iPhone 锁定时的音频画外音

Question

Solution below

I am working on a running app which gives voiceover instruction at the start of each exercise. It works as intended when the device is active but doesn't work when the phone is locked. [ Current audio continues but no future audio plays. ]

Question How can I start playing an audio voiceover while the users iPhone is locked.

Currently I track the workout using Timer.scheduledTimer for 2 timers I display [current activity and the overall workout]. When the timer hits a pre-defined time towards the end of one activity, the voiceover audio plays to introduce the next activity. The timer firing is what starts the audio and I believe this is the issue . I can pause the workout, skip sections and all works fine - until the device is locked.

If an audio voiceover is playing when I press the home button or lock the iPhone it continues to play as expected. The issue is then that the timer doesn't fire so the next audio voiceover [in say 90 seconds] is never played.

Some of the answers and comments I've read have said that this functionality just isn't possible in iOS. The Couch to 5k app https://itunes.apple.com/gb/app/one-you-couch-to-5k/id1082307672?mt=8 uses this functionality so I know it's possible to achieve, I just don't know how they're doing it. [It will be a dedicated member that downloads it to see what I mean :)]

From searching SO I've read a lot of post saying that Timer or NSTimer can't run when the app is in the background or the phone is locked. Any posts that say it can work are old and based on iOS4/5

I've read about suggestions of using silence and essentially having 1 long audio file. While this would pose some new challenges for skipping the sections in the workout I've also read comments that say this behaviour would not pass Apple's testing of the app for the AppStore. The 3rd downside being that my audio file size would increase.

An option I've seen is local notifications, however I'm yet to see an example of one used to play audio voiceover when the app is in the background.

How can I achieve the same functionality as the Couch to 5k app?

Thanks

Answer 1

Ok, I worked up a solution to play audio prompts to the user throughout the course of a 30 minute workout, even if the iPhone is locked or the app is in the background. The app has been submitted and approved for the AppStore .

To get the app to perform as required I use 2 AVAudioPlayers ; one plays the voiceover, the other plays a 15 minute track of silence [as it's a separate track and just silent it's a low quality .mp3 and only added 900kb to the app size]. 15 minutes well exceeds any interval between voiceovers so it works perfect.

When the user starts an exercise the first audio prompt is played. From that point, until the user stops the exercise there will be audio playing in the background. Because background audio is playing, the Timer still operates.

When a voiceover finishes, the AVAudioPlayerDelegate method audioPlayerDidFinishPlaying is called. Within this method I reset the audio properties [I'll explain why shortly], instantiate my silent AVAudioPlayer , and start playing the 15 minute track of silence. Playing another track using this delegate method means that there is no real break in background audio so the app is kept alive. With the app 'alive' the timer continues to countdown. When it's time for another voiceover the timer calls a method which resets the audio category, instantiates my voiceover AVAudioPlayer and plays a voiceover. This process is repeated until the final voiceover. When that completes no further silence is played and the app can safely be backgrounded.

The audio voiceover needs to duck the users music [reduces the music volume so that the audio can be clearly heard]. The silent audio shouldn't do that, music should continue to play at full volume. As both AVAudioPlayers use the AVAudioSession.sharedInstance I found that I had to deactivate the sharedInstance, reset the properties, with or without .duckOthers , and then reactivate the sharedInstance before instantiating my AVAudioSession. That then gave me the desired result. Constant background audio that only ducked the users music when a voiceover played.

do{
    try AVAudioSession.sharedInstance().setActive(false)
    try AVAudioSession.sharedInstance().setCategory(.playback, mode: .default, options: [.interruptSpokenAudioAndMixWithOthers, .duckOthers])
    try AVAudioSession.sharedInstance().setActive(true)
} catch{
        print(error.localizedDescription)
}

I know that some said Apple do not allow silent audio, however, in this circumstance the silent audio only plays for the duration of the exercise routine. As soon as it ends the app can terminate, thereby not keeping the app alive any longer than necessary.

Answer 2

My understanding of VoiceOver leads me to the way that only a foreground application can control what it reads out.

You need focusing with VoiceOver to make it speak and you focus nothing when the device is locked or your app is in the background mode.

I suggest to take a look at the speech synthesis or the AVAudioSession class to enable background audio : both might be good workarounds to take over in the background mode: just try and let us know. ;o)