Why isn't my ImageView doing anything, at least redrawing? - iphone

Question

I made a function that will cycle through images (1.png, 2.png, 3.png, 4.png, 5.png) and display them to a UIImageView. The idea is that I am loading a webpage and I want this imageview to run through this function, basically as a loading icon. However I dont understand why it isn't working, as far as the imageview goes, i dont see anything. Please help! My function's code is below. I know it is something wrong i'm doing here because everything else i test including the webview delegate works just fine.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
for(int c = 1; !webviewLoaded; c++)
{
c = c % 6;
if(c == 6)
c = 1;
NSString *numberString = [NSString stringWithFormat:#"%d.png", c];
UIImage *img = [UIImage imageNamed:numberString];
[self.loadingImage setImage:img];
sleep(1);
}
});

Answer 1

Because you are doing UI stuff on a background thread, you should't change UI components from a background thread.
You can change your code as following:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_async(dispatch_get_main_queue(), ^{
for(int c = 1; !webviewLoaded; c++)
{
c = c % 6;
if(c == 6)
c = 1;
NSString *numberString = [NSString stringWithFormat:#"%d.png", c];
UIImage *img = [UIImage imageNamed:numberString];
[self.loadingImage setImage:img];
sleep(1);
}
}
});
This piece of code will work, but is kind of useless and it will give you results not as good as you're expecting because what you are doing is create a separate thread (operation) just to call the UI thread from it and this is not efficient.
What you should do is entirely remove the dispatch like this:
for(int c = 1; !webviewLoaded; c++)
{
c = c % 6;
if(c == 6)
c = 1;
NSString *numberString = [NSString stringWithFormat:#"%d.png", c];
UIImage *img = [UIImage imageNamed:numberString];
[self.loadingImage setImage:img];
sleep(1);
}
This will also work, but make sure you have those files #"%d.png added to your project.
And because you are not familiar with GCD here is a good tutorial