问如何使用Photo Kit获取相册海报图像？

EN

    PHFetchOptions *userAlbumsOptions = [PHFetchOptions new];
    userAlbumsOptions.predicate = [NSPredicate predicateWithFormat:@"estimatedAssetCount > 0"];

    PHFetchResult *userAlbums = [PHAssetCollection fetchAssetCollectionsWithType:PHAssetCollectionTypeAlbum subtype:PHAssetCollectionSubtypeAny options:userAlbumsOptions];

    [userAlbums enumerateObjectsUsingBlock:^(PHAssetCollection *collection, NSUInteger idx, BOOL *stop) {
    NSLog(@"album title %@", collection.localizedTitle);
    PHFetchResult *assetsFetchResult = [PHAsset fetchAssetsInAssetCollection:collection options:nil];
    PHAsset *asset = [assetsFetchResult objectAtIndex:0];
    NSInteger retinaMultiplier = [UIScreen mainScreen].scale;
    CGSize retinaSquare = CGSizeMake(80 * retinaMultiplier, 80 * retinaMultiplier);

    [[SDWebImageManager sharedManager] downloadImageWithURL:[NSURL URLWithString:asset.localIdentifier] options:SDWebImageProgressiveDownload targetLocalAssetSize:retinaSquare progress:^(NSInteger receivedSize, NSInteger expectedSize) {

    } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
        if (image) {
            albumCoverImg.image = image;
        }
    }];
}];

static func fetchThumbnail(collection: PHCollection, targetSize: CGSize, completion: @escaping (UIImage?) -> ()) {

    func fetchAsset(asset: PHAsset, targetSize: CGSize, completion: @escaping (UIImage?) -> ()) {
        let options = PHImageRequestOptions()
        options.deliveryMode = PHImageRequestOptionsDeliveryMode.highQualityFormat
        options.isSynchronous = false
        options.isNetworkAccessAllowed = true

        // We could use PHCachingImageManager for better performance here
        PHImageManager.default().requestImage(for: asset, targetSize: targetSize, contentMode: .default, options: options, resultHandler: { (image, info) in
            completion(image)
        })
    }

    func fetchFirstImageThumbnail(collection: PHAssetCollection, targetSize: CGSize, completion: @escaping (UIImage?) -> ()) {
        // We could sort by creation date here if we want
        let assets = PHAsset.fetchAssets(in: collection, options: PHFetchOptions())
        if let asset = assets.firstObject {
            fetchAsset(asset: asset, targetSize: targetSize, completion: completion)
        } else {
            completion(nil)
        }
    }

    if let collection = collection as? PHAssetCollection {
        let assets = PHAsset.fetchKeyAssets(in: collection, options: PHFetchOptions())

        if let keyAsset = assets?.firstObject {
            fetchAsset(asset: keyAsset, targetSize: targetSize) { (image) in
                if let image = image {
                    completion(image)
                } else {
                    fetchFirstImageThumbnail(collection: collection, targetSize: targetSize, completion: completion)
                }
            }
        } else {
            fetchFirstImageThumbnail(collection: collection, targetSize: targetSize, completion: completion)
        }
    } else if let collection = collection as? PHCollectionList {
        // For folders we get the first available thumbnail from sub-folders/albums
        // possible improvement - make a "tile" thumbnail with 4 images
        let inner = PHCollection.fetchCollections(in: collection, options: PHFetchOptions())
        inner.enumerateObjects { (innerCollection, idx, stop) in
            self.fetchThumbnail(collection: innerCollection, targetSize: targetSize, completion: { (image) in
                if image != nil {
                    completion(image)
                    stop.pointee = true
                } else if idx >= inner.count - 1 {
                    completion(nil)
                }
            })
        }
    } else {
        // We shouldn't get here
        completion(nil)
    }
}