retrieve metadata from mp3s

metadata.h

#import <Foundation/Foundation.h>

@interface metadataRetriever : NSObject

+ (NSDictionary *)metadataForFile:(NSString *)filePath;
+ (NSData *)id3TagForFile:(NSString *)filePath;

@end

metadata.m

#import "metadataRetriever.h"
#import <CoreFoundation/CoreFoundation.h>
#import <AudioToolbox/AudioToolbox.h>

static NSString * const kRaiseMessage = @"Failed to read ID3 tag.";

@implementation metadataRetriever

+ (AudioFileID)openFile:(NSString *)path {
    NSURL *fileURL = [NSURL fileURLWithPath:path];
    
    AudioFileID fileID = nil;
    OSStatus err = noErr;
    
    err = AudioFileOpenURL((__bridge CFURLRef)fileURL, kAudioFileReadPermission, 0, &fileID);
    if (err != noErr) [NSException raise:kRaiseMessage format:@"Failed to open audio file."];
    return fileID;
}

+ (NSDictionary *)metadataForFile:(NSString *)filePath {
    AudioFileID fileID = [self openFile:filePath];
    CFDictionaryRef piDict = nil;
    UInt32 piDataSize = 0;
    
    OSStatus err = AudioFileGetProperty(fileID, kAudioFilePropertyInfoDictionary, &piDataSize, &piDict);
    if (err != noErr) [NSException raise:kRaiseMessage format:@"Failed to read info dictionary."];
    
    return (__bridge NSDictionary *)piDict;
}

+ (NSData *)id3TagForFile:(NSString *)filePath {
    AudioFileID fileID = [self openFile:filePath];
    UInt32 id3DataSize = 0;
    OSStatus err;
    
    err = AudioFileGetPropertyInfo(fileID, kAudioFilePropertyID3Tag, &id3DataSize, NULL);
    if (err != noErr) [NSException raise:kRaiseMessage format:@"Failed to read tag size."];
    
    char *rawID3Tag = (char *)malloc(id3DataSize*sizeof(char));
    
    err = AudioFileGetProperty(fileID, kAudioFilePropertyID3Tag, &id3DataSize, rawID3Tag);
    if (err != noErr) [NSException raise:kRaiseMessage format:@"Failed to read raw ID3 tag."];
    
    UInt32 id3TagSize = 0;
    UInt32 id3TagSizeLength = 0;
    err = AudioFormatGetProperty(kAudioFormatProperty_ID3TagSize, id3DataSize, rawID3Tag, &id3TagSizeLength, &id3TagSize);
    
    if (err != noErr) {
        switch (err) {
            case kAudioFormatUnspecifiedError:
                [NSException raise:kRaiseMessage format:@"Unspecified error."];
                break;
            case kAudioFormatUnsupportedPropertyError:
                [NSException raise:kRaiseMessage format:@"Unsupported property."];
                break;
            case kAudioFormatBadPropertySizeError:
                [NSException raise:kRaiseMessage format:@"Bad property size."];
                break;
            case kAudioFormatBadSpecifierSizeError:
                [NSException raise:kRaiseMessage format:@"Bad specifier size"];
                break;
            case kAudioFormatUnsupportedDataFormatError:
                [NSException raise:kRaiseMessage format:@"Unsupported data."];
                break;
            case kAudioFormatUnknownFormatError:
                [NSException raise:kRaiseMessage format:@"Unkown audio format."];
                break;
            default:
                [NSException raise:kRaiseMessage format:@"Unknown error."];
                break;
        }
    }
    return [NSData dataWithBytesNoCopy:rawID3Tag length:id3DataSize freeWhenDone:YES];
}

@end