restrict a custom view in a relative layout

I am new to android and I am a bit stuck. I am trying to create a simple drawing app which shows an example on the top of the page and a square space for it in below it. Aim is to display a letter and a kid needs to practice in replicating it.

I have troubles including the drawing class in the layout which needs to restrict its boundaries.

this is the main layout

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:id="@+id/exampleText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="32dp"
android:text="TextView"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<androidx.constraintlayout.widget.ConstraintLayout
enter code here` android:id="@+id/drawingBoard"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/exampleText">
</androidx.constraintlayout.widget.ConstraintLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

this is the PaintView class

import android.app.ActionBar;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
public class PaintView extends View {
public ViewGroup.LayoutParams param;
private Path path = new Path();
private Paint brush = new Paint();
public PaintView(MainActivity context) {
super(context);
brush.setAntiAlias(true);
brush.setColor(Color.RED);
brush.setStyle(Paint.Style.STROKE);
brush.setStrokeJoin(Paint.Join.ROUND);
brush.setStrokeWidth(8f);
param = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
float pointX = event.getX();
float pointY = event.getY();
switch (event.getAction()){
case MotionEvent.ACTION_DOWN:
path.moveTo(pointX,pointY);
return true;
case MotionEvent.ACTION_MOVE:
path.lineTo(pointX,pointY);
break;
default:
return  false;
}
postInvalidate();
return  false;
}
@Override
protected void onDraw(Canvas canvas) {
canvas.drawPath(path,brush);
}

this is how it is called in the Main Activity

  protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
PaintView paintView = new PaintView(this);
setContentView(paintView);
}

I need to fit the drawing board into the "drawingBoard".

It is not necessary the right approach, but this is as far as I got.

Thanks in advance

Attach the customview to a fragment.
This is a completely different approach so you will have to make a lot of changes to your code.

The other approach would be adding the view programmatically to the ConstraintLayout.
In MainActivity,

protected void onCreate(Bundle  savedInstanceStatee) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//remove those lines.
/*
PaintView paintView = new PaintView(this);
setContentView(paintView);*/
ConstraintLayout parentLayout = (ConstraintLayout)findViewById(R.id.drawingBoard);
ConstraintSet set = new ConstraintSet();
PaintView paintView = new PaintView(this);
// set view id, else getId() returns -1
paintView.setId(View.generateViewId());
layout.addView(paintView, 0);
set.clone(parentLayout);
// connect start and end point of views, in this 
case top of child to top of parent.
set.connect(paintView.getId(),
ConstraintSet.TOP,parentLayout.getId(), 
ConstraintSet.TOP, 60);
// ... similarly add other constraints
set.applyTo(parentLayout);
}

In the activity you are overwriting the content view with just the paint view:
setContentView(paintView);

Remove that line.

Add the PaintView in XML instead:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:id="@+id/exampleText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="32dp"
android:text="TextView"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<!-- TODO: Replace with your package name -->
<com.example.PaintView
android:id="@+id/drawingBoard"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/exampleText" />
</androidx.constraintlayout.widget.ConstraintLayout>

With this approach you have to inflate the custom view in its constructors:

public class PaintView extends ConstraintLayout {
public PaintView(@NonNull Context context) {
super(context);
init();
}
public PaintView(@NonNull Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
init();
}
public PaintView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
public PaintView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
super(context, attrs, defStyleAttr, defStyleRes);
init();
}
public void init() {
inflate(getContext(), R.layout.layout_product_item, this);
}
}

To interact with the PaintView from the activity code you can do:

PaintView paintView = (PaintView) findViewById(R.id.drawingBoard);