In the last few weeks, I've used the [ALAssetsLibrary loadImages:callback:] method to load a list of images. For what I was doing, I wanted to wait till all the images were loaded before proceeding. I found that I could solve this problem very elegantly using a recursive solution:
- (void)loadImages:(NSArray *)imageUrls loadedImages:(NSArray *)loadedImages
callback:(void(^)(NSArray *))callback
{
if (imageUrls == nil || [imageUrls count] == 0) {
callback(loadedImages);
}
else {
NSURL *head = [imageUrls head];
__unsafe_unretained id unretained_self = self;
ALAssetsLibrary* library = [[ALAssetsLibrary alloc] init];
[library assetForURL:head resultBlock:^(ALAsset *asset) {
ALAssetRepresentation *assetRepresentation = asset.defaultRepresentation;
UIImage *image = [UIImage imageWithCGImage:assetRepresentation.fullResolutionImage
scale:assetRepresentation.scale
orientation:(UIImageOrientation)assetRepresentation.orientation];
[unretained_self loadImages:[imageUrls tail] loadedImages:[loadedImages
arrayByAddingObject:image] callback:callback];
} failureBlock:^(NSError *error) {
[unretained_self loadImages:[imageUrls tail] loadedImages:loadedImages callback:callback];
}];
}
}
In general, this scenario where you want to call a blocking method over a list of items seems to be better handled by recursion. This is just another place where functional programming proves superior in handling asynchronous tasks.
By the way, here is a reference to my NSArray+FP category which defines the head and tail code I use above.