Using Coroutines with Third party library that's using callback handlers

Question

Here is a breakdown of how the current third party SDK implementation works.

class Handler(val context: Context) {


    val device = Controller.getInstance(context,Listener())

    fun connectBT(BTDevice:BluetoothDevice){
        device.connectBT(BTDevice)
    }


}

and then the Listener implementation

class Listener: BBDeviceController.BBDeviceControllerListener{
    override fun onBTConnected(device: BluetoothDevice?) {
        println("Device Connected")
        // Send back to function that device is connect
    }
}

This is a straightforward example, but the idea is, when you press a button it will call connectBT() and then contain the result like so:

val handler = Handler(this)
val res = handler.connectBT(btDevice)

I know you can use suspendCoroutine on the function handler.connectBT() however the issue is how do I get the listeners result from the SDK to return back to the main function that called it?

Phil Dukhov · Accepted Answer · 2022-03-29 04:33:58Z

2

When using suspendCoroutine, you need to call resume/resumeWithException/etc on the continuation object. You can store/pass this object anywhere, for example to your listener:

class Handler(val context: Context) {
    val listener = Listener()
    val device = Controller.getInstance(context, listener)

    suspend fun connectBT(BTDevice:BluetoothDevice){
        suspendCoroutine<Unit> { continuation ->
            listener.continuation = continuation
            device.connectBT(BTDevice)
        }
    }
}

class Listener: BBDeviceController.BBDeviceControllerListener{
    var continuation: Continuation<Unit>? = null
    
    override fun onBTConnected(device: BluetoothDevice?) {
        println("Device Connected")
        if (continuation != null) {
            continuation?.resume(Unit)
            continuation = null
        }
    }
}

answered Mar 29, 2022 at 4:33

Phil Dukhov

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2 Comments

Derek Over a year ago

Thank you for this, I think I was over-engineering it. A follow up question. Can this still work if different functions return different values. For example. After the device is connected. There is a function to getDeviceInfo() And it has a callback that returns a string of data. How would using the coroutines allow for this flow to work?

Phil Dukhov Over a year ago

@Derek you have to manually manage it, e.g. store each value in your listener and resume only when all the data is gathered. Continuation is a pretty simple thing - it can only be resumed only once, because it will resume suspended function execution.

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