Passing a Value Asynchronously - objective-c

Question

I have the following method that uses blocks and completion handlers:
// Returns YES if photo is stored in a virtual vacation.
- (BOOL) photoIsOnVacation
{
__block BOOL photoOnFile = NO;
// Identify the documents folder URL.
NSFileManager *fileManager = [[NSFileManager alloc] init];
NSError *errorForURLs = nil;
NSURL *documentsURL = [fileManager URLForDirectory:NSDocumentDirectory
inDomain:NSUserDomainMask
appropriateForURL:nil
create:NO
error:&errorForURLs];
if (documentsURL == nil) {
NSLog(#"Could not access documents directory\n%#", [errorForURLs localizedDescription]);
} else {
// Retrieve the vacation stores on file.
NSArray *keys = [NSArray arrayWithObjects:NSURLLocalizedNameKey, nil];
NSArray *vacationURLs = [[NSFileManager defaultManager] contentsOfDirectoryAtURL:documentsURL
includingPropertiesForKeys:keys
options:NSDirectoryEnumerationSkipsHiddenFiles
error:nil];
if (!vacationURLs) photoOnFile = NO;
else {
// Search each virtual vacation for the photo.
for (NSURL *vacationURL in vacationURLs) {
NSError *errorForName = nil;
NSString *vacationName = nil;
[vacationURL getResourceValue:&vacationName forKey:NSURLNameKey error:&errorForName];
[VacationHelper openVacationWithName:vacationName usingBlock:^(UIManagedDocument *vacationDocument) {
NSError *error = nil;
NSManagedObjectContext *moc = vacationDocument.managedObjectContext;
// Build fetch request.
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:#"Photo"];
NSString *currentPhotoID = [self.chosenPhoto objectForKey:FLICKR_PHOTO_ID];
request.predicate = [NSPredicate predicateWithFormat:#"unique = %#", currentPhotoID];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:#"unique" ascending:YES];
request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
// Execute fetch request.
NSArray *checkPhotos = [moc executeFetchRequest:request error:&error];
if (error) {
NSLog(#"Error searching for photo:%#",error);
} else {
Photo *checkPhoto = [checkPhotos lastObject];
if ([checkPhoto.unique isEqualToString:currentPhotoID]) photoOnFile = YES;
}
}];
if (photoOnFile) break;
}
}
}
return photoOnFile;
}
My problem is that photoOnFile is always false because execution reaches the return before the block that contains the fetch request. I've tried embedding the photoOnFile assignment within dispatch_async(dispatch_get_main_queue(),^{ but that hasn't helped. Any guidance appreciated.
Update: here is the reworked code successfully incorporating Ken's recommended solution:
- (void)checkIfPhotoIsOnVacationAndDo:(void(^)(BOOL photoIsOnVacation))completionBlock
{
__block BOOL photoIsOnVacation = NO;
// Identify the documents folder URL.
NSFileManager *fileManager = [[NSFileManager alloc] init];
NSError *errorForURLs = nil;
NSURL *documentsURL = [fileManager URLForDirectory:NSDocumentDirectory
inDomain:NSUserDomainMask
appropriateForURL:nil
create:NO
error:&errorForURLs];
if (documentsURL == nil) {
NSLog(#"Could not access documents directory\n%#", [errorForURLs localizedDescription]);
} else {
// Retrieve the vacation stores on file.
NSArray *keys = [NSArray arrayWithObjects:NSURLLocalizedNameKey, nil];
NSArray *vacationURLs = [[NSFileManager defaultManager] contentsOfDirectoryAtURL:documentsURL
includingPropertiesForKeys:keys
options:NSDirectoryEnumerationSkipsHiddenFiles
error:nil];
if (!vacationURLs) photoIsOnVacation = NO;
else {
// Search each virtual vacation for the photo.
for (NSURL *vacationURL in vacationURLs) {
NSError *errorForName = nil;
NSString *vacationName = nil;
[vacationURL getResourceValue:&vacationName forKey:NSURLNameKey error:&errorForName];
[VacationHelper openVacationWithName:vacationName usingBlock:^(UIManagedDocument *vacationDocument) {
NSError *error = nil;
NSManagedObjectContext *moc = vacationDocument.managedObjectContext;
// Build fetch request.
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:#"Photo"];
NSString *currentPhotoID = [self.chosenPhoto objectForKey:FLICKR_PHOTO_ID];
request.predicate = [NSPredicate predicateWithFormat:#"unique = %#", currentPhotoID];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:#"unique" ascending:YES];
request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
// Execute fetch request.
NSArray *checkPhotos = [moc executeFetchRequest:request error:&error];
if (error) {
NSLog(#"Error searching for photo:%#",error);
} else {
Photo *checkPhoto = [checkPhotos lastObject];
if ([checkPhoto.unique isEqualToString:currentPhotoID]) {
photoIsOnVacation = YES;
completionBlock(photoIsOnVacation);
}
}
}];
if (photoIsOnVacation) break;
}
completionBlock(photoIsOnVacation);
}
}
}

Answer 1

Asynchronicity tends to spread. Once you make an API asynchronous, all of its callers have to be redesigned to work asynchronously, too. Therefore, a method like your - (BOOL) photoIsOnVacation is untenable because its interface is synchronous – the caller expects to have an answer as soon as the call completes – but the implementation doesn't work that way.
You have to redesign to something like - (void) checkIfPhotoIsOnVacationAndDo:(void(^)(BOOL photoIsOnVacation))block. That takes a block from the caller and invokes the block with the answer when it is known.

Answer 2

This is what semaphores are for:
bool waitForBlockToExecute()
{
__block bool value = false;
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
// sleep for a bit
sleep(1);
value = true;
// notify that the block is finished
dispatch_semaphore_signal(semaphore);
});
// wait for the semaphore
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_release(semaphore); // clean up the semaphore
return value;
}
Obviously, the dispatch_async block would be replaced with your callback block, but I assume you get the picture from the above code.