How to stall program while waiting for image to load - ios

Question

How would you stall the program from executing? Doesn't sleep function do that? If it does, what's wrong with this code:
AsyncImageView *tmpImage = [imagesArray objectAtIndex:index];
int waitCounter = 0;
while (waitCounter < 10 && !tmpImage.imageIsLoaded) {
waitCounter++;
sleep(0.5);
}
NSLog(#"%i",waitCounter);
if (tmpImage.imageIsLoaded) {
return [tmpImage image];
} else {
return [UIImage imageNamed:#"img_noimg.png"];
}
The AsyncImageView class loads an image from an URL then sets imageIsLoaded property to true. This usually happens quite fast (under a second), but if I scroll though the images like a madman, then it doesn't have time to load (it loads 10 images ahead).
I've added the while to prevent that from happening, but the log displays multiple cases of waitCounter being 10 while there was no sleep time (the interface didn't freeze).

Answer 1

One approach to solve a problem like this is to use a completion handler. A completion handler signals the call-site when an asynchronous task is complete and passes the result of the task as a parameter to the completion handler.
Your AsyncImageView class does have indeed such a completion handler. It's a selector. Today, one would use a block instead. The principle is the same, though.
Continuing after an async task has finished is called "Continuation". With blocks, you code looks as follows:
typedef void (^completion_handler_block)(id result);
void doSomethingAsync(completion_handler_block)completionHandler;
- (void) foo {
doSomethingAsync(^(id result) {
// Continuation code
...
});
}
With blocks implementing continuation is especially easy, since it keeps the context (referenced variables defined in the call-site) within the block (closure).

Answer 2

Where are you doing this? in method cellForRowAtIndexPath ?
If so, you must not sleep here.
For loading an image asynchronously you must
1) pass the real UIImageView to your AsyncImageView and
2) when you load the UIImage from URL, instead of set *_imageIsLoaded = TRUE*, you can load it directly in the passed UIImageView using *_imageView.image = image*
For instance you can do something like this, taking into account that maybe you already have the image loaded, etc. Create this method in AsyncImageView like this
-(void) reloadWithImageView:(UIImageView*)imageView {
_imageView = imageView;
}
and when you later load the UIImage (probably in the connectionDidFinishLoading method)
- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection {
connection = nil;
UIImage *image = [UIImage imageWithData:data];
_imageView.image = image;
}

Answer 3

Try This, I guess it will work. Instead of
sleep(0.5);
try
[NSThread sleepForTimeInterval:.5f];