How to get the duration of an audio file in iOS?

NSDictionary* fileAttributes = 
    [[NSFileManager defaultManager] attributesOfItemAtPath:filename 
                                                     error:nil]

From the file attribute keys, you can get the date, size, etc. But how do you get the duration?

In the 'File Attribute Keys' of the NSFileManager class reference you can see that there is no key to use that will return the duration of a song. All the information that the NSFileManager instance gets about a file is to do with the properties of the actual file itself within the operating system, such as its file-size. The NSFileManager doesn't actually interpret the file.

In order to get the duration of the file, you need to use a class that knows how to interpret the file. The AVFoundation framework provides the exact class you need, AVAsset. You can instantiate an instance of this abstract class using the concrete subclass AVURLAsset, and then provide it an NSURL which points to the audio file you wish to get the duration. You can then get the duration from the AVAsset instance by querying its duration property.

For example:

AVURLAsset* audioAsset = [AVURLAsset URLAssetWithURL:audioFileURL options:nil];
CMTime audioDuration = audioAsset.duration;
float audioDurationSeconds = CMTimeGetSeconds(audioDuration);

Note that AVFoundation is designed as a heavily asynchronous framework in order to improve performance and the overall user experience. Even performing simple tasks such as querying a media file's duration can potentially take a long period of time and can cause your application to hang. You should use the AVAsynchronousKeyValueLoading protocol to asynchronously load the duration of the song, and then update your UI in a completion handler block. You should take a look at the 'Block Programming Guide' as well as the WWDC2010 video titled, 'Discovering AV Foundation', which is available free at https://developer.apple.com/videos/wwdc/2010.

For anyone still looking for this. Based on the answer, the code for Swift 4 (including the async loading of values taken from Apple's documentation):

let audioAsset = AVURLAsset.init(url: yourURL, options: nil)

audioAsset.loadValuesAsynchronously(forKeys: ["duration"]) {
    var error: NSError? = nil
    let status = audioAsset.statusOfValue(forKey: "duration", error: &error)
    switch status {
    case .loaded: // Sucessfully loaded. Continue processing.
        let duration = audioAsset.duration
        let durationInSeconds = CMTimeGetSeconds(duration)
        print(Int(durationInSeconds))
        break              
    case .failed: break // Handle error
    case .cancelled: break // Terminate processing
    default: break // Handle all other cases
    }
}

For completeness - There is another way to get the duration for a mp3 file:

NSURL * pathToMp3File = ...
NSError *error = nil;
AVAudioPlayer* avAudioPlayer = [[AVAudioPlayer alloc]initWithContentsOfURL:pathToMp3File error:&error];

double duration = avAudioPlayer.duration; 
avAudioPlayer = nil;

I have used this with no discernible delay.

You can achieve the same in Swift using :

let audioAsset = AVURLAsset.init(url: audioFileURL, options: nil)
let duration = audioAsset.duration
let durationInSeconds = CMTimeGetSeconds(duration)