Running background threads that include blocks - ios

Question

I have a singleton that loads up an a bunch of ALAssets when my app launches. This is causing the main thread to freeze for more then 10 seconds while it loads each image into memory. Obviously a big no no.
I tried to put it on a background thread, but it only partially executes.
+ (CCPhotos*) sharedPhotos
{
static CCPhotos* shared = nil;
if (!shared)
{
shared = [[CCPhotos alloc] init];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
[shared loadPhotosArray];
});
}
return shared;
}
- (void) loadPhotosArray
{
NSLog(#"Loading photos");
_photos = [[NSMutableArray alloc] init];
NSData* data = [[NSUserDefaults standardUserDefaults] objectForKey: #"savedImages"];
if (data)
{
NSArray* storedUrls = [[NSArray alloc] initWithArray: [NSKeyedUnarchiver unarchiveObjectWithData: data]];
// reverse array
NSArray* urls = [[storedUrls reverseObjectEnumerator] allObjects];
for (NSURL* assetUrl in urls)
{
NSLog(#"Looking up %#", assetUrl);
// Block to handle image handling success
// This initializes, but doesn't get called
////-->>
ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset)
{
ALAssetRepresentation *rep = [myasset defaultRepresentation];
CGImageRef iref = [rep fullScreenImage];
if (iref) {
UIImage* tempImage = [UIImage imageWithCGImage:iref];
UIImage* image = [[UIImage alloc] initWithCGImage: tempImage.CGImage scale: 1.0 orientation: UIImageOrientationUp];
// Set image in imageView
[_photos addObject: image];
NSLog(#"Added photo with url: %#", [rep url]);
[[NSNotificationCenter defaultCenter] postNotificationName: #"PhotosChanged" object: self];
}
};
// Handles failure of getting image
ALAssetsLibraryAccessFailureBlock failureblock = ^(NSError *myerror)
{
NSLog(#"Can't get image - %#",[myerror localizedDescription]);
};
// Load image then call appropriate block
ALAssetsLibrary* assetslibrary = [[ALAssetsLibrary alloc] init];
[assetslibrary assetForURL: assetUrl
resultBlock: resultblock
failureBlock: failureblock];
}
}
else
{
NSLog(#"Photo storage is empty");
}
}
I've narrowed down the problem to ALAssetsLibraryAssetForURLResultBlock resultblock which doesn't get called. Multiple threads spawn at the beginning, and each one gets to this line, initializes the result block, but doesn't call it. I think it has to do with the thread safety on the block. Any thoughts?

Answer 1

Try doing this when you post the notification:
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName: #"PhotosChanged" object: self];
});
I believe the reason it is not working for you is because this is not being called on the main thread. All UI changes/updates (which I am assuming this leads to) must be executed on the main thread. This will force the notification to happen on the necessary thread and should work properly!
This is a pretty common thing to do. I actually created a code snippet because I found myself typing this out so often:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
<#on back thread#>
dispatch_async(dispatch_get_main_queue(), ^{
<#on main thread#>
});
});
I then set the completion short cut to dispatch_async

Answer 2

From what I'm understanding, you are spinning up multiple instances of your singleton. In that case you will probably want to put a #synchronized block in there. For example:
+ (CCPhotos*) sharedPhotos
{
static CCPhotos* shared = nil;
#synchronized ([CCPhotos class])
{
if (!shared)
{
shared = [[CCPhotos alloc] init];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
[shared loadPhotosArray];
});
}
}
return shared;
}
Hope that helps.