Question

I have the variant of the following codes, triggered by a user input.

NSLog(@"WHY YOU iOS!!!");
[scanButton setSelected:YES];
[overlayImageView setImage:image];
[overlayView setHidden:YES];

The thing is that the above routine executes with seemingly random delay from five seconds to more than a minute.

Sometimes, they don’t execute at all, except, mysteriously, the NSLog method. If I set a breakpoint at the NSLog line, it just executes immediately and I can see “WHY YOU …” printed out in the console. However, even though the debugger runs smoothly past the below three lines without errors or warnings, the effect of the the next three lines takes place after random delay, or infinite delay. What’s wrong? How could it be possible that they have random delay?

I suspected that UI part of Cocoa SDK suspends due to my poor program design or whatever, but even during the random delay, other buttons and functionalities work so fine, while the effect of setImage takes place suddenly in 47.2 seconds. What’s wrong with iOS, or with me?

What confounds me more is that the above exact routine works fine without delay if the image parameter (UIImage*) comes from a different source. If that’s the case, we can attribute the cause of the problem to the image, however, why do scanButton and overlayView also respond with the same delay, even though they have nothing to do with the image? Total mystery to me.

Answer 1

I have no idea on whats going on, as there is almost no hint in the code that you’ve provided, but why you dont try something like this:

dispatch_async(dispatch_get_main_queue(), ^{

        NSLog(@"WHY YOU iOS!!!");
        [scanButton setSelected:YES];
        [overlayImageView setImage:image];
        [overlayView setHidden:YES];

        [overlayImageView setNeedsDisplay];
        [overlayImageView setNeedsLayout];

        [scanButton setNeedsDisplay];
        [scanButton setNeedsLayout];

        [overlayView setNeedsDisplay];
        [overlayView setNeedsLayout];

});

Basically ensure always to update de UI in the main thread (in case that you’re using a background thread) and set the UI controls as needed to be redrawn.