迅捷shouldPerformSegueWithIdentifier获取数据异步

Question

Im making a very basic app which has a search field to get data that is passed to a tableview.

What I want to do is run an Async task to get the data and if the data is succesfully fetched go to the next view, during the loading the screen must not freeze thats why the async part is needed.

When the user pressed the searchbutton I run the following code to get data in my

override func shouldPerformSegueWithIdentifier(identifier: String, sender: AnyObject?) -> Bool {

method.

    var valid  = true
    let searchValue = searchField.text
    let session = NSURLSession.sharedSession()

    let url = NSURL(string: "https://someapi.com/search?query=" + searchValue!)



    let task = session.dataTaskWithURL(url!, completionHandler: {(data: NSData?, response: NSURLResponse?, error: NSError?) -> Void in
        if let theData = data {
            dispatch_async(dispatch_get_main_queue(), {
                //for the example a print is enough, later this will be replaced with a json parser
                print(NSString(data: theData, encoding: NSUTF8StringEncoding) as! String)

            })
        }
        else
        {
            valid =  false;
            print("something went wrong");
        }
    })

    UIApplication.sharedApplication().networkActivityIndicatorVisible = true
    task.resume()
    return valid;

I removed some code that checks for connection/changes texts to show the app is loading data to make the code more readable. This is the part where I have the problem, it comes after all the checks and im sure at this part I have connection etc.

What happens is it returns true (I can see this because the view is loaded), but also logs "Something went wrong" from the else statement.

I Understand this is because the return valid (at the last line) returns valid before valid is set to false. How can I only return true (which changes the view) if the data is succesfully fetched and dont show the next view if something went wrong?

Answer 1

Because you want the data fetching to be async you cannot return a value, because returning a value is sync (the current thread has to wait until the function returns and then use the value). What you want instead is to use a callback. So when the data is fetched you can do an action. For this you could use closures so your method would be:

func shouldPerformSegue(identifier: String, sender: AnyObject?, completion:(success:Bool) -> ());

And just call completion(true) or completion(false) in your session.dataTaskWithURL block depending on if it was successful or not, and when you call your function you give a block for completion in which you can perform the segue or not based on the success parameter. This means you cannot override that method to do what you need, you must implement your own mechanism.