iOS4 & background [UIImage setImage:]

Question

Up to iOS 3.2, I used this kind of code to load UIImageView image in background, and it worked fine...

Code:

- (void)decodeImageName:(NSString *)name
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    UIImage *newImage = [UIImage imageNamed:name];
    [myImageView setImage:newImage];
    [pool release];
}
...
[self performSelectorInBackground:@selector(decodeImageName:) withObject:@"ID"]

... even if [UIImageView setImage:] was not thread-safe !

But since iOS 4, it doesn't work any more... Images appear on screen two seconds after setImage call. And if I do a [myImageView performSelectorOnMainThread:@selector(setImage:) withObject:newImage waitUntilDone:YES] instead of [myImageView setImage:newImage], images appear immediately but seem to be re-decoded again on-the-fly (ignoring the previous [UIImage imageNamed:] which should have already decoded the image data), causing a pause on my main thread... Even if documentation says The underlying image cache is shared among all threads..

Any thought ?

Answer 1

Don’t do it in the background! It’s not thread-safe. Since an UIImageView is also an NSObject, I think that using -[performSelectorOnMainThread:withObject:waitUntilDone:] on it might work, like:

[myImageView performSelectorOnMainThread:@selector(setImage:) withObject:newImage waitUntilDone:NO];

And it’s UIImage which is newly made thread-safe. UIImageView is still not thread-safe.

Answer 2

performSelectorInBackground: runs a selector in a background thread. Yet setImage: is a UI function. UI functions should only be run on the main thread. I do not have insight into the particular problem, but this is the first gut feel about this code, and it may be that iOS4 handles the (non-supported) mechanism of running UI functions in background threads somehow differently.

Answer 3

If you're using iOS 4.0, you should really consider reading up on blocks and GCD. Using those technologies, you can simply replace your method with:

- (void)decodeImageName:(NSString *)name
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    UIImage *newImage = [UIImage imageNamed:name];

    dispatch_async(dispatch_get_main_queue(), ^{
        [myImageView setImage:newImage];
    }

    [pool release];
}