使用多个对象集合中的第一项创建对象数组？

Question

I am new to Objective C and iOS programming, and have wandered into waters that are a bit over my head. Any help is appreciated.

I am having an issue sorting an array to use in a UITableView. Basically, I want to query a set of albums, and sort alphabetically or by date (ascending and descending) based on user settings. First I synthesize some global arrays, and mediaquery the albums:

@synthesize allSongs, arrayForTableView;
MPMediaQuery *songsQuery = [MPMediaQuery songsQuery];
[songsQuery setGroupingType: MPMediaGroupingAlbum];
arrayForTableView = [songsQuery collections];
allSongs = [songsQuery items];

So far, so good. Albums sort alphabetically just like they should. The allSongs array is used for some playback functions elsewhere. So now, I add in detection for the settings, as well as sort by the "year" property:

@synthesize allSongs, arrayForTableView;
MPMediaQuery *songsQuery = [MPMediaQuery songsQuery];
[songsQuery setGroupingType: MPMediaGroupingAlbum];
NSArray *allAlbums = [songsQuery collections];
allSongs = [songsQuery items];

if ([[[NSUserDefaults standardUserDefaults] stringForKey:@"SortAlbumsMethod"] isEqualToString:@"0"]) {
    arrayForTableView = [songsQuery collections];
} else if ([[[NSUserDefaults standardUserDefaults] stringForKey:@"SortAlbumsMethod"] isEqualToString:@"1"]) {
    NSSortDescriptor *sortDescriptor;
    sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"year"
                                                 ascending:YES];
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
    arrayForTableView = [allAlbums sortedArrayUsingDescriptors:sortDescriptors];
} else {
    NSSortDescriptor *sortDescriptor;
    sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"year"
                                                 ascending:NO];
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
    arrayForTableView = [allAlbums sortedArrayUsingDescriptors:sortDescriptors];
}

This is where I run into problems. The above doesn't work because each collection (album) does not have a year property. If I replace "allAlbums" with "allSongs," the code functions correctly. Except that I now have an entry for every song. The label info displayed is only relevant to the album, and the navigation is based on AlbumPersistentID, so everything works great, I just have way too many duplicate entries (that appear identical, because the song name is not displayed).

It seems like the obvious solution then is to create an array, lets call it "albumRepresentatives" with a single item from each collection in "allAlbums", and use that in the sort descriptor. Each item would actually be a song and not an album, but the look and functionality would work nonetheless. Ideally, I'd like to compare all songs in each album, select one with the highest "year" property and add it to "albumRepresentatives". Plan B would be to simply take the song at index 0 for each album collection.

Problem is that actually doing that is beyond my abilities at this point. Any help would be greatly appreciated!

Answer 1

One option could be to use set because they do not store duplicated objects and init it with content of your array. You might need to override (possible add to category) isEqual to achieve it.

Second option:

//Could be added as category to MPMediaItem for example
+(MPMediaItem*)latestSongInAlbum:(NSArray*)songsInAlbum
    MPMediaItem *latestSong = nil;
    for (MPMediaItem *song in songsInAlbum) {
     if(!latestSong) {
      latestSong = song;
     } else if([[latestSong valueForProperty: MPMediaItemPropertyReleaseDate] compare:
      [song valueForProperty: MPMediaItemPropertyReleaseDate]] == NSOrderedAscending]) {
     latestSong = song;
     }
    }
 return latestSong;
}

than do something similar to this pseudocode:

NSMutableArray* latestSongFromeachAlbum = [[NSMutableArray alloc] initWithCapacity:[allAlbums count]];
for( album in allAlbums) {
 NSIndexSet* indexes = [allSongs indexesOfObjectsPassingTest:(^)(id obj, NSUInteger idx, BOOL *stop)) {
      return album == obj.album;
 }];
 NSArray* songsInAlbum = [allSongs objectsAtIndexes:indexes];

 MPMediaItem* latestSong = [ExtendedClass latestSongInAlbum:songsInAlbum];

 //now you have pair latestSong(you can use it's date) in album and the album. You can store as you prefer
 [latestSongFromeachAlbum addObject:latestSong];
}

Hope it's complete enough.