view in xml layout

I've created my own view and I can't figure out how to add it from the main.xml layout file

setContentView(vi); its work but ı want setContentView(R.layout.activity_main);

public class Gui extends View {

public Gui(Context context) {
    super(context);

}
@Override
protected void onDraw(Canvas canvas) {


    Paint myPaint = new Paint();
    Paint myPaintFill= new Paint();


    myPaintFill.setColor(Color.GREEN);

    myPaint.setColor(Color.BLACK);
    myPaint.setStyle(Paint.Style.STROKE);
    myPaint.setStrokeWidth(1);


    RectF r = new RectF(0,0,50,50);
    canvas.drawRoundRect(r, 0, 0, myPaint);



    }

 }

}