getting the posterImage of ALAssetsGroupAlbum by name of album - ios

Question

Struggling a bit here with blocks and the ALAssetLibrary routines. I've been through the WWDC + Stanford videos on blocks and read up a bit but it hasn't quite clicked for me yet.
What I am trying to do is get the posterImage for a custom photo library. The first routine below is in my main viewController and calls the function getPosterImageForAlbum: which is in an ALAssetLibrary class extension.
The things I am struggling with/confused about/have tried:
Should I pass in a reference (CGImageRef or UIImage) and have the
ALAssetLibrary method set it or define a return value for the
ALAssetLibrary method and then handle the image in my main class?
I've tried it both ways without success.
The asynchronous nature of the ALAssetLibrary enumeration methods are
kind of hard to work with - so I guess I am doing it wrong.
Defining a block to be passed as a param: do I always need to
typedef it?
I think I've got all the conceptual bits and pieces swirling around but I haven't yet been able to wrangle them into a clear understanding of working with blocks. Any tips or pointers* to good articles would be much appreciated.
//
- (IBAction)getPosterImage:(id)sender {
NSString *groupName = self.groupNameField.text;
NSLog(#"%#", groupName);
__weak typeof(self) weakSelf = self;
CGImageRef tmpImg = [weakSelf.library getPosterImageForAlbum:groupName withCompletionBlock:(CGImageRef)(^GetPosterImageCompletion)(NSError *error){
if (error!=nil) {
NSLog(#"getPosterImage error: %#", [error description]);
} else {
if (tmpImg != nil){
UIImage * posterImg = [UIImage imageWithCGImage:tmpImg];
weakSelf.pImage.image = posterImg;
}
}
}];
}
// This is in an extension of the ALAssetLibrary
typedef CGImageRef(^GetPosterImageCompletion)(NSError* error);
-(CGImageRef)getPosterImageForAlbum:(NSString*)albumName
withCompletionBlock:(GetPosterImageCompletion)completionBlock
{
__block BOOL albumWasFound = NO;
__block CGImageRef thePosterImage = nil;
SaveImageCompletion test;
//search all photo albums in the library
[self enumerateGroupsWithTypes:ALAssetsGroupAlbum
usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
NSLog(#"this group name: %#",
[group valueForProperty:ALAssetsGroupPropertyName]);
//compare the names of the albums
if ([albumName compare:
[group valueForProperty:ALAssetsGroupPropertyName]]==NSOrderedSame) {
printf("matches \n"); //target album is found
albumWasFound = YES;
thePosterImage = group.posterImage;
*stop = true;
return ;
}
} failureBlock: test];
if (albumWasFound==NO) {
NSLog(#"%#", #"No group found");
}
return thePosterImage;
}

Answer 1

I'm editing my original answer in a big way because only after I posted it did I really understand what you are trying to do -- write a class extension on ALAssetLibrary that finds an album's poster image by name, rather than just find an album's poster image by name in any way possible.
I would handle this by writing an extension method that takes, as a parameter, a completion block that accepts a single parameter of type UIImage (or optionally an NSError parameter as well). In that completion block, the caller can do whatever they want with the image that's asynchronously returned. There is no need to typedef the block -- you could write the method this way:
- (void) getPosterImageForAlbumNamed: (NSString *) albumName completionBlock: (void (^)(UIImage *, NSError *)) completionBlock
{
__block ALAssetsGroup * foundAlbum = nil;
[self enumerateGroupsWithTypes: ALAssetsGroupAlbum
usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
if (group) {
if ([(NSString *) [group valueForProperty: ALAssetsGroupPropertyName] compare: albumName] == NSOrderedSame) {
*stop = YES;
foundAlbum = group;
completionBlock([UIImage imageWithCGImage: [group posterImage]], nil);
}
} else{
if (! foundAlbum) {
NSLog(#"Album wasn't found");
// completionBlock(nil, XX SOME ERROR XX);
}
}
} failureBlock:^(NSError *error) {
NSLog(#"Couldn't access photo library");
// completionBlock(nil, XX SOME ERROR XX);
}];
}
Then, you could call the method this way:
-(IBAction) getPosterForAlbum: (id) sender
{
NSString * albumName = self.textField.text;
ALAssetsLibrary * library = [[ALAssetsLibrary alloc] init];
__weak typeof(self) weakSelf = self;
[library getPosterImageForAlbumNamed: albumName completionBlock:^(UIImage * image, NSError * error) {
if (! error) {
[weakSelf doSomethingWithPosterImage: image];
}
}];
}
Is that along the lines of what you are trying to do? (Sorry for the major edit...)