NSURLSessionDataTask dataTaskWithURL completion handler not getting called

Question

I have been learning Objective C lately, and I decided to try connections.

Everything was great with NSURLConnection, until I discovered it was outdated, and tried to work with NSURLSession.

I am trying a very simple example, but can't seem to get my app to run the code inside the completion block.

Here is the code used:

NSURL * url = [NSURL URLWithString:@"http://api.openweathermap.org/data/2.5/weather?q=London,uk"];

NSLog(@"2");
NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
NSLog(@"3");

NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject
                                                             delegate: nil
                                                        delegateQueue: [NSOperationQueue mainQueue]];

NSLog(@"4");
NSURLSessionDataTask * dataTask = [defaultSession dataTaskWithURL:url
                                                completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                                    dispatch_sync(dispatch_get_main_queue(), ^{
                                                        NSLog(@"11");
                                                        if(error == nil)
                                                        {
                                                            NSString * text = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
                                                            NSLog(@"Data = %@",text);
                                                        }
                                                        NSLog(@"22");
                                                    });

                                                }];

NSLog(@"5");
[dataTask resume];
NSLog(@"6");

I get all the numbers printed from the main execution, but the completionHandler is never executed. I also tried this using a delegate, with no success.

Thanks in advance.

EDIT As suggested, I have changed my function to the following:

-(void) doGET{
    NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration     defaultSessionConfiguration];
    NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject];
    NSURLSessionDataTask * dataTask = [defaultSession dataTaskWithURL:[self url]
                                                    completionHandler:^(NSData *data,    NSURLResponse *response, NSError *error) {
                                                            NSLog(@"11");
                                                            if(error == nil)
                                                            {
                                                                NSString * text = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
                                                                NSLog(@"Data = %@",text);
                                                            }
                                                            NSLog(@"22");

                                                    }];
    [dataTask resume];

}

My completion manager is still not getting run. I also tried with the sharedSession instead of passing a configuration but no luck either.

Thanks for the help

Answer 1

If you're going to use the completion block rendition of the data task, rather than specifying a delegate of nil, like this:

NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject
                                                             delegate: nil
                                                        delegateQueue: [NSOperationQueue mainQueue]];

You should instead instantiate your session using the method that does not take a delegate at all:

NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject];

Or, alternatively, given that you're not customizing your configuration at all, you can skip the instantiation of the NSURLSessionConfiguration altogether and use the shared NSURLSession:

NSURLSession *defaultSession = [NSURLSession sharedSession];

But, bottom line, you should not use the sessionWithConfiguration:delegate:queue: rendition unless you're going to implement the delegates, in which case you wouldn't use the rendition of the NSURLSessionDataTask method with the completionHandler parameter.

Also, make sure your device/simulator is running iOS 7.0 or greater.

Answer 2

Check the location of your code.

Although it is not stated in OP where the code was located, I found, after much looking and many iterations, that putting sample code in -viewDidLoad never executed the completion handler. Moving the code into

- (void)viewWillAppear:(BOOL)animated { }

solved the never completing task issue. (I still don't understand why though.)