objective c - Receive remote control events without audio

Question

Here is some background information, otherwise skip ahead to the question in bold. I am building an app and I would like it to have access to the remote control/lock screen events. The tricky part is that this app does not play audio itself, it controls the audio of another device nearby. The communication between devices is not a problem when the app is in the foreground. As I just found out, an app does not assume control of the remote controls until it has played audio with a playback audio session, and was the last do so. This presents a problem because like I said, the app controls ANOTHER device's audio and has no need to play its own.

My first inclination is to have the app play a silent clip every time it is opened in order to assume control of the remote controls. The fact that I have to do this makes me wonder if I am even going to be allowed to do it by Apple or if there is another way to achieve this without fooling the system with fake audio clips.

QUESTION(S): Will Apple approve an app that plays a silent audio clip in order to assume control of the remote/lock screen controls for the purpose of controlling another device's audio? Is there any way of assuming control of the remote controls without an audio session?

P.S. I would prefer to have this functionality on iOS 4.0 and up.

P.P.S I have seen this similar question and it has gotten me brainstorming but the answer provided is not specific to what I need to know.

Answer 1

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NOTE: As of iOS 7.1, you should be using MPRemoteCommandCenter instead of the answer below.

You create various system-provided subclasses of MPRemoteCommand and assign them to properties of the [MPRemoteCommandCenter sharedCommandCenter].

I'm keeping the rest of this around for historical reference, but the following is not guaranteed to work on recent iOS versions. In fact, it just might not.

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You definitely do need an audio player but not necessarily an explicit session to take control of the remote control events. (AVAudioSession is implicit to any app that plays audio.) I spent a decent amount of time playing with this to confirm this.

I've seen a lot of confusion on the internet about where to set up the removeControlEventRecievedWithEvent: method and various approaches to the responder chain. I know this method works on iOS 6 and iOS 7. Other methods have not. Don't waste your time handling remote control events in the app delegate (where they used to work) or in a view controller which may go away during the lifecycle of your app.

I made a demo project to show how to do this.

Here's a quick rundown of what has to happen:

1. You need to create a subclass of UIApplication. When the documentation says UIResponder, it means UIApplication, since your application class is a subclass of UIResponder. In this subclass, you're going to implement the remoteControlReceivedWithEvent: and canBecomeFirstResponder methods. You want to return YES from canBecomeFirstResponder. In the remote control method, you'll probably want to notify your audio player that something's changed.

2. You need to tell iOS to use your custom class to run the app, instead of the default UIApplication. To do so, open main.m and change this:

    return UIApplicationMain(argc, argv, nil, NSStringFromClass([RCAppDel`egate class]));
   

   to look like this:

   return UIApplicationMain(argc, argv, NSStringFromClass([RCApplication class]), NSStringFromClass([RCAppDelegate class]));
   

   In my case RCApplication is the name of my custom class. Use the name of your subclass instead. Don't forget to #import the appropriate header.

3. OPTIONAL: You should configure an audio session. It's not required, but if you don't, audio won't play if the phone is muted. I do this in the demo app's delegate, but do so where appropriate.

4. Play something. Until you do, the remote controls will ignore your app. I just took an AVPlayer and gave it the URL of a streaming site that I expect to be up. If you find that it fails, put your own URL in there and play with it to your heart's content.

This example has a little bit more code in there to log out remote events, but it's not all that complicated. I just define and pass around some string constants.

I bet that a silent looping MP3 file would help work towards your goal.