/Twitter-kit在我的应用中使用Twitter API。我想获取一个名人的个人资料图片的图像URL。以下是我的代码。

func getImageURL(celebrity :String) -> String{

    var imageURL = ""
    let client = TWTRAPIClient()
    let statusesShowEndpoint = userSearch
    let params = ["q": celebrity,
                  "page" : "1",
                  "count" : "1"
                  ]
    var clientError : NSError?

    let request = client.URLRequestWithMethod("GET", URL: statusesShowEndpoint, parameters: params, error: &clientError)

     client.sendTwitterRequest(request) { (response, data, connectionError) -> Void in
        if connectionError != nil {
            print("Error: \(connectionError)")
        }

        do {
            let json = try NSJSONSerialization.JSONObjectWithData(data!, options: [])
            print("json: \(json)")

            let profileImageURL: String? = json[0].valueForKey("profile_image_url_https") as? String

            imageURL =  self.cropTheUrlOfImage(profileImageURL!)
            print(imageURL)

            //how do i return the value back. confused ?????
            return imageURL

        } catch let jsonError as NSError {
            print("json error: \(jsonError.localizedDescription)")
        }
    }

    return imageURL

}

由于闭包未完全执行，我不知道如何返回值。我是Swift新手，感谢任何帮助。

我想在tableview的cellForRowIndexPath中使用此方法来从imageURL下载图像。

- Ajay

---

请参阅如何从Swift的void闭包中返回值？。 - OOPer

http://stackoverflow.com/questions/38160959/void-inside-of-return-function/38161092#38161092 - Pranav Wadhwa

Answer 1

你需要从一个 @escaping 闭包中返回。 修改该函数。

func getImageURL(celebrity: String) -> String {

}

到

func getImageURL(celebrity: String, completion: @escaping(String)->()) {

      // your code
      completion(imgURL)
}

你可以按照以下方式使用它

getImageURL(celebrity: String) { (imgURL) in

      self.imgURL = imgURL // Using self as the closure is running in background
}

这里是一个例子，我用闭包写多个方法以供完成。

class ServiceManager: NSObject {

//  Static Instance variable for Singleton
static var sharedSessionManager = ServiceManager()

//  Function to execute GET request and pass data from escaping closure
func executeGetRequest(with urlString: String, completion: @escaping (Data?) -> ()) {

    let url = URL.init(string: urlString)
    let urlRequest = URLRequest(url: url!)

    URLSession.shared.dataTask(with: urlRequest) { (data, response, error) in
        //  Log errors (if any)
        if error != nil {
            print(error.debugDescription)
        } else {
            //  Passing the data from closure to the calling method
            completion(data)
        }
    }.resume()  // Starting the dataTask
}

//  Function to perform a task - Calls executeGetRequest(with urlString:) and receives data from the closure.
func downloadMovies(from urlString: String, completion: @escaping ([Movie]) -> ()) {
    //  Calling executeGetRequest(with:)
    executeGetRequest(with: urlString) { (data) in  // Data received from closure
        do {
            //  JSON parsing
            let responseDict = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String: Any]
            if let results = responseDict!["results"] as? [[String:Any]] {
                var movies = [Movie]()
                for obj in results {
                    let movie = Movie(movieDict: obj)
                    movies.append(movie)
                }
                //  Passing parsed JSON data from closure to the calling method.
                completion(movies)
            }
        } catch {
            print("ERROR: could not retrieve response")
        }
    }
  }
}

以下是我使用它传递值的示例。

ServiceManager.sharedSessionManager.downloadMovies(from: urlBase) { (movies : [Movie]) in   // Object received from closure
      self.movies = movies
      DispatchQueue.main.async {
            //  Updating UI on main queue
            self.movieCollectionView.reloadData()
      }
}

我希望这能帮助寻找同样解决方案的任何人。

- caffieneToCode

---

为什么不使用异步函数来检索数据？ - Casey

Answer 2

您是正确的，sendTwitterRequest函数会在您的函数返回后才返回，因此外部函数无法返回imageURL。

相反，在闭包中，取出您想要在单元格中返回的值并将其存储在某个地方（成员变量中），然后让tableView自己更新它（例如使用tableView.reloadData()）。

这将导致它再次获取单元格（cellForRow ...）。更改实现以使用从调用中存储的值的成员变量。

- Lou Franco