Exploring MusicKit: Adding Crossfade

In the latest update to MusicKit announced at WWDC 2024, the team introduced a new feature: crossfade. While this appears to be the only major change for us MusicKit developers this year, it is a welcome addition for audio playback. Also, I assume that there are numerous performance improvements and optimizations happening under the hood. Let’s dive into the details of this new feature and how to implement it in our iOS app.

The New Crossfade Feature

The crossfade feature allows for a smooth transition between tracks, creating a better listening experience. This functionality is implemented through a new Transition enum and associated properties in the ApplicationMusicPlayer class.

The ApplicationMusicPlayer Class

The ApplicationMusicPlayer class has been updated to include a new transition property:

@available(iOS 15.0, tvOS 15.0, visionOS 1.0, macOS 14.0, *)
@available(watchOS, unavailable)
public class ApplicationMusicPlayer : MusicPlayer {
    public static let shared: ApplicationMusicPlayer
    public var queue: ApplicationMusicPlayer.Queue

    @available(iOS 18.0, *)
    @available(macOS, unavailable)
    @available(macCatalyst, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    @available(visionOS, unavailable)
    public var transition: MusicPlayer.Transition
}

The transition property allows you to set the transition style between playing items. By default, it is set to .none, meaning there is no transition between tracks.

The Transition Enum

The new Transition enum is where the magic happens:

@available(iOS 18.0, *)
@available(macOS, unavailable)
@available(macCatalyst, unavailable)
@available(tvOS, unavailable)
@available(watchOS, unavailable)
@available(visionOS, unavailable)
extension MusicPlayer {
    public enum Transition : Equatable, Hashable, Sendable {
        case none
        case crossfade(options: MusicPlayer.Transition.CrossfadeOptions)
        
        public static let crossfade: MusicPlayer.Transition
        public static func crossfade(duration: TimeInterval?) -> MusicPlayer.Transition
    }
}

This enum introduces the crossfade case, which can be used with or without specifying a duration.

Implementing Crossfade in Your App

Using the new crossfade feature in your app is straightforward. Here is a simple example of how to enable crossfade with a 2-second duration:

if #available(iOS 18.0, *) {
    ApplicationMusicPlayer.shared.transition = .crossfade(duration: 2)
}

This sets a 2-second crossfade transition between the music items. Remember to wrap this in an availability check, as the feature is only available from iOS 18.0 onwards!