Objective-C: UIImageWriteToSavedPhotosAlbum() + asynchronous = problems - objective-c

Question

Update: This problem was also reported here, with a more detailed treatment of the cause:
UIImageWriteToSavedPhotosAlbum saves only 5 image out of 10. Why?
In my case as well, the error was: "Write busy" - this seems to be an issue related to device speed. There is probably some solution that involves manually handling threading or similar - but, inspired by Tommy's answer below, I serialized the saving of images, and that works around the problem.
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Situation:
I'm trying to save a large-ish collection of images to the camera roll using a for-loop (number of images depends on user input - may be 1, 2, 3, or theoretically hundreds). Images are pointed to by an NSSet (though I could just as easily do an NSArray), and the NSSet is set to nil after the for-loop completes.
I'm using UIImageWriteToSavedPhotosAlbum() to save out the images on a detached thread (using NSThread's detachNewThreadSelector:toTarget:withObject: class method), and starting a UIActivityIndicator spinner on the main thread.
Problem:
When I attempt to save out more than ~5, any image after the ~5th will output this error in the log:
*** -[NSKeyedUnarchiver initForReadingWithData:]: data is NULL
For example, let's say I exported 9 images. The for-loop will run through all 9 images (confirmed by NSLogging), but I'll get around 4 iterations of the error above (and only 5 images saved to the camera roll).
If I add a breakpoint in the loop and wait a second or two in between each iteration, however, they are all saved correctly without complaint. So..
Theory:
Based on my logs and observations, UIImageWriteToSavedPhotosAlbum() is clearly running asynchronously and is somehow 'too slow' to keep up with my application.
Is there a simple way to force it to run synchronously (ideally on the main thread)? I've been experimenting with adding reference counts to the images I'm trying to save out, but (1) this feels hacky, and (2) I haven't solved the problem regardless.
Any advice would be great. Thanks!

Answer 1

I use ALAssetsLibrary and a dispatch_queue to make sure that images are saved sequentially:
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
dispatch_queue_t queue = dispatch_queue_create("com.myApp.saveToCameraRoll", NULL);
[images enumerateObjectsUsingBlock:^(UIImage *image, NSUInteger idx, BOOL *stop) {
dispatch_async(queue, ^{
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
[library writeImageToSavedPhotosAlbum:image.CGImage metadata:metaData completionBlock:^(NSURL *assetURL, NSError *writeError) {
if (writeError) {
// handle the error
}
else {
if (image == [images lastObject]) {
dispatch_async(dispatch_get_main_queue(), ^{
// perhaps indicate to the user that save has finished
});
}
}
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
dispatch_release(sema);
});
}];

Answer 2

If you're desperate not to write multiple images at once then you could use UIImageWriteToSavedPhotosAlbum's completion target and selector to implement a form of tail recursion. Something like:
- (void)writeSetToSavedPhotosAlbum:(NSMutableSet *)images
{
if(![images count]) return;
UIImage *imageToSave = [[[images anyObject] retain] autorelease];
[images removeObject:imageToSave];
NSLog(#"I shall now write image %#", imageToSave);
UIImageWriteToSavedPhotosAlbum(
imageToSave,
self,
#selector(writeSetToSavedPhotosAlbum:),
images);
}
EDIT: it may also be worth seeing whether you get the same results with ALAssetsLibrary's -writeImageToSavedPhotosAlbum:orientation:completionBlock:, which takes a block for completion so is even more straightforward to work with.

Answer 3

The docs do mention you'll be notified asynchronously. I wonder if this might work: for each image you want to save create a block operation and add it to main operation queue. Kind of like:
for (UIImage *toSave in imagesToSave)
{
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
UIImageWriteToSavedPhotosAlbum(toSave, target, selector, ctx);
}];
}
Might be worth a shot, let me know if it helps!

Answer 4

In this Code Work Save image :
UIImageWriteToSavedPhotosAlbum([self screenshot], nil, nil, nil);
UIAlertView *alert=[[UIAlertView alloc]initWithTitle:#"Save" message:#"Photo saved to album" delegate:self cancelButtonTitle:#"OK" otherButtonTitles:nil, nil];
[alert show];