CODEWITHSUNDEEP
iosobjective-cnsdata
Harrison Weinerman
Harrison Weinerman

Reputation: 86

Updating a progress indicator with synchronous downloads in iOS, not able to update UI on main thread?

I'm having a problem where I'm unable to update UI when performing synchronous downloads. I would expect that using synchronous APIs would ensure that code executes in order (which it doesn't seem to be doing), which is really confusing me.

The following code is in a UICollectionView's didSelectItemAtIndexPath and is not wrapped in any asynchronous block or anything.

Any ideas on what I can do to be able to update the UI (most importantly a progress indicator) as these tasks occur? I think that the way it is currently laid out should work, but for some reason it's not able to update until the code has all 'executed'.

if ([internetReachable isReachable]) {
    //does not become visible until after
    self.circleProgress.alpha = 1.0;

    //lots of downloading and saving with NSData dataWithContentsOfURL followed by this: 

    for (int i = 1; i < pages.count; i++) {
        NSString *number;
        if (i < 10) {
            number = [NSString stringWithFormat:@"00%d", i];
        }
        else if (i < 100) {
            number = [NSString stringWithFormat:@"0%d", i];
        }
        else {
            number = [NSString stringWithFormat:@"%d", i];
        }
        NSURL *imageURL = [NSURL URLWithString:[NSString stringWithFormat:@"http://books.hardbound.co/%@/%@-%@.png", slug, slug, number]];
        NSData *imageData = [NSData dataWithContentsOfURL:imageURL];

        [df setObject:imageData forKey:[NSString stringWithFormat:@"%@-%@", slug, number]];

        CGFloat progress = ((CGFloat)i / pages.count);
//only runs for the last iteration, rather than calling the method to update the progress indicator each iteration and allowing it to update before going back to the next iteration as I would expect
        [self updateProgressBarWithAmount:[NSNumber numberWithFloat:progress]];

        NSLog(@"progress after: %f", self.circleProgress.progress);
    }   
}

Upvotes: 1

Views: 666

Answers (2)

Andrew
Andrew

Reputation: 15377

I am not by any means qualified to explain what exactly happens during each render loop and why updateProgress doesn't actually let a screen render occur before you block the main thread again, but I am able to provide a solution.

After you update the progress of the progress view, you want the changes to get rendered "right now". This means you have to tell the current run loop to run one iteration, and then return to you so you can do another long running task.

[[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode beforeDate: [NSDate date]];

Call that whoever you want the progress view to update, and it will do a screen render and then return to you.

I got this from this answer

However, you really should be doing this asynchronously.

(Apologies for any typos, as this is being typed on my phone)

Upvotes: 1

p4sh4
p4sh4

Reputation: 3291

UI can only be executed on the main thread. Since the main thread is busy doing the downloading, it can't update the UI. It's almost never a good idea to perform any long running operations on the main thread. You should make the download asynchronous, and update the UI on the main thread.

The loop in the code you posted will only be executed after lots of downloading and saving with NSData dataWithContentsOfURL is performed, all the while the application will be unresponsive, and that's very poor UX. Take a look at this question for a much better implementation of a progress bar.

Upvotes: 3

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