从完成处理程序返回值 - Swift [英] Return value from completion handler - Swift

问题描述

我在Utilities类中使用了loadImage方法，并且在通过闭包返回图像时遇到了一些麻烦。基本上是因为我的代码可以返回图像或错误，在调用方法时将其分配给图像属性将不起作用。

I'm using a loadImage method in a Utilities class and am having some trouble with returning images via closures. Basically because my code could return either an image or an error, assigning it to an image property when the method is called will not work.

我正在使用的方法在类的方法声明中有错误，或者我应该以不同的方式调用方法来预测可能不同的结果？谢谢

Is the approach I'm using wrong in the method declaration of the class, or should I be calling the method differently to anticipate potentially differing results? Thanks

public class UtilitiesService: NSObject {
    public class func loadImage(urlString:String)
    {

    var imgURL: NSURL = NSURL(string: urlString)!
    let request: NSURLRequest = NSURLRequest(URL: imgURL)
    NSURLConnection.sendAsynchronousRequest(
        request, queue: NSOperationQueue.mainQueue(),
        completionHandler: {(response: NSURLResponse!,data: NSData!,error: NSError!) -> Void in
            if error == nil {
                self.returnImage(data)
            }
    })
}

public class func returnImage(imageData: NSData) -> UIImage {

    return UIImage(data: imageData)!

}
}

//// view controller
class someView: UIViewController {
var image.image = loadImage(url) ///will throw a return type error
 }

推荐答案

为 loadImage func添加处理程序：

Add a handler to your loadImage func:

Swift 3

Swift 3

 func loadImage(_ urlString: String, handler:@escaping (_ image:UIImage?)-> Void)
    {

        let imageURL: URL = URL(string: urlString)!

        URLSession.shared.dataTask(with: imageURL) { (data, _, _) in
            if let data = data{
                handler(UIImage(data: data))
            }
        }.resume()
    }

调用func像这样：

loadImage("SomeURL") { (image) -> Void in
            if let image = image{
                DispatchQueue.main.async {
                    self.imageView.image = image
                }
            }
        }

Swift 2.3

Swift 2.3

func loadImage(urlString: String, handler: (image:UIImage?)-> Void)
    {

        let imageURL: NSURL = NSURL(string: urlString)!

        NSURLSession.sharedSession().dataTaskWithURL(imageURL) { (data, _, _) in
            if let data = data{
                handler(image: UIImage(data: data))
            }
            }.resume()
    }

像这样调用func：

  loadImage("someURL") { (image) -> Void in
            if let image = image{
                dispatch_async(dispatch_get_main_queue()) {
                    self.imageView.image = image
                }
            }
        }

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