使用MPMediaQuery通过评级查询ios库

Question

我正在开发一个iphone 4应用程序,它根据不同的标准过滤本地媒体库.我已经能够按歌曲名称,艺术家和流派进行过滤.现在我需要添加一个评级过滤机制.我想的是"评级> = N星"(N:0..5)

我用来过滤的代码是:

allMedia = [MPMediaQuery songsQuery];
MPMediaPropertyPredicate *mpp1 = [MPMediaPropertyPredicate predicateWithValue:@"2"     forProperty:MPMediaItemPropertyRating comparisonType:MPMediaPredicateComparisonEqualTo];
[allMedia addFilterPredicate:mpp1];

但是MPMediaPropertyPredicate不允许通过MPMediaItemPropertyRating进行过滤(实际上它适用于艺术家和歌曲标题).

2011-05-12 11:37:39.060 Radio3[1525:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'MPMediaPropertyPredicate cannot filter using the rating property.'

我通过MPMediaItemPropertyRating谷歌,但似乎我应该找到一种替代方法来按轨道评级过滤.

有人可以给我一个提示吗？

谢谢

更新:这是我的代码来解决这个问题:

allMedia = [MPMediaQuery songsQuery];
//MPMediaPropertyPredicate *mpp1 = [MPMediaPropertyPredicate predicateWithValue:@"2"   forProperty:MPMediaItemPropertyRating comparisonType:MPMediaPredicateComparisonEqualTo];
//MPMediaPropertyPredicate *mpp2 = [MPMediaPropertyPredicate predicateWithValue:@"Pop" forProperty:MPMediaItemPropertyGenre comparisonType:MPMediaPredicateComparisonContains];
//[allMedia addFilterPredicate:mpp1];
//[allMedia addFilterPredicate:mpp2];

//[myPlayer setQueueWithQuery:allMedia];


NSArray *itemsFromGenericQuery = [allMedia items];

NSMutableArray *mArray = [[NSMutableArray alloc] init];
int i = 0;
int j=0;

NSLog(@"itemCount: %d",[itemsFromGenericQuery count]);

float playsQuery = sliderPlays.value;
if(playsQuery == 20){playsQuery = 10000;}

NSLog(@"sliderRating.value %f  sliderPlays.value %.1f", [sliderRating value],  playsQuery);

while(i++ < 1000){
    int trackNumber = arc4random() % [itemsFromGenericQuery count];
    MPMediaItem *song = [itemsFromGenericQuery objectAtIndex:trackNumber];

    NSString *artistName = [song valueForProperty: MPMediaItemPropertyArtist];
    NSString *title = [song valueForProperty: MPMediaItemPropertyTitle];
    NSString *rating = [song valueForKey:MPMediaItemPropertyRating];
    double lengh = [[song valueForProperty:MPMediaItemPropertyPlaybackDuration] doubleValue];
    NSNumber *playCount = [song valueForKey:MPMediaItemPropertyPlayCount];



    if ([rating intValue] >= sliderRating.value && [playCount intValue] <= playsQuery) {
        if(j++ > 50){break;}
        NSLog (@"tracknumber: %d j: %d  artistName: %@ title: %@ lengh: %.1f rating: %@ playcount: %d",trackNumber, j, artistName, title, lengh, rating, [playCount intValue]);            
        [mArray addObject:song];            
    }

    if(i++ > 1000)break;
}


MPMediaItemCollection *itemCol = [[MPMediaItemCollection alloc] initWithItems:mArray];


[myPlayer setQueueWithItemCollection:itemCol];



[myPlayer setShuffleMode: MPMusicShuffleModeSongs];
[myPlayer setRepeatMode: MPMusicRepeatModeNone];

Answer 1

MPMediaItemPropertyRating是一个用户定义的属性，根据Apple 文档：

用户定义的属性不能用于构建媒体属性谓词。

解决这个问题的一种方法是首先迭代所有歌曲，将评级存储在数据库（或其他东西）中，然后对数据进行排序。