How to draw a line in android

If you're working with ConstraintLayout you need to define at least 2 constraints for the line to show up. Like this:

<View
        android:layout_width="0dp"
        android:layout_height="1dp"
        android:background="@android:color/black"
        app:layout_constraintEnd_toEndOf="@+id/someView1"
        app:layout_constraintStart_toStartOf="@+id/someView2"
        app:layout_constraintTop_toBottomOf="@+id/someView3" />

Though I defined 3 constraints.

If anyone can benefit from a simplified solution:

Point class not needed. Directly in MotionEvent.ACTION_MOVE move to the start X and Y coordinates that you get from MotionEvent.ACTION_DOWN. This method works better since we use drawPath instead of drawLine, which is less computationally intensive and therefore performs better.

class DrawingView(context: Context,
                  attributeSet: AttributeSet): View(context, attributeSet) {

    private val mPaint = Paint()
    private var mPath = Path()
    var startX = 0f
    var startY = 0f
    var endX = 0f
    var endY = 0f
    init {
        mPaint.color = Color.GREEN
        mPaint.style = Paint.Style.STROKE
        mPaint.strokeJoin = Paint.Join.ROUND
        mPaint.strokeCap = Paint.Cap.ROUND
        mPaint.strokeWidth = 50f

    }
    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        canvas.drawPath(mPath, mPaint)
    }

    override fun onTouchEvent(event: MotionEvent): Boolean {
       when (event.action) {
            MotionEvent.ACTION_DOWN -> {
                startX = event.x
                startY = event.y
                invalidate()
            }
            MotionEvent.ACTION_MOVE -> {
                endX = event.x
                endY = event.y
                val path = Path()
                path.moveTo(startX, startY) <- // Important!
                path.lineTo(endX, endY)
                mPath = path
                invalidate()
            }
        }
        return true
    }
}