iOS 7 Update UI During NSURLConnection Async Request

Question

After my UIViewController segues... inside the viewDidAppear I am making an asynchronous NSURLConnection call to retrieve some data. One of the pieces of data is a URL for an image that I will show the user on this new view.

I pass my method doing the NSURLConnection a block that I want to use as a callback after all the data is returned from the server; I call it in connectionDidFinishLoading.

Here is the block I am sending as my callback for connectionDidFinishLoading to call:

^(BOOL success, NSDictionary *response, NSError *error) {

            if (success) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    [loadingImageView startAnimating];
                    [self setImage:[UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:userSubmission.storageUri]]]];
                });

                NSLog(@"Is Main Thread: %ld", (long)[NSThread isMainThread]);
                userSubmission = [[Submission alloc] initWithDictionary:response];
                if (userSubmission.votes) {
                    _numberOfVotesSubmissions.text = [NSString stringWithFormat:@"%ld", (long)userSubmission.votes];
                } else {
                    _numberOfVotesSubmissions.text = @"0";
                }
                _votesOrSubmissionsImage.image = [UIImage imageNamed:@"list_votes_icon"];


            } else {
                _numberOfVotesSubmissions.text = [NSString stringWithFormat:@"%ld", (long) _clue.submissions];
                _votesOrSubmissionsImage.image = [UIImage imageNamed:@"list_photo_icon"];
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                [loadingImageView stopAnimating];
            });
        }];

I put a break point at the point where I try to load the image and at the point where I try to stop the "loadingImageView" from animating and I NEVER see the UIActivityViewController.

I have a UIImageView that is 300x300 and I place my UIActivityViewController in the middle of it (via addSubView).

This work beautifully if I start the UIActivityViewController spinning inside of viewDidAppear, however, there's not always going to be a scenario where I'll have an image to show and in that case the default "no image found" image is what I want to present.

I assume my problem is that I am running the NSURLConnection call and the run loop wants to finish my block callback before it starts the other stuff.

Any help is greatly appreciated.

Thanks.

Answer 1

The problem is you are loading the image data on the main thread - that's bad. You also try to do a lot of UI work in the background. Try this:

^(BOOL success, NSDictionary *response, NSError *error) {
        if (success) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [loadingImageView startAnimating];
            });

            NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:userSubmission.storageUri]];
            NSLog(@"Is Main Thread: %ld", (long)[NSThread isMainThread]);
            userSubmission = [[Submission alloc] initWithDictionary:response];

            dispatch_async(dispatch_get_main_queue(), ^{
                [self setImage:[UIImage imageWithData:imageData]];
                if (userSubmission.votes) {
                    _numberOfVotesSubmissions.text = [NSString stringWithFormat:@"%ld", (long)userSubmission.votes];
                } else {
                    _numberOfVotesSubmissions.text = @"0";
                }
                _votesOrSubmissionsImage.image = [UIImage imageNamed:@"list_votes_icon"];
                [loadingImageView stopAnimating];
            });
        } else {
            dispatch_async(dispatch_get_main_queue(), ^{
                _numberOfVotesSubmissions.text = [NSString stringWithFormat:@"%ld", (long) _clue.submissions];
                _votesOrSubmissionsImage.image = [UIImage imageNamed:@"list_photo_icon"];
            });
        }
    }];