Programming iOS 6, 3rd Edition

Playing Sound in the Background

In the multitasking world, when the user switches away from your app to another app, by default, your app is suspended and stops producing sound. But if the business of your app is to play sound, you might like your app to continue playing sound in the background. In earlier sections of this chapter, I’ve spoken about how your app, in the foreground, relates its sound production to background sound such as the Music app. Now we’re talking about how your app can be that background sound, possibly playing sound while some other app is in the foreground.

To play sound in the background, your app must do these things:

• In your Info.plist, you must include the “Required background modes” key (UIBackgroundModes) with a value that includes “App plays audio” (audio).
• Your audio session’s policy must be Playback (and must be active, of course).

If those things are true, then the sound that your app is playing when the user clicks the Home button and dismisses your application, or switches to another app, will go right on playing.

Note

When the screen is locked, your app can continue to play sound only if it is capable of playing sound in the background.

Moreover, your app may be able to start playing in the background even if it was not playing previously — namely, if it is mixable (AVAudioSessionCategoryOptionMixWithOthers, see earlier in this chapter), or if it is capable of being the remote control target. Indeed, an extremely cool feature of playing sound ...