How to load several photos with assetForURL with a list of URLs - objective-c

Question

For a list of URLs I need to load the photos with ALAssetsLibrary:assetForURL, and this within one method.
Since this method works async but it is not iterating over the passed list of URLs, as we all know.
I found this snippet (which should work):
- (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];
}];
}
}
How do I write the method definition in the form (above all the callback)
void loadImages(NSArray *imageUrls, NSArray *loadedImages, ...) ?
How do I call this method from another method (again mainly the callback part) ?
Can the callback be in the calling method or a 3rd method needed for this? and how does this method need to be written?
I have found the snippet here: http://www.calebmadrigal.com/functional-programming-deal-asynchronicity-objective-c/

Answer 1

Use NSThread to call the loadImages method.
NSMutableArray *imageCollection = [NSThread detachNewThreadSelector:#selector (loadImages:)
toTarget:self
withObject:imageUrlsCollection];
- (NSMutableArray *)loadImages:(NSArray *)imageUrls
{
ALAssetsLibrary* library = [[ALAssetsLibrary alloc] init];
NSMutableArray *loadedImages = [[NSMutableArray alloc] init];
#try
{
for(int index = 0; index < [imageUrls count]; index++)
{
NSURL *url = [imageUrls objectAtIndex:index];
[library assetForURL:url resultBlock:^(ALAsset *asset) {
ALAssetRepresentation *assetRepresentation = asset.defaultRepresentation;
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *image = [UIImage imageWithCGImage:assetRepresentation.fullResolutionImage scale:assetRepresentation.scale orientation:(UIImageOrientation)assetRepresentation.orientation];
[loadedImages addObject:image];
});
} failureBlock:^(NSError *error) {
NSLog(#"Failed to get Image");
}];
}
}
#catch (NSException *exception)
{
NSLog(#"%s\n exception: Name- %# Reason->%#", __PRETTY_FUNCTION__,[exception name],[exception reason]);
}
#finally
{
return loadedImages;
}
}
Note: With ARC,take care about invalid attempt to access ALAssetPrivate past the lifetime of its owning ALAssetsLibrary issue
Here is the fix :)