How to limit the canvas (or canvas) where I am drawing

I am creating an application where in one of the activity I need the user to sign. With the code I have, I can sign on the whole screen, but I want that space to be limited. That is to say that there is a box on the screen and that the signature can not exceed that space. How could I do it?

This is my code:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Vista vista = new Vista(this);
    setContentView(vista);
}

class Vista extends View{
    float x = 0;
    float y = 0;
    String accion = "accion";
    Path path = new Path();
    public Vista(Context context){
        super(context);
    }

    public void onDraw(Canvas canvas){

        //Definir caracteristicas del pincel
        Paint paint = new Paint();
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(3);//Grosor línea
        paint.setColor(Color.BLACK);//Color línea



        if (accion == "down")
            path.moveTo(x,y);
        if (accion == "move")
            path.lineTo(x,y);

        canvas.drawPath(path,paint);


    }
    public boolean onTouchEvent(MotionEvent e){
        x = e.getX();
        y = e.getY();

        if (e.getAction() == MotionEvent.ACTION_DOWN)
            accion = "down";
        if (e.getAction() == MotionEvent.ACTION_MOVE)
            accion = "move";

        invalidate();
        return true;


    }


}

}