High memory usage looping through PHAssets and calling requestImageForAsset

Question

I'm using an image picker library to allow the user to select many images from their photo library. They are returned as an array of PHAssets. Then, I want to convert all the PHAssets to UIImages and write them to the app's storage.

At the moment, I'm looping through all the assets and calling requestImageForAsset synchronously. My issue is that there is incredibly high memory usage spike when this loop is being run (with 30 images, it spikes up to 130MB). I would like to prevent this.

Here is my code:

for(PHAsset *asset in self.assets) {
        NSLog(@"started requesting image %i", i);
        [[PHImageManager defaultManager] requestImageForAsset:asset targetSize:PHImageManagerMaximumSize contentMode:PHImageContentModeAspectFit options:[self imageRequestOptions] resultHandler:^(UIImage *image, NSDictionary *info) {
            dispatch_async(dispatch_get_main_queue(), ^{
                assetCount++;
                NSError *error = [info objectForKey:PHImageErrorKey];
                if (error) NSLog(@"Image request error: %@",error);
                else {
                    NSString *imagePath = [appDelegate.docsPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%i.png",i]];
                    NSData *imageData = UIImagePNGRepresentation(image);
                    if(imageData) {
                        [imageData writeToFile:imagePath atomically:YES];
                        [self.imagesArray addObject:imagePath];
                    }
                    else {
                        NSLog(@"Couldn't write image data to file.");
                    }
                    [self checkAddComplete];
                    NSLog(@"finished requesting image %i", i);
                }
            });
        }];
    i++;
}

Based on the logs, I see that all of the "starting requesting image x" are called first, then all of the completion blocks ("finished requesting image x"). I think that this might be contributing to the memory issue. It would probably be less memory intensive to ensure that the completion block for each iteration is called before freeing those resources and moving to the next iteration. How can I do this?

Answer 1

I solved it with a recursive method that ensures that the completion block is finished before next iteration. This way it is possible to get thousands of photos with very low memory. Here is what it does:

Given an array of indices of selected photos in the camera roll

1. Request first object in array.
2. In completion block, remove first object in array and call same method again.
3. Repeat until array is empty

Here is the code.

- (void)processPhotos
{
  NSIndexPath *indexPath = [_selectedPhotosArray objectAtIndex:0];
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    PHAsset *asset = [_allPhotos objectAtIndex:indexPath.row];
    [self.imageManager requestImageForAsset:asset
                                 targetSize:PHImageManagerMaximumSize
                                contentMode:PHImageContentModeAspectFill
                                    options:self.imageRequestOptions
                              resultHandler:^(UIImage * _Nullable result, NSDictionary * _Nullable info) {
      NSError *error = [info objectForKey:PHImageErrorKey];
      if (_selectedPhotosArray.count > 0) {
        [_selectedPhotosArray removeObjectAtIndex:0];
      }
      if (error) { 
        NSLog(@"[CameraRoll] Image request error: %@",error);
      } else {
        if (result != nil) {
          [self processImage:result];
        }
      }
      if (_selectedPhotosArray.count > 0) {
        // Recurring loop
        [self processPhotos];
      }
    }];
  });
}

Make sure to check if array not empty before calling this method for the first time.

Answer 2

@Inder Kumar Rathore trick does not work for me. So i tried read more about PHImageManager here

I found that if i switch from

- requestImageForAsset:targetSize:contentMode:options:resultHandler:

to

- requestImageDataForAsset:options:resultHandler:

i will receive the image with the same dimension {5376, 2688} but the size in byte is much smaller. So the memory issue is solved.

hope this help !!

(note : [UIImage imageWithData:imageData] use this to convert NSData to UIImage)

Answer 3

Please use autoreleasepool for memory management.

for(PHAsset *asset in self.assets) {
    // This autorelease pool seems good (a1)
    @autoreleasepool {
        NSLog(@"started requesting image %i", i);
        [[PHImageManager defaultManager] requestImageForAsset:asset targetSize:PHImageManagerMaximumSize contentMode:PHImageContentModeAspectFit options:[self imageRequestOptions] resultHandler:^(UIImage *image, NSDictionary *info) {
            dispatch_async(dispatch_get_main_queue(), ^{
                //you can add autorelease pool here as well (a2)
                @autoreleasepool {
                    assetCount++;
                    NSError *error = [info objectForKey:PHImageErrorKey];
                    if (error) NSLog(@"Image request error: %@",error);
                    else {
                        NSString *imagePath = [appDelegate.docsPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%i.png",i]];
                        NSData *imageData = UIImagePNGRepresentation(image);
                        if(imageData) {
                            [imageData writeToFile:imagePath atomically:YES];
                            [self.imagesArray addObject:imagePath];
                        }
                        else {
                            NSLog(@"Couldn't write image data to file.");
                        }
                        [self checkAddComplete];
                        NSLog(@"finished requesting image %i", i);
                    }
                } //a2 ends here
            });
        }];
        i++;
    } // a1 ends here
}