Memory Warning on fetching images from device photo library

Question

I have used following code to fetch images from photo library, resizing it before displaying it is receiving memory warnings. Also it gets terminated due to memory pressure.

-(void)readImages:(int)getAlbumImages
{

    imagesArray = [[NSMutableArray alloc]init];
    allImagesArray = [[NSMutableArray alloc] init];

    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
    void (^assetEnumerator)( ALAsset *, NSUInteger, BOOL *) = ^(ALAsset *result, NSUInteger index, BOOL *stop) {
        if(result != nil) {
             if([[result valueForProperty:ALAssetPropertyType] isEqualToString:ALAssetTypePhoto]) {
                        [allImagesArray addObject:[result valueForProperty:ALAssetPropertyURLs]];

                        NSURL *url= (NSURL*) [[result defaultRepresentation]url];
                        [library assetForURL:url
                                 resultBlock:^(ALAsset *asset) {
                                     [imagesArray addObject:[UIImage imageWithCGImage:[[asset defaultRepresentation] fullScreenImage]]];
                                     [picsTbl reloadData];
                                     [loadingView setHidden:YES];
                                 }
                                failureBlock:^(NSError *error){ NSLog(@"test:Fail"); } ];
                    }

                }
                else if(result == NULL){
                    [loadingView setHidden:YES];
                }
            };

            NSMutableArray *assetGroups = [[NSMutableArray alloc] init];
            void (^ assetGroupEnumerator) ( ALAssetsGroup *, BOOL *)= ^(ALAssetsGroup *group, BOOL *stop) {
                if(group != nil) {
                    [group enumerateAssetsUsingBlock:assetEnumerator];
                    [assetGroups addObject:group];
                    NSLog(@"Number of assets in group :%ld",(long)[group numberOfAssets]);
                    NSLog(@"asset group is:%@",assetGroups);
                }
                NSLog(@"[group numberOfAssets] %d",[group numberOfAssets]);

            };

            assetGroups = [[NSMutableArray alloc] init];

            [library enumerateGroupsWithTypes:ALAssetsGroupAll
                                   usingBlock:assetGroupEnumerator
                                 failureBlock:^(NSError *error) {NSLog(@"A problem occurred");}];
        }

Please help.

Answer 1

It should be quite obvious that you can't open and store all photos as fullscreen images in an array since this will cause memory pressure and finally a memory failure, which leads to a crash.

Additionally, the statement

[picsTbl reloadData];

might be problematic since it is not clear on which execution context it is executed since it is a completion handler of a system method. Here, if picsTbl is a UITableView is must be the main thread. You should make sure it actually is the case.

You need a different approach for your problem. Specifically, since you are potentially working with large data (given a restricted device) you must ensure you only process one image and only keep one image in memory at a time.

You can get some ideas how to sequentially invoke asynchronous methods - that is, effectively serializing asynchronous methods, here:

Force async tasks to run in sequence

ios programming: Using threads to add multiple images to library

Answer 2

• Crash is happening due to excess memory from UIImage
• We have use @autorelasePool.
• If you are saving the imgaes into an array then resize it before saving.

• For Non-ARC

    block = ^(ALAsset *myasset)
  {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
  
    // rest of block here
  
    [pool release];     }
  

• For ARC

    block = ^(ALAsset *myasset)
{
@autoreleasepool{
  // rest of block here

}
}

• Below is the sample Code

  {

      if(result != nil) 
          if([[result valueForProperty:ALAssetPropertyType] isEqualToString:ALAssetTypePhoto]) {
              [assetURLDictionaries addObject:[result valueForProperty:ALAssetPropertyURLs]];
  
              NSURL *url= (NSURL*) [[result defaultRepresentation]url];
  
              [library assetForURL:url
                       resultBlock:^(ALAsset *asset) {
                               @autoreleasepool {
  
                                   UIImage *imageToSave = [UIImage imageWithCGImage:[[asset defaultRepresentation] fullScreenImage]];
                                   CGSize sizeToSave = CGSizeMake(30,30);
                                   UIGraphicsBeginImageContextWithOptions(sizeToSave, NO, 0.f);
                                   [imageToSave drawInRect:CGRectMake(0.f, 0.f, sizeToSave.width, sizeToSave.height)];
                                   UIImage *finalImageToSave = UIGraphicsGetImageFromCurrentImageContext();
                                   UIGraphicsEndImageContext();
                                   NSLog(@"mutableArray.count:%d",mutableArray.count);
                                   if (finalImageToSave!=nil) {
                                       [mutableArray addObject:finalImageToSave];
  
                                   }
                                   if ([mutableArray count]==89)//count
                                   {
                                       [imageArray addObjectsFromArray:mutableArray];
                                       [self allPhotosCollected:imageArray];
                                   }
                               }
                       }
                      failureBlock:^(NSError *error){
                          [[[UIAlertView alloc]initWithTitle:@"" message:@"operation was not successfull!" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil, nil]show];
                          NSLog(@"operation was not successfull!");
                      } ];
          }
      }else{
  
          //handle UI
      }