iphone - how to find Bluetooth audio devices in iOS

Question

Okay, I'm working on a fun project that has a hurdle where I need to enable Bluetooth audio support for my iOS app.

The hurdle I'm at is that I simply can't even begin to get a list of connected Bluetooth audio devices. Even though my iPhone 5S recognizes my earpiece (a ~3 - 4 year old LG HBM-230, to be precise) and plays audio through it for phone calls, BOTH External Accessory and CoreBluetooth are giving me nothing useful when I query both.

I'm basing my own code off questions & answers I found for both the CoreBluetooth and External Accessory frameworks.

When my code simply tries to "scanForPeripheralsWithServices:nil" for any Bluetooth devices which Settings->Bluetooth say are visible and connected, the below code simply is NOT coming up with a single hit beyond the "CBCentralManagerStatePoweredOn" message in the console.

And this line in my code (with a valid EAAccessoryManager instance)

NSArray * connectedDevices = [self.eAAccessoryManager connectedAccessories];

also comes back with a nil array.

What could I be doing wrong?

B.T.W., I've made this code available as a GitHub project.

@implementation BluetoothManager

+ (BluetoothManager *)sharedInstance
{
    static dispatch_once_t pred = 0;
    __strong static id _bluetoothMGR = nil;

    dispatch_once(&pred, ^{
        _bluetoothMGR = [[BluetoothManager alloc] init];
    });

    return _bluetoothMGR;
}

- (id)init
{
    self = [super init];
    if(self)
    {
        dispatch_queue_t centralQueue = dispatch_queue_create("com.yo.mycentral", DISPATCH_QUEUE_SERIAL);

        // whether we try this on a queue of "nil" (the main queue) or this separate thread, still not getting results
        self.cbManager = [[CBCentralManager alloc] initWithDelegate:self queue:centralQueue options:nil];
    }
    return self;
}

// this would hit.... if I instantiated this in a storyboard of XIB file
- (void)awakeFromNib
{
    if(!self.cbManager)
        self.cbManager = [[CBCentralManager alloc] initWithDelegate:self queue:nil options:nil];
}

- (void)centralManager:(CBCentralManager *)central didDiscoverPeripheral:(CBPeripheral *)peripheral advertisementData:(NSDictionary *)advertisementData RSSI:(NSNumber *)RSSI {

    NSLog(@"hey I found %@",[advertisementData description]);
}

- (void)centralManager:(CBCentralManager *)central didRetrieveConnectedPeripherals:(NSArray *)peripherals
{
    NSLog( @"I retrieved CONNECTED peripherals");
}

-(void)centralManager:(CBCentralManager *)central didRetrievePeripherals:(NSArray *)peripherals{
    NSLog(@"This is it!");
}

- (void)centralManagerDidUpdateState:(CBCentralManager *)central{
    NSString *messtoshow;

    switch (central.state) {
        case CBCentralManagerStateUnknown:
        {
            messtoshow=@"State unknown, update imminent.";
            break;
        }
        case CBCentralManagerStateResetting:
        {
            messtoshow=@"The connection with the system service was momentarily lost, update imminent.";
            break;
        }
        case CBCentralManagerStateUnsupported:
        {
            messtoshow=@"The platform doesn't support Bluetooth Low Energy";
            break;
        }
        case CBCentralManagerStateUnauthorized:
        {
            messtoshow=@"The app is not authorized to use Bluetooth Low Energy";
            break;
        }
        case CBCentralManagerStatePoweredOff:
        {
            messtoshow=@"Bluetooth is currently powered off.";
            break;
        }
        case CBCentralManagerStatePoweredOn:
        {
            messtoshow=@"Bluetooth is currently powered on and available to use.";
            NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], CBCentralManagerScanOptionAllowDuplicatesKey, nil];

            [_cbManager scanForPeripheralsWithServices:nil options:options];

            break;
        }   

    }
    NSLog(@"%@", messtoshow);
}

@end

Answer 1

First you will need to configure your applications audio session to allow bluetooth connections that support audio. You can do this in, for example, your application delegates - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions method. Make sure you link the AVFoundation Framework and import in headers that will use it.

#import <AVFoundation/AVFoundation.h>// place in .h

[self prepareAudioSession];// called from application didFinishLaunchingWithOptions

- (BOOL)prepareAudioSession {

     // deactivate session
     BOOL success = [[AVAudioSession sharedInstance] setActive:NO error: nil];
     if (!success) {
         NSLog(@"deactivationError");
     }

     // set audio session category AVAudioSessionCategoryPlayAndRecord options AVAudioSessionCategoryOptionAllowBluetooth
     success = [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord withOptions:AVAudioSessionCategoryOptionAllowBluetooth error:nil];
     if (!success) {
         NSLog(@"setCategoryError");
     }

     // activate audio session
     success = [[AVAudioSession sharedInstance] setActive:YES error: nil];
     if (!success) {
         NSLog(@"activationError");
     }

return success;

}

Every application has an audio session singleton that you can configure. The sessions category and mode (in this example I did not set the mode so it reverts to the default mode) declare your applications intentions as to how you would like audio routing to be handled. It follows an important rule of last in wins. This means that if the user plugs in a headset or in this case a bluetooth device that is a hands free peripheral (HFP) the system will automatically route the audio to the headset or bluetooth device. The users physical actions are used to determine audio routing. However if you wish to give the user a list of available routes Apple recommend using MPVolumeView class.

An example for adding MPVolumeView could be put in a UIViewController subclasses viewDidLoad method.

#import <MediaPlayer/MediaPlayer.h> // place in .h
// prefered way using MPVolumeView for user selecting audio routes
self.view.backgroundColor = [UIColor clearColor];
CGRect frameForMPVV = CGRectMake(50.0, 50.0, 100.0, 100.0);
MPVolumeView *routeView = [[MPVolumeView alloc] initWithFrame:frameForMPVV];
[routeView setShowsVolumeSlider:NO];
[routeView setShowsRouteButton:YES];
[self.view addSubview: routeView];

As of iOS 7 you can get all inputs like this

// portDesc.portType could be for example - BluetoothHFP, MicrophoneBuiltIn, MicrophoneWired
NSArray *availInputs = [[AVAudioSession sharedInstance] availableInputs];
int count = [availInputs count];
for (int k = 0; k < count; k++) {
    AVAudioSessionPortDescription *portDesc = [availInputs objectAtIndex:k];
    NSLog(@"input%i port type %@", k+1, portDesc.portType);
    NSLog(@"input%i port name %@", k+1, portDesc.portName);
}

The portType you would be interested in is "BluetoothHFP". The portName property typically is the manufacturer/model which is what you would show to the user. (I've checked this with a non-LE bluetooth Motorola dinosaur and it works)

Because of the last in wins rule you will need to observe these two notifications (iOS 7 included). One to handle interruptions (such as phone calls or an alarm) and the second to be notified of route changes. Route change notifications is the one related to this question.

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(myInterruptionSelector:)
                                             name:AVAudioSessionInterruptionNotification
                                           object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(myRouteChangeSelector:)
                                             name:AVAudioSessionRouteChangeNotification
                                           object:nil];

For iOS 6.x you could read the currentRoute property of AVAudioSession inside the myRouteChange: selector to get the new route, as this will get called when a headset or bluetooth device is connected.

- (void)myRouteChangeSelector:(NSNotification*)notification {
 AVAudioSessionRouteDescription *currentRoute = [[AVAudioSession sharedInstance] currentRoute];
      NSArray *inputsForRoute = currentRoute.inputs;
      NSArray *outputsForRoute = currentRoute.outputs;
      AVAudioSessionPortDescription *outPortDesc = [outputsForRoute objectAtIndex:0];
      NSLog(@"current outport type %@", outPortDesc.portType);
      AVAudioSessionPortDescription *inPortDesc = [inputsForRoute objectAtIndex:0];
      NSLog(@"current inPort type %@", inPortDesc.portType);
}

Any iOS version < 6.0 you'll need the 'now deprecated' AudioSessionServices class. This class is a C api that instead of notifications it allows you to add property listeners.

I'll finish on this note - YOU DONT ALWAYS GET WHAT YOU WANT from the system. There are interruption handling notifications to observe and lots of error checking needed. I think this is a really good question and I hope this sheds some light on what it is you are trying to achieve.