Question

I’m working on a radio alarm clock, and i have some issues.
I am using local notifications for the alarms, so it has a gentle fallback if the app is not running.

I am well aware of the limitations of the device, and i know what i can and cannot do when the device has gone into background.

But my question is this:
I have seen other apps starting an audio streamer when i’ve locked the device. How is this possible? May this be inside an execution-timeframe?

How is the best way to implement this? Is it any way i can activate a streaming session when the device is locked?

Edit

To clarify: I know how i make audio play in the background. But the issue is triggering the audio-playback when an local notification or some other event fires.

One app that seems to do this, is Radio Alarm Clock. I haven’t tried it for long period of times yet. But it seems to do this. A video demo of the app: http://www.youtube.com/watch?v=KJQiFOcdBWk

Answer 1

Have you already declared your background task?

Support for some types of background execution must be declared in advance by the app that uses them. An app declares support for a service using its Info.plist file. Add the UIBackgroundModes key to your Info.plist file and set its value to an array containing one or more of the following strings:

audio — The app plays audible content to the user while in the background. (This content includes streaming audio or video content using AirPlay.)

iOS App Programming Guide – Implementing Long Running Background Tasks

You can add this by clicking on your main project icon, then under the Info tab you can add "Required Background Modes" to the "Custom iOS Target Properties" section. "App Plays Audio" will be one of the three default values.

Big Edit With New Answer:

If everything else is already in order, you can keep your app running in the background using the UIApplication method

- (UIBackgroundTaskIdentifier)beginBackgroundTaskWithExpirationHandler:(void (^)(void))handler

detailed here: UIApplication Class Reference

with an example here: Hour 21: Building Background-Aware Applications

This allows you to run an instance of NSTimer which triggers your music player. The difference between this approach and UILocalNotifications is that this method never lets the app fully enter the background mode, the music player exists the entire time which subverts the need to create it from the background, which looks to be impossible.

There may be limitations to how long of a timer you can set, I haven’t tested this past 14 minutes out.