Passing Event Information from Custom View to ViewModel

Question

Basically I am implementing a paint app, so I made a custom View to handle event and draw on canvas. Now I would like to display the coordinate of the points being drawn on a TextView in real time. So I am trying to use a ViewModel with LiveData to do the updating. But I can't figure out a way to give the information from my custom View to the ViewModel. How would you do that ? I also tried to use BindingAdapter but I can't find a way to do that.

Here the code for my custom View:

class PaintView(context: Context, attrs: AttributeSet): View(context,attrs) {

    var params: LayoutParams
    private val path : Path = Path()
    private val brush: Paint  = Paint()

    init {
        params = LayoutParams(LayoutParams.MATCH_PARENT, 
        LayoutParams.WRAP_CONTENT)

        brush.isAntiAlias = true
        brush.color = Color.BLACK
        brush.style = Paint.Style.STROKE
        brush.strokeJoin = Paint.Join.ROUND
        brush.strokeWidth = 8f

    }

    override fun onTouchEvent(event: MotionEvent): Boolean {
        val pointX = event.x
        val pointY = event.y

        when (event.action) {
            MotionEvent.ACTION_DOWN -> {
                path.moveTo(pointX, pointY)
                return true
            }
            MotionEvent.ACTION_MOVE -> {
                path.lineTo(pointX, pointY)
            }
            else -> return false
        }
        postInvalidate()
        return false
    }

    fun clearPath(){
        path.reset()
        invalidate()
    }

    public override fun onDraw(canvas: Canvas) {
        canvas.drawPath(path, brush)
    }
}

Here is a binding adapter that I found following this post: Android data binding view.onTouchListener . I can Log the points coordinate, but I cant't send them to my ViewModel, so it is quite empty right now.

@BindingAdapter("touchListener")
fun setTouchListener(self: View, bool: Boolean) {
    self.setOnTouchListener(
        object : View.OnTouchListener {
            override fun onTouch(view: View, event: MotionEvent): Boolean {
                val pointX = event.x
                val pointY = event.y
                when (event.action) {
                    MotionEvent.ACTION_DOWN -> {
                        Log.i("Points", " ($pointX, $pointY)")
                    }
                    MotionEvent.ACTION_MOVE -> {
                        Log.i("Points", " ($pointX, $pointY)")
                    }
                    MotionEvent.ACTION_UP -> {
                        Log.i("Points", " ($pointX, $pointY)")
                    }
                }
                return false
            }
        })
}

Answer 1

Sorry for sharing this in JAVA... But I guess you can capture the ideia.

On CustomView, create a custom interface:

public class PaintView {

    private OnCoordinateUpdate mCoordinatesListener;

    override fun onTouchEvent(event: MotionEvent): Boolean {
        val pointX = event.x
        val pointY = event.y
        ....
        if(listener != null) {
            listener.onUpdate(pointX, pointY);
        }
    }
    ...

    public void setCoordinatesListener(OnCoordinateUpdate listener) {
        mCoordinatesListener = listener;
    }

    public interface OnCoordinateUpdate {
        void onUpdate(int x, int y);
    }
}

Then, on your activity:

public class MainActivity {
    ...
    mPaintView.setCoordinatesListener(new PaintView.OnCoordinateUpdate {
        @Override
        void onUpdate(int x, int y) {
            if(mTextView != null) {
                mTextView.setText("X: " + x + " Y: " + y);
            }
        }
    });
}

This way, your PaintView can invoke the onUpdate whenever you want. Then, in your activity, everytime the onUpdate is called, the TextView will have its content updated.

I guess this is a good a approach because you create a custom interface ( OnCoordinateUpdate ) and it only makes sense to your custom view only.

Answer 2

Finally it worked the following way. I took inspiration from the previous post and just used the binded viewmodel to call a function and then get the data.

here is the MainActivity code:

class MainActivity : AppCompatActivity () {

    private lateinit var binding: ActivityMainBinding
    private lateinit var viewModel: PaintViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        viewModel = ViewModelProviders.of(this).get(PaintViewModel::class.java)

        binding.viewmodel = viewModel
        binding.paintView.setCoordinatesListener(object : 
            PaintView.OnCoordinateUpdate {
            override fun onUpdate(x: Float, y: Float) {
                binding.textView.text = "X:  $x \nY:  $y"
                binding.viewmodel?.onUpdateCoordinate(x,y)
            }
        })
        binding.clearButton.setOnClickListener { binding.paintView.clearPath() }
    }
}

Here is the ViewModel code, the challenge is to call the onUpdate function right in the ViewModel.

class PaintViewModel : ViewModel() {
    private val _pointX = MutableLiveData<Float>()
    private val _pointY = MutableLiveData<Float>()

    val pointX : LiveData<Float> = _pointX
    val pointY : LiveData<Float> = _pointY

    init {
        _pointX.value = 0f
        _pointY.value = 0f
    }

    fun onUpdateCoordinate(x: Float, y: Float) {
            _pointX.value = x
            _pointY.value = y
        }
}