Musings of a Mobile Developer

Resume Presentations Android iOS

19 Jun 2016
PHImageManager

ALAsset framework has been deprecated. PHImageManager is a new API that acts as a centralized coordinator for image and video assets. From NSHispter:

Previously, each app was responsible for creating and caching their own image thumbnails. In addition to requiring extra work on the part of developers, redundant image caches could potentially add up to gigabytes of data across the system. But with PHImageManager, apps don’t have to worry about resizing or caching logistics, and can instead focus on building out features.

It provides synchronous opportunistic and asynchronous loading, as well as caching mechanisms. But first, we’ll cover how to retrieve the assets.

Fetching assets

Here is an example for pre-fetching 5 images, sorted by descending creation date order:

@import Photos;

@property (nonatomic, strong) NSArray<PHAsset *> *assets;

PHFetchOptions *options = [[PHFetchOptions alloc] init];
options.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"creationDate" ascending:NO]];
            
PHFetchResult *results = [PHAsset fetchAssetsWithMediaType:PHAssetMediaTypeImage options:options];
long numberImages = 5;

[results enumerateObjectsUsingBlock:^(id  _Nonnull object, NSUInteger idx, BOOL * _Nonnull stop) {
    if ([object isKindOfClass:[PHAsset class]]) {
        [self.assets addObject:object];
    }
    if (idx == numberImages - 1) {
        *stop = YES;
    }
}];

If you are looking to reverse enumerate, use enumerateObjectsWithOptions:usingBlock: passing NSEnumerationReverse for the options.

Asynchronous loading

The basic usage example is as follows (taken from nshipster):

@property (nonatomic, strong) PHImageManager* manager;

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"Cell" forIndexPath:indexPath];

    if (cell.tag) {
        [self.manager cancelImageRequest:(PHImageRequestID)cell.tag];
    }

    PHAsset *asset = self.assets[indexPath.row];

    cell.tag = [manager requestImageForAsset:asset
                                  targetSize:CGSizeMake(100.0, 100.0)
                                 contentMode:PHImageContentModeAspectFill
                                     options:nil
                               resultHandler:^(UIImage * _Nullable result, NSDictionary * _Nullable info) {
                                   cell.imageView.image = result;
                               }];
    return cell;
}

By default, the call will be asynchronous, which means resultHandler will be called after a result is available. This means that the imageViews might display a small delay before displaying the asset (See Opportunistic synchronous loading to avoid this problem).

Pre-caching

If you are certain all your assets will need to be displayed, or have a limit of images to load, you can cache them using a PHCachingImageManager. Here is an example:

PHCachingImageManager* cachingImageManager = [PHCachingImageManager new];
cachingImageManager.allowsCachingHighQualityImages = YES; //Set to NO if fast scrolling
[cachingImageManager startCachingImagesForAssets:self.assets
                                                  targetSize:CGSizeMake(100.0, 100.0)
                                                 contentMode:PHImageContentModeAspectFill
                                                     options:nil];

This way, the image will be ready when requested. It is also encouraged to call stopCachingImagesForAssets:targetSize:contentMode:options or stopCachingImagesForAllAssets. This is important when making multiple startCachingImagesForAssets requests with different parameters.

Opportunistic synchronous loading

To avoid the problem of asynchronous loading of requestImageForAsset:, it is possible to ask for a low-quality photo to put as placeholder. To do that:

PHImageRequestOptions* options = [PHImageRequestOptions new];
options.synchronous = YES;
options.deliveryMode = PHImageRequestOptionsDeliveryModeOpportunistic;
[[PHImageManager defaultManager] requestImageDataForAsset:asset options:options resultHandler:^(NSData * _Nullable imageData, NSString * _Nullable dataUTI, UIImageOrientation orientation, NSDictionary * _Nullable info) {
		 //Handle image
            }];

In this case, resultHandler will be called twice:

  • A first time, with the low-quality image, synchronously, called on the calling thread. This means that it is necessary to dispatch to main queue to interact with UIKit.
  • A second time, with a higher quality image, called on the main thread. synchronous has to be set to true for this to work.

PS: I know I am not supposed to tell you what to do, but, you know, you could always follow me on Twitter