Rx Kotlin: map function can't infer return type

Question

In an app that connects to bluetooth devices, I am using the following function using RxKotlin:

private fun startBluetoothPair(device: BluetoothDevice) {
    Observable.just(device)
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .map {
            var uuid: UUID = BLUETOOTH_UUID
            var socket = it.createRfcommSocketToServiceRecord(uuid)
            socket.connect()
            return socket
        }
        .subscribe {
            // Do something with the BluetoothSocket
        }
}

This function should simply connect with the bluetoothdevice on the background and then do something with the socket (in the mainthread again). However, the map can't handle the return socket part, telling me there is a Type mismatch, it found a BluetoothSocket where it required a Unit.

What is going wrong here? I thought a map should be able to infer the return type.

Answer 1

Just simply skip using the return keyword in this case! The last statement will be the return expression:

.map {
  val uuid: UUID = BLUETOOTH_UUID
  val socket = it.createRfcommSocketToServiceRecord(uuid)
  socket.connect()
  socket
}

It's recommended to avoid using labeled expressions like return@map, because of the maintainability and less complexity.

It's also a good practice using val instead of var for these variables since you don't reassign them.

Answer 2

In your map function replace the statement

return socket

with

return@map socket

Because return statement is normally used to return values for top-level functions. For lambda expressions and inline (nested) functions, use returns at labels i.e. return@{method_name}. You can also leave last line just to socket in the case, and the compiler will handle it for you assuming the last line as return value for the inline function. But for better readability, I will definitely prefer qualified return syntax. You can read more details in kotlin documentation here and here