Place time-consuming operations in non-main threads, and those that require UI updates in the main thread.

__weak typeof(self) weakSelf = self; dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ // not in main block task [weakSelf getImageForCollectionView]; dispatch_async(dispatch_get_main_queue(), ^{ // main block. change ui NSLog(@"%@", weakSelf.photos); }); });

EDIT:

- (void)getImageForCollectionView:(void(^)(void))callback { _library = [[ALAssetsLibrary alloc] init]; self.photos = [NSMutableDictionary dictionary]; dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ [_library enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) { if (group) { NSMutableArray *array = [NSMutableArray array]; [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) { if (result) { [array addObject:result]; } }]; [self.photos setValue:array forKey:[group valueForProperty:@"ALAssetsGroupPropertyName"]]; } } failureBlock:^(NSError *error) { }]; dispatch_async(dispatch_get_main_queue(), ^{ callback(); }); }); }

[self getImageForCollectionView:^{ // ... NSLog(@"%@", self.photos); }];