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iOS Photo Library in its simplest

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ALAssetsLibrary class provides access to the videos and photos in iOS device media library. Those can be e.g. in the Saved Photos album, coming from iTunes, or imported into the device.

The ALAssetsLibrary library API has been around since iOS 4.0, though considering amount of new related questions apparently it still has its touch of novelty.

“OK I got it. Now, can you show me a simpler code”? Heard this more then once when consulting on the topic, so eventually decided to put together a simple enough example that shows the ALAssetsLibrary basics without cluttering things with too much details.

This sample project shows how to iterate through the Library Saved Photos, using UIImageView and UIStepper. The relevant code is below.

#import <AssetsLibrary/AssetsLibrary.h>
#import "ViewController.h"

typedef void(^ImageProcessingBlock)(UIImage *);

@interface ViewController ()
    @property (nonatomic, strong) NSMutableArray *imagesURLArray;
    @property (nonatomic, copy) ImageProcessingBlock imageProcessingBlock;
@end

typedef NS_ENUM(NSUInteger, ImageFormats) {
    kImageThumbnail = 0,
    kImageFullScreen
};

@implementation ViewController

#pragma mark -
#pragma mark - Initialization
- (NSMutableArray *)imagesURLArray {
    if (!_imagesURLArray) {
        _imagesURLArray = [[NSMutableArray alloc] init];
    }
    return _imagesURLArray;
}

#pragma mark - AssetsLibrary images processing
// iterate over ALAssetsLibrary images, storing their URL to _imagesURLArray for later retrieval
// for the first image, runs it through imageProcessingBlock to initialize the imageView
- (void)fetchAssetsLibraryPhotosURLWithCompletionBlock:(void(^)())completionBlock {
    ALAssetsLibrary *assetLibrary = [[ALAssetsLibrary alloc] init];
    ALAssetsLibraryGroupsEnumerationResultsBlock libGroupEnumerationResult = ^(ALAssetsGroup *group, BOOL *stop) {
        if (group) {
            NSUInteger numAssets = group.numberOfAssets;
            [group enumerateAssetsUsingBlock:^(ALAsset *asset, NSUInteger index, BOOL *innerstop) {
                if (asset) {
                    ALAssetRepresentation *rep = [asset defaultRepresentation];
                    NSURL *imageURL = [rep url];
                    UIImage *image = nil;
                    if (index == 0) {
                        CGImageRef iref = [rep fullScreenImage];
                        if (iref) {
                            image = [UIImage imageWithCGImage:iref scale:rep.scale
                                                           orientation:(UIImageOrientation)rep.orientation];
                        }
                    }
                    dispatch_async(dispatch_get_main_queue(), ^{
                        [self.imagesURLArray addObject:imageURL];
                        if (image) {
                            self.imageProcessingBlock(image);
                        }
                        if (index + 1 == numAssets) {
                            completionBlock();
                        }
                    });
                }
            }];
        }
    };
    [assetLibrary enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos
                                  usingBlock:libGroupEnumerationResult
                                  failureBlock:^(NSError *error) {
          NSLog(@"failure: %@", [error localizedDescription]);
     }];
}
- (void)assetLibraryProcessImageAtURL:(NSURL *)assetLibraryPath
                          ImageFormat:(ImageFormats)format
                          WithBlock:(void(^)(UIImage *theImage))block {
    if (!assetLibraryPath) return;

    /// -- failure block --- ///
    ALAssetsLibraryAccessFailureBlock failureblock  = ^(NSError *theError) {
        NSLog(@"Can't get image from the asset library %@", [theError localizedDescription]);
    };
    /// -- full screen image block --- ///
    ALAssetsLibraryAssetForURLResultBlock fullScreenProcessResultBlock = ^(ALAsset *theAsset) {
        ALAssetRepresentation *rep = [theAsset defaultRepresentation];
        CGImageRef iref = [rep fullScreenImage];
        if (iref) {
            UIImage *image = [UIImage imageWithCGImage:iref scale:rep.scale
                                           orientation:(UIImageOrientation)rep.orientation];
            block (image);
        }
    };
    /// -- thumbnail image block --- ///
    ALAssetsLibraryAssetForURLResultBlock thumbnailProcessResultblock = ^(ALAsset *theAsset) {
        CGImageRef iref = [theAsset thumbnail];
        if (iref) {
            UIImage *image = [UIImage imageWithCGImage:iref];
            block (image);
        }
    };
    /////////
    ALAssetsLibrary *assetslibrary = [[ALAssetsLibrary alloc] init];
    [assetslibrary assetForURL:assetLibraryPath
                   resultBlock:(format == kImageThumbnail) ? thumbnailProcessResultblock : fullScreenProcessResultBlock
                   failureBlock:failureblock];
}


#pragma mark - view lifecycle
- (void)viewDidLoad {
    [super viewDidLoad];

    // initialize the image processing block
    __weak typeof (self) weakSelf = self;
    self.imageProcessingBlock =  ^(UIImage *theImage) {
        [weakSelf.imageView setImage:theImage];
    };

    // the navigation stepper will be set & enabled after processing images URL
    _navigationStepper.enabled = NO;

    [self fetchAssetsLibraryPhotosURLWithCompletionBlock:^{
        NSUInteger imagesCount = [self.imagesURLArray count];
        if (imagesCount > 1) {
            _navigationStepper.enabled = YES;
            _navigationStepper.maximumValue = imagesCount - 1;
        }
    }];
}

#pragma mark - use the stepper to navigate images, loading them for display as needed
- (IBAction)navigationStepperValueChanged:(id)sender {
    NSUInteger stepperValue = _navigationStepper.value;

    NSURL *imageURL = (NSURL *)[self.imagesURLArray objectAtIndex:stepperValue];
    [self assetLibraryProcessImageAtURL:imageURL ImageFormat:kImageFullScreen WithBlock:self.imageProcessingBlock];

    // for thumbnail image, just use below
    // [self assetLibraryProcessImageAtURL:imageURL ImageFormat:kImageThumbnail WithBlock:imageProcessingBlock];
}


@end

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