如何使用在与 onCreate() 不同的线程中启动的 lateinit 变量？

Question

Working on a small app that listens for data from an Arduino device that it is connected to via bluetooth. The Arduino has an LED on board that is controlled by a button. When the LED is turned on or off the Arudino sends a 1(on) or 0(off) to indicate a state change. My app when it sees that state change should display or hide an image.

It does not.

I believe the problem is that my bluetoothSocket is called from a private inner class that is running on a different thread than the main thread. But I am stumped as to how to correct this. Or I am completely wrong in my assumption.

The app does connect via bluetooth and I can see the LED state change messages in LogCat.

This function is listening for those LED state changes:

private fun readBlueToothDataFromMothership(bluetoothSocket: android.bluetooth.BluetoothSocket) {
    val bluetoothSocketInputStream = bluetoothSocket.inputStream
    val buffer = ByteArray(256)
    var bytes: Int

    while (true) {
        try {
            bytes = bluetoothSocketInputStream.read(buffer)
            val readMessage = String(buffer, 0, bytes)

            Log.i(LOGTAG, readMessage)

            if (readMessage == "1") {
                imageView_mothership_LED_state.showOrHideImage(true)
            } else {
                imageView_mothership_LED_state.showOrHideImage(false)
            }
        } catch (e: IOException) {
            e.printStackTrace()
            break
        }
    }
}

This is the showOrHideImage() function that I tried to use moving the if else statement down here.

private fun View.showOrHideImage(imageShow: Boolean) {
    visibility = if (imageShow) {
        View.VISIBLE
    } else {
        View.INVISIBLE
    }
}

Private inner class that sets up my bluetoothSocket connect when requested.

private inner class ConnectThread(device: BluetoothDevice): Thread() {
    private var newSocket = device.createRfcommSocketToServiceRecord(uuid)

    override fun run() {
        try {
            Log.d(LOGTAG, "Connecting bluetoothSocket")
            handler.post {
                connectedOrNotTextView.text = getString(R.string.connecting)
                connectToDeviceButton.isEnabled = false
            }
            bluetoothSocket = newSocket
            bluetoothSocket.connect()
            Log.d(LOGTAG, "Socket connected")
            handler.post {
                connectedOrNotTextView.text = getString(R.string.connected)
                connectToDeviceButton.isEnabled = false; disconnectButton.isEnabled = true
            }
        }catch (e1: Exception){
            Log.e(LOGTAG, "Error connecting bluetoothSocket, $e1")
            handler.post {
                connectedOrNotTextView.text = getString(R.string.connection_failed)
                connectToDeviceButton.isEnabled = true; disconnectButton.isEnabled = false
            }
        }
        readBlueToothDataFromMothership(bluetoothSocket)
    }
}

I'm calling readBlueToothDataFromMothership() from here. But I know this is wrong.

I've tried to call if from onCreate but that fails since at that point I have not initialized bluetoothSocket.

Which is where I get hung up. I am unclear when or how/where to do the initilization.

This is the onCreate(), but without the call to readBlueToothDataFromMothership()

private lateinit var selectedBluetoothDevice: BluetoothDevice
private lateinit var bluetoothAdapter: BluetoothAdapter
private lateinit var bluetoothSocket: bluetoothSocket

class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    bluetoothAdapter = BluetoothAdapter.getDefaultAdapter()
    if (!bluetoothAdapter.isEnabled) {
        bluetoothAdapter.enable()
    }

    val pairedDevices: Set<BluetoothDevice> = bluetoothAdapter.bondedDevices
    pairedDevices.forEach {
        // MAC of test HC-05 Bluetooth shield. MAC of HC-05 on Motherboard: 00:14:03:06:1B:AE
        if (it.address == "00:14:03:06:21:64") {
            selectedBluetoothDevice = it
            bluetoothNameTextView.text = selectedBluetoothDevice.name
            bluetoothAddressTextView.text = selectedBluetoothDevice.address
        }
        else if (it.address == "00:00:00:00:00:00") { // CHANGE BLUETOOTH ADDRESS HERE
            selectedBluetoothDevice = it
            bluetoothNameTextView.text = selectedBluetoothDevice.name
            bluetoothAddressTextView.text = selectedBluetoothDevice.address
        }
    }
    AcceptThread().start()
    handler = Handler()

    connectToDeviceButton.setOnClickListener{
        ConnectThread(selectedBluetoothDevice).start()
    }
    disconnectButton.setOnClickListener{
        Log.d(LOGTAG, "Closing bluetoothSocket and connection")
        bluetoothSocket.close()
        connectedOrNotTextView.text = getString(R.string.notConnected)
        connectToDeviceButton.isEnabled = true; disconnectButton.isEnabled = false
    }
}

If I run the app with the call to readBlueToothDataFromMothership(bluetoothSocket) in the inner Class as it is above, it will run until I have pressed the Arduino's button 4 times. Then crashes with the following:

2019-12-05 11:03:18.641 4025-5005/com.example.pigcatcher I/MainActivity: 0
2019-12-05 11:03:19.740 4025-5005/com.example.pigcatcher I/MainActivity: 1
2019-12-05 11:03:20.839 4025-5005/com.example.pigcatcher I/MainActivity: 0
2019-12-05 11:03:21.747 4025-5005/com.example.pigcatcher I/MainActivity: 1
2019-12-05 11:03:21.762 4025-5005/com.example.pigcatcher E/AndroidRuntime: FATAL EXCEPTION: Thread-6
Process: com.example.pigcatcher, PID: 4025
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a 
view hierarchy can touch its views.

If I move the call up to onCreate() it crashes immediately with the following:

Process: com.example.pigcatcher, PID: 29343
java.lang.RuntimeException: Unable to start activity 

ComponentInfo{com.example.pigcatcher/com.example.pigcatcher.MainActivity}: 
kotlin.UninitializedPropertyAccessException: lateinit property bluetoothSocket has not been 
initialized

Which I can understand why. But I am unclear on how to initilize this properly without messing up the inner class? I am obviously am confused here. :)

Answer 1

Actually the whole code is quite confusing and fundamentally incorrect.

1. If you want to change UI elements from UI thread, it's not necessary to run the code onCreate, you can just call runOnUiThread Android function and it will run the code on UI thread. But I am not recommending using threads for it. Why launching a separate thread for bluetooth connection? I do recommend Kotlin Coroutines better for easier concurrency control and easier thread switch.

2. Also it's very weird, that you change outside values from a Thread wrapper.

3. Error clearly states, that the bluetoothSocket is not initialized (and probably this is the part where you press disconnect). So probably your Thread is not called at all, because it should not affect that much that the object is created from another thread if there are no race conditioning.