Resuming execution of code after interruption while app is in background

Question

I have spent days researching on SO and other websites for the answer to this but without any luck.

Essentially, the challenge I've set for myself is to create an alarm clock app for iOS that will sound no matter where the user may be (foreground or background). This I have already accomplished by using an AVAudioPlayer instance and starting to play an empty sound file when the user sets the alarm in order for the app to keep running in the background. When it is time for the alarm to go off (ie when the NSTimer is fired) a second player, which has already been initiated and prepared to play, starts playing the ringtone to which the user wakes up.

Also, I have managed to handle interruptions by a phone call, system timer or alarm clock by implementing the AVAudioSessionDelegate methods beginInterruption and endInterruptionWithFlags. It works both in background and foreground modes, but the strangest thing happens:

When the interruption ends, the AVAudioPlayer resumes playing BUT I cannot execute any other code in my app unless I bring the app to the foreground again.

To get to the bottom of this, I have experimented with a much simpler project which I am posting below. What this app does is, as soon as you enter the app, an instance of the AVAudioPlayer class starts looping a certain sound. Then when you bring it to the background, the player continues to loop the sound. When an interruption occurs I pause the player and when it ends I use a dispatch to wait a couple of seconds before it calls two methods, ie (void)playPlayer, a method that contains the code to resume playing the file and (void)tester, a method that contains a timer, which is set to stop the player 5 seconds after the interruption (or 7 seconds to be exact) has ended. Both the methods get called as indicated by the NSLogs I have put in both of them, but the timer never gets fired and the player continues to play indefinitely.

Here is the code for the .h file:

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <AudioToolbox/AudioToolbox.h>

@interface InterruptionTest3ViewController : UIViewController <AVAudioSessionDelegate,      AVAudioPlayerDelegate>
{
    AVAudioSession *mySession;
    AVAudioPlayer *myPlayer;
}

-(void) playPlayer;
-(void) pausePlayer;
-(void) tester;

@end

Here is the code for the .m file:

#import "InterruptionTest3ViewController.h"

@interface InterruptionTest3ViewController ()

@end

@implementation InterruptionTest3ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    mySession = [AVAudioSession sharedInstance];
    NSError *setActiveError = nil;
    [mySession setActive:YES withFlags:AVAudioSessionSetActiveFlags_NotifyOthersOnDeactivation error:&setActiveError];
    if (setActiveError) {
        NSLog(@"Session failed to activate within viewDidLoad");
    }
    else {
        NSLog(@"Session was activated within viewDidLoad");
    }
    NSError *setCategoryError = nil;
    [mySession setCategory:AVAudioSessionCategoryPlayback error:&setCategoryError];
    if (setCategoryError) {
        NSLog(@"Category failed to be set");
    }
    else {
        NSLog(@"Category has been set");
    }

    [mySession setDelegate:self];


    NSString *path = [[NSBundle mainBundle] pathForResource:@"headspin" ofType:@"wav"];
    NSError *initMyPlayerError = nil;
    myPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL fileURLWithPath:path]     error:&initMyPlayerError];
    if (initMyPlayerError) {
        NSLog(@"myPlayer failed to initiate");
    }
    else {
        NSLog(@"myPlayer has been initiated");
    }

    [myPlayer prepareToPlay];
    [self playPlayer];

    OSStatus propertySetError = 0;
    UInt32 allowMixing = true;

    propertySetError = AudioSessionSetProperty (
                                                kAudioSessionProperty_OverrideCategoryMixWithOthers, 
                                                sizeof (allowMixing),                                 
                                                &allowMixing                                          
                                                );

    [myPlayer setNumberOfLoops:-1];
    [[UIApplication sharedApplication] beginReceivingRemoteControlEvents];
}

-(void) beginInterruption
{
    [myPlayer pause];
}

-(void) endInterruptionWithFlags:(NSUInteger)flags
{
    if (flags) {
        if (AVAudioSessionInterruptionFlags_ShouldResume) 
        {
            {
                dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC),dispatch_get_main_queue(), ^{

                    [self playPlayer];
                    [self tester];
                });
}
        }
    }
}

-(void) tester
{
    [NSTimer timerWithTimeInterval:5.0 target:self selector:@selector(pausePlayer) userInfo:nil repeats:NO];
    NSLog(@"tester method has been called");
}

-(void) playPlayer
{
    [NSTimer timerWithTimeInterval:5.0 target:myPlayer selector:@selector(stop) userInfo:nil repeats:NO];
    [myPlayer play];
    NSLog(@"playPlayer method has been called");
}

-(void) pausePlayer
{
    [myPlayer pause];
}



//viewDidUnload etc not listed.

So, this is it folks. Again, why is the timer not being fired after an interruption while the app is in the background? Do I need to set something in the applicationDidEnterBackground method?

Thank you very much in advance!

Answer 1

please use "[NSTimer scheduledTimerWithTimeInterval:5.0 target:myPlayer selector:@selector(stop) userInfo:nil repeats:NO]";

Answer 2

Not to avoid the question, but the play-an-empty-sound-in-the-background bit is a hack. The ability to play a sound in the background is provided so that you can play music or other sound for the user's benefit, not to keep a background process alive.

Consider using local notifications instead if you want to play a sound or otherwise alert the user at a particular time.

why is the timer not being fired after an interruption while the app is in the background?

If I remember correctly, timers are either suspended or cancelled when your app enters the background. The documentation explicitly says that you should "stop timers and other periodic tasks" when your app is interrupted, i.e. when it's sent to the background.