iOS Components and Frameworks: Understanding the Advanced Features of the iOS SDK (2014)

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After the background task work is done, the last counter is updated in NSUserDefaults so that it can start over correctly, and the UI is updated to allow the user to start the background task again.

NSLog(@"Background Completed tasks");

[userDefaults setInteger:0
forKey:kLastCounterKey];

[userDefaults synchronize];

dispatch_sync(dispatch_get_main_queue(), ^{
[self.backgroundButton setEnabled:YES];
[self.backgroundButton setTitle:@"Start Background Task"
forState:UIControlStateNormal];
});

Finally there are two key items that need to take place to finish the background task: tell the application to end the background task, and invalidate the background task identifier. Every line of code between obtaining the background task ID and ending it will execute in the background.

[[UIApplication sharedApplication] endBackgroundTask:self.backgroundTask];

self.backgroundTask = UIBackgroundTaskInvalid;

Implementing Background Activities

iOS supports a very specific set of background activities that can continue processing without the limitations of the background task identifier approach. These activities can continue running or being available without a time limit, and need to avoid using too many system resources to keep the app from getting terminated.

Types of Background Activities

These are the background activities:

Playing background audio

Tracking device location

Supporting a Voice over IP app

Downloading new Newsstand app content

Communication with an external or Bluetooth accessory

Fetching content in the background

Initiating a background download with a push notification

To support any of these background activities, the app needs to declare which background activities it supports in the Info.plist file. To do this, select the app target in Xcode, and select the Capabilities tab. Turn “Background Modes” to On, then check the desired modes to support. Xcode will add the entries to the Info.plist file for you. Or, edit the Info.plist file directly by selecting the app target in Xcode, then select the Info tab. Look for the Required Background Modes entry in the list; if it is not present, hover over an existing entry and click the plus sign to add a new entry, and then select Required Background Modes. An array entry will be added with one empty NSString item. Select the desired background mode, as shown in Figure 16.2.

Figure 16.2 Xcode’s Info editor showing required background modes.

After the Required Background Modes entry is established, activity-specific logic can be built into the app and it will function when the app is in the background.

Playing Music in the Background

To play music in the background, the first step is to adjust the audio session settings for the app. By default, an app uses the AVAudioSessionCategorySoloAmbient audio session category. This ensures that other audio is turned off when the app is started, and that the app audio is silenced when the screen is locked or the ring/silent switch on the device is set to silent. This session will not work, since audio will be silenced when the screen is locked or when another app is brought to the foreground. The viewDidLoad method in ICFViewController adjusts the audio session category to AVAudioSessionCategoryPlayback, which will ensure that the audio will continue playing when the app is in the background or the ring/silent switch is set to silent.

AVAudioSession *session = [AVAudioSession sharedInstance];

NSError *activeError = nil;
if (![session setActive:YES error:&activeError])
{
NSLog(@"Failed to set active audio session!");
}

NSError *categoryError = nil;
if (![session setCategory:AVAudioSessionCategoryPlayback
error:&categoryError])
{
NSLog(@"Failed to set audio category!");
}

The next step in playing audio is to initialize an audio player. This is also done in the viewDidLoad method so that the audio player is ready whenever the user indicates that audio should be played.

NSError *playerInitError = nil;

NSString *audioPath =
[[NSBundle mainBundle] pathForResource:@"16_audio"
ofType:@"mp3"];

NSURL *audioURL = [NSURL fileURLWithPath:audioPath];

self.audioPlayer = [[AVAudioPlayer alloc]
initWithContentsOfURL:audioURL
error:&playerInitError];

The Play Background Music button is wired to the playBackgroundMusicTouched: method. When the user taps that button, the logic checks to see whether audio is currently playing. If audio is currently playing, the method stops the audio and updates the title of the button.

if ([self.audioPlayer isPlaying])
{
[self.audioPlayer stop];

[self.audioButton setTitle:@"Play Background Music"
forState:UIControlStateNormal];

}
else
{ ...
}

If music is not currently playing, the method starts the audio and changes the title of the button.

[self.audioPlayer play];

[self.audioButton setTitle:@"Stop Background Music"
forState:UIControlStateNormal];

While the audio is playing, the user can press the lock button on the device or the home button to background the app, and the audio will continue playing. A really nice feature when audio is playing and the screen is locked is to display the currently playing information on the lock screen. To do this, first set up a dictionary with information about the playing media.

UIImage *lockImage = [UIImage imageNamed:@"book_cover"];

MPMediaItemArtwork *artwork =
[[MPMediaItemArtwork alloc] initWithImage:lockImage];

NSDictionary *mediaDict =
@{
MPMediaItemPropertyTitle: @"BackgroundTask Audio",
MPMediaItemPropertyMediaType: @(MPMediaTypeAnyAudio),
MPMediaItemPropertyPlaybackDuration:
@(self.audioPlayer.duration),
MPNowPlayingInfoPropertyPlaybackRate: @1.0,
MPNowPlayingInfoPropertyElapsedPlaybackTime:
@(self.audioPlayer.currentTime),
MPMediaItemPropertyAlbumArtist: @"Some User",
MPMediaItemPropertyArtist: @"Some User",
MPMediaItemPropertyArtwork: artwork };

There are a number of options that can be set. Note that a title and an image are specified; these will be displayed on the lock screen. The duration and current time are provided and can be displayed at the media player’s discretion, depending on the state of the device and the context in which it will be displayed. After the media information is established, the method starts the audio player, then informs the media player’s MPNowPlayingInfoCenter about the playing media item info. It sets self to be the first responder, since the media player’s info center requires the view or view controller playing audio to be the first responder in order to work correctly. It tells the app to start responding to “remote control” events, which will allow the lock screen controls to control the audio in the app with delegate methods implemented.

[[MPNowPlayingInfoCenter defaultCenter]
setNowPlayingInfo:mediaDict];

[self becomeFirstResponder];

[[UIApplication sharedApplication]
beginReceivingRemoteControlEvents];

Now when the audio is playing in the background, the lock screen will display information about it, as shown in Figure 16.3.

Figure 16.3 Lock screen showing sample app playing background audio.

In addition, the Control Center will display information about the audio, as shown in Figure 16.4.

Figure 16.4 Lock screen control center showing sample app playing background audio.

Summary

This chapter illuminated two approaches to executing tasks while the app is in the background, or while it is not the current app that the user is interacting with.

The first approach to background task processing described was finishing a long-running task in the background. The sample app demonstrated how to check whether the device is capable of running a background task, showed how to set up and execute a background task, and explained how to handle the case when iOS terminates the app and notifies the background task that it will be ended (or expired).

The second approach to background tasks described was supporting very specific types of background activities allowed by iOS, like playing music, interacting with Bluetooth devices, monitoring the GPS for large changes, or maintaining a persistent network connection to allow VoIP-type apps to work. The chapter explained how to configure an Xcode project to allow a background activity to take place, and then the sample app demonstrated how to play audio in the background while displaying information about the audio on the lock screen.

Exercises

1. Enhance the background task processing to download a series of images, or download and process a large JSON file in the background.

2. Add the capability to monitor the GPS for large location changes while the app is in the background, and log the changes as they happen. Refer to Chapter 2, “Core Location, MapKit, and Geofencing,” for more information on location information.

3. Update the audio handling to support the controls shown on the lock screen and Control Center. Hint: Implement the remoteControlReceivedWithEvent: method in the view controller that becomes the first responder.