iOS 8专辑艺术封面

Question

自iOS 8以来，iPad&iPhone设备返回一个空数组。当我从本地文件（NSBundle）中提取时，我可以获得专辑艺术或封面艺术，但从iTunes或设备本身购买的任何歌曲都是空的。

我已经更新到最新的XCode，两台设备上的最新iOS，以及iTunes。我在iPad 4、iPad Air、iPhone 5、iPhone 6上进行了测试。希望有人知道发生了什么，这似乎是iOS 8中的一个已知错误。此外，我可以播放资产并检索歌曲名称和艺术家等信息。

MPMediaQuery *songQuery = [MPMediaQuery songsQuery];
NSArray *itemsFromGenericQuery = [songQuery items];
NSMutableArray *songsList = [[NSMutableArray alloc] initWithArray:itemsFromGenericQuery];
MPMediaItem *mediaItem = (MPMediaItem *)[songsList objectAtIndex:0];
NSURL *url = [mediaItem valueForProperty:MPMediaItemPropertyAssetURL];
AVAsset *asset = [[AVURLAsset alloc] initWithURL:url options:nil];
NSArray *commonArray = [assets commonMetadata];
//Test A
NSArray *albumArray = [AVMetadataItem metadataItemsFromArray:commonArray filteredByIdentifier:AVMetadataIdentifieriTunesMetadataCoverArt];
NSLog(@"commonArray = %lu",(unsigned long)[commonArray count]); //Returns 3
NSLog(@"albumArray has %lu",(unsigned long)[albumArray count]); //Returns 0 or null
//Test B
for (AVMetadataItem *metadataItem in asset.commonMetadata) {
    if ([metadataItem.commonKey isEqualToString:@"artwork"]){
        NSDictionary *imageDataDictionary = (NSDictionary *)metadataItem.value;
        NSData *imageData = [imageDataDictionary objectForKey:@"data"];
        UIImage *image = [UIImage imageWithData:imageData];
        coverArtImage.image = image;
    }
}
//Test C
for (AVMetadataItem *item in commonArray) {
    if ([item.keySpace isEqualToString:AVMetadataKeySpaceiTunes]) {
        NSData *newImage = [item.value copyWithZone:nil];
        coverArtImage.image = [UIImage imageWithData:newImage];
    }
}
//Test D
for (AVMetadataItem *item in asset.metadata) {
    if ([item.commonKey isEqualToString:@"artwork"]){
        NSDictionary *imageDataDictionary = (NSDictionary *)item.value;
        NSData *imageData = [imageDataDictionary objectForKey:@"data"];
        UIImage *image = [UIImage imageWithData:imageData];
        coverArtImage.image = image;
    }
    if ([item.keySpace isEqualToString:AVMetadataKeySpaceiTunes]) {
        NSData *newImage = [item.value copyWithZone:nil];
        coverArtImage.image = [UIImage imageWithData:newImage];
    }
}

所有封面艺术图像都返回null或从未被调用。此外，按照苹果的建议，从直接文件中提取封面是异步的，但无论我在什么设备上尝试过，似乎都至少需要10秒。iOS 7允许我们直接从mediaItem中提取封面，而且它是即时的，我不明白他们为什么会削弱这个功能。

Answer 1

NSString *queryString = [url query];
if (queryString == nil) // shouldn't happen
    return nil;
NSArray *components = [queryString componentsSeparatedByString:@"="];
if ([components count] < 2) // also shouldn't happen
    return nil;
id trackId = [components objectAtIndex:1];
MPMediaQuery *query = [[MPMediaQuery alloc] init];
[query addFilterPredicate:[MPMediaPropertyPredicate predicateWithValue:trackId forProperty:MPMediaItemPropertyPersistentID]];
NSArray *items = [query items];
[query release];
if ([items count] < 1) // still shouldn't happen
    return nil;
MPMediaItem* mpitem= [items objectAtIndex:0];
MPMediaItemArtwork* mpArt = [mpitem valueForProperty:MPMediaItemPropertyArtwork];
UIImage* mpImage = [mpArt imageWithSize:mpArt.bounds.size];

Answer 2

我建议尝试新的Swift实现：

let query = MPMediaQuery.songsQuery()
let mediaCollection = MPMediaItemCollection(items: query.items!)
print(mediaCollection.items)

这应该为下载了歌曲的物理设备返回一个非空数组。