objective c - Peripheral and central at the same time on iOS

Question

I've looked everywhere and tried everything, but nothing seems to work :(

On iOS, I'm making an app (for iOS 6 and above) in which iOS devices need to exchange data. Therefore, both devices need to be peripheral and central at the same time. I've done exactly as specified in the WWDC video, but the devices can't connect successfully with each other.

When I make one device only central and the other only peripheral, the central connects seamlessly to the peripheral.

However, when both devices are peripheral and central at the same time, I get random errors: at any stage (discovering services/characteristics or setting notify value to YES) errors sometimes happen, and sometimes discoverServices doesn't even call didDiscoverServices

Is there something different I should be doing? I simply merged the peripheral and central code into one view controller. I've noticed that if device "a" connects to device "b", and then device "b" connects to device "a", it works more often than not. I manage this by using NSThread sleepForTimeInterval: manually for different amounts of time on each device, but how could I get one device to connect first (and then the other) in a reliable (and not manually pre-defined) way?

If I do get errors, usually they're simply Unknown error

Please let me know if you need any code or any other information :)

Answer 1

Yes, it can be in both roles at the same time. You just have to initialize a CBPeripheralManager and a CBCentralManager. As soon as the peripheral manager is initialized and you receive the POWER ON state the device starts acting as a peripheral. You can add your services at this point and receive connections from other devices. At the same time you can use the central manager to scan and initiate connections to other peripherals.

Note that you cannot connect to your own device even if it acts as a peripheral.

For your errors, I suggest:

1. Turn off scanning before initiating a connection. That is, scan, find peripheral, stop scan, connect. Connection and scanning do not like each other.
2. Use a dedicated queue for handling bluetooth events, not the main queue. [[CBCentralManager alloc] initWithDelegate:self queue:my_dedicated_bluetooth_q]
3. Unfortunately, the stack sometimes become unstable. Even restarts are possible. But this usually happens only under heavy loads or several simultaneous connections. Hopefully, this will be improved in iOS7.
4. The unfamous Unknown error started to appear for several developers recently. Judging from your description there are probably a number of reasons why your setup may fail and it would require much more info that what fits well into a SO question.

For more info I suggest you search the bluetooth-dev mailing list archives https://lists.apple.com/archives/Bluetooth-dev or send a mail Bluetooth-dev@lists.apple.com. The community provides great help if you approach with reasonable questions like this.