How to know when UIimageView finished loading? - iphone

Question

In my view controller, how can I know when a certain UIImageView has finished loading (large jpeg from documents directory)? I need to know so that I can then swap a placeholder low-res imageview with this hi-res imageview. Do I need to create a custom callback to know this? Any way is fine.
By the way, here is a snippet of code where I load the image:
NSString *fileName = [NSString stringWithFormat:#"hires_%i.jpg", currentPage];
NSString *filePath = [NSString stringWithFormat:#"%#/BookImage/%#", [self documentsDirectory], fileName];
hiResImageView.image = [[[UIImage alloc] initWithContentsOfFile:filePath] autorelease];

Answer 1

UIImageView isn't doing any loading at all. All the loading is being done by [[UIImage alloc] initWithContentsOfFile:filePath], and your thread is blocked while the file is loaded (so the load is already complete by the time that call finally returns).
What you want to do is something like this:
- (void)loadImage:(NSString *)filePath {
[self performSelectorInBackground:#selector(loadImageInBackground:) withObject:filePath];
}
- (void)loadImageInBackground:(NSString *)filePath {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
UIImage *image = [[UIImage alloc] initWithContentsOfFile:filePath];
[self performSelectorOnMainThread:#selector(didLoadImageInBackground:) withObject:image waitUntilDone:YES];
[image release];
[pool release];
}
- (void)didLoadImageInBackground:(UIImage *)image {
self.imageView.image = image;
}
You would set up self.imageView to display the low-res image and then call loadImage: to load the high-res version.
Note that if you call this repeatedly before didLoadImageInBackground: gets called from earlier calls, you may cause the device to run out of memory. Or you might have the image from the first call take so much longer to load than image from the second call that didLoadImageInBackground: gets called for the second image before it gets called for the first. Fixing those issues is left as an exercise for the reader (or for another question).