Function does not wait until the data is downloaded

问题

I have the following function which downloads an image from server;

func getImageFromServerById(imageId: String) -> UIImage? {
    let url:String = "https://dummyUrl.com/\(imageId).jpg"
    var resultInNSDataformat: NSData!

    let task = NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: url)!) {(data, response, error) in
        if (error == nil){
            resultInNSDataformat = data
        }
    }
    task.resume()
    return UIImage(data: resultInNSDataformat)
}

The function does not wait for the download task to be completed before returning the image. Therefore my app always crashes. Any ideas for how to wait for the download?

回答1:

The other answer is not a good replacement for the code you already had. A better way would be to continue using NSURLSession's data tasks to keep the download operation asynchronous and adding your own callback block to the method. You need to understand that the contents of the download task's block are not executed before you return from your method. Just look at where the call to resume() is for further evidence.

Instead, I recommend something like this:

func getImageFromServerById(imageId: String, completion: ((image: UIImage?) -> Void)) {
    let url:String = "https://dummyUrl.com/\(imageId).jpg"

    let task = NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: url)!) {(data, response, error) in
        completion(image: UIImage(data: data))
    }

    task.resume()
}

Which can be called like this

getImageFromServerById("some string") { image in
    dispatch_async(dispatch_get_main_queue()) {
        // go to something on the main thread with the image like setting to UIImageView
    }
}

来源：https://stackoverflow.com/questions/27208198/function-does-not-wait-until-the-data-is-downloaded

标签

swift

nsurlsession