Updating UIProgressView within dispatch_async

Question

I have my app making a google API call using dispatch_async and getting the JSON results that I am parsing through. I want to implement a progress view such that it updates depending on how many results from JSON response that I have parsed through.

So my dispatch_async Google API call is:

dispatch_async(myQueue, ^{

    NSData* data1 = [NSData dataWithContentsOfURL:googleURL];
    [self performSelectorOnMainThread:@selector(fetchResultsForGoogle:) withObject:data1 waitUntilDone:YES];
});

My fetchResultsForGoogle: withObject: method parses through the results, and I want to display the progress view on the screen with the number of processed results. So within the fetchResultsForGoogle... method, I want to have:

float percentage = (float)counter/(float)totalItems;
        self.progressView.progress = percentage;
        NSString *percentText = [NSString stringWithFormat:@"%f %% %@", percentage, @" complete"];
        self.progressViewLabel.text = percentText;

But I think understand that when the dispatch_async is performing on another thread, you can't update view on the main thread without using dispatch_async(dispatch_get_main_queue() . In an attempt to solve this, I tried two ways of implementing this (shown below) but either of these ways don't work for me ( progressView and progressViewLabel don't update at all).

Plan A:

dispatch_async(myQueue, ^{

        NSData* data1 = [NSData dataWithContentsOfURL:googleURL];
        [self performSelectorOnMainThread:@selector(fetchResultsForGoogle:) withObject:data1 waitUntilDone:YES];

dispatch_async(dispatch_get_main_queue(), ^{

            float percentage = (float)counter/(float)totalItems;
            self.progressView.progress = percentage;
            NSString *percentText = [NSString stringWithFormat:@"%f %% %@", percentage, @"complete"];
            NSLog(@"Percentage: %@", percentText);
            self.progressViewLabel.text = percentText;
        });

    });

Plan B:

dispatch_async(myQueue, ^{

            NSData* data1 = [NSData dataWithContentsOfURL:googleURL];
            [self performSelectorOnMainThread:@selector(fetchResultsForGoogle:) withObject:data1 waitUntilDone:YES];

        });

And within fetchResultsForGoogle... method:

dispatch_async(dispatch_get_main_queue(), ^{

                float percentage = (float)counter/(float)totalItems;
                self.progressView.progress = percentage;
                NSString *percentText = [NSString stringWithFormat:@"%f %% %@", percentage, @"complete"];
                NSLog(@"Percentage: %@", percentText);
                self.progressViewLabel.text = percentText;
            });

So any ideas or tips regarding the correct way to implement this is very much appreciated!

EDIT-SOLVED IT I fixed it. In dispatch_async block, I was passing it to performSelectorOnMainThread . Therefore when I was trying to update the progress view in performSelectorOnMainThread , the UI won't update until the dispatch_async was completed.

So, I changed it to performSelectorInBackgroundThread inside the dispatch_async block and now the performSelectorOnMainThread updates the UI like the way it should.

I am new to all this, but I am glad I am still learning new things.

Answer 1

You're right you can't update things in the main thread in the way you're doing it. You have to perform a selector on the main thread like so:

I'm not exactly sure where your percentage changes, but add the line below there. And it'll update your label and progressView on the main thread

 [self performSelectorOnMainThread:@selector(updateProgress:) withObject:[NSNumber numberWithFloat:percentage] waitUntilDone:NO];


-(void) updateProgress:(NSNumber *) num {
        float percentage = [num floatValue];
        self.progressView.progress = percentage;
        NSString *percentText = [NSString stringWithFormat:@"%f %% %@", percentage, @" complete"];
        self.progressViewLabel.text = percentText;
 }