How can I check if AVMetadataItem items like Title, Artist, etc , are null or don't exist?

I am building a music app and I am trying to get the MP3 files metadata with this piece of code:

for (NSString *format in [assest availableMetadataFormats]) {

        for (AVMetadataItem *item in [assest metadataForFormat:format]) {
            if ([[item commonKey] isEqualToString:@"title"]) {

                [SongNameMutableArray addObject:[NSString stringWithFormat:@"%@",[item value]]];
            }
            if ([[item commonKey] isEqualToString:@"artist"]) {

                [ArtistMutableArray addObject:[NSString stringWithFormat:@"%@",[item value]]];
            }
            if ([[item commonKey] isEqualToString:@"albumName"]) {

            }
            if ([[item commonKey] isEqualToString:@"artwork"]) {
                UIImage *img = nil;
                if(floor(NSFoundationVersionNumber) > NSFoundationVersionNumber_iOS_7_1) {
                    img = [UIImage imageWithData:item.dataValue];

                }
                else {
                    NSDictionary *dict ;
                    [item.value copyWithZone:nil];
                    img = [UIImage imageWithData:[dict objectForKey:@"data"]];

                }
            }
        }
    }

The code works fine. Only thing is that when the mutable arrays are filled, they do it in the following way:

SongNameMutableArray => [song1, song2, song3, songX];
ArtistMutableArray => [Artists1, Artist3, Artist 10, ArtistX];

Because not all songs have an artist in the metadata.

According to this specific part of the code

if ([[item commonKey] isEqualToString:@"artist"]) {
    [ArtistMutableArray addObject:[NSString stringWithFormat:@"%@",[item value]]];
}

If the item found is the artist, insert it in the array.

Now my question is how to detect when a song has no artist in the metadata? So that i can insert "null" in ArtistMutableArray => [Artists1, null, Artist3, null, Artist 5, ArtistX];

I tried with:

if ([[item commonKey] isEqualToString:@"null"])
if ([[item value] isEqualToString:@"null"])

I am not sure what I am missing in here.

Solution

Everything you are doing needs to be done differently. Don't split the data into separate arrays, especially if you want each array to have the same number of elements and each corresponding index refers to the same asset.

Instead, define a class that contains the few properties you want to track for each asset. Then have a single array of those class instances. Then there is no need to worry about keeping multiple arrays in sync and perform tricks by adding "null" values to an array where needed.

There are many other benefits to having one array of data. For example, sorting. By having one array, you can sort that one array of data by artist, for example. Trying to keep multiple arrays in sync when sorting or filtering is a huge pain.

Since you are getting just the common metadata there is no need to iterate over all formats and look at the properties of each format. Just use the commonMetadata property of the asset.

When getting the commonKey value, do not compare it against hardcoded strings. Use the constants provided by the API such as AVMetadataCommonKeyAlbumName.

And since each metadata item can only have one value for commonKey, use if/else to make your code more efficient. And use the stringValue property of the metadata item to get a string value instead of using stringWithFormat.

Here is a start for your metadata class. Name as you see fit and add whatever other properties you need.

@interface MyMetaData: NSObject

@property (nonatomic, copy) NSString *title;
@property (nonatomic, copy) NSString *artist;
@property (nonatomic, copy) NSString *albumName;
@property (nonatomic, copy) UIImage *artwork;

@end

@implementation MyMetaData
@end

And here's a replacement for the code you posted in your question:

NSMutableArray *metadata = [NSMutableArray array]; // your array of obtained metadata

AVAsset *asset = // some asset

MyMetaData *data = [[MyMetaData alloc] init];

NSArray *commonData = asset.commonMetadata;
for (AVMetadataItem *item in commonData) {
    AVMetadataKey key = item.commonKey;
    if ([key isEqual: AVMetadataCommonKeyTitle]) {
        data.title = item.stringValue;
    } else if ([key isEqual:AVMetadataCommonKeyArtist]) {
        data.artist = item.stringValue;
    } else if ([key isEqual:AVMetadataCommonKeyAlbumName]) {
        data.albumName = item.stringValue;
    } else if ([key isEqual:AVMetadataCommonKeyArtwork]) {
        data.artwork = // your code to get the image
    }
}

[metadata addObject:data];

This is much cleaner and you now only have one array to deal with. If a given asset doesn't have data for, say the artist, that value simply remain nil on the given MyMetaData instance.