Layout definito proprio, metodo onDraw () non richiamato

Ho definito una classe come quella ：

public class TestMyFrameLayout extends FrameLayout{

    Paint mPaint;
    public TestMyFrameLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public TestMyFrameLayout(Context context) {
        super(context);
        mPaint = new Paint();
        mPaint.setColor(Color.GREEN);
    }

    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawCircle(50f, 50f, 30, mPaint);
    }   
}

e l'ho chiamato come:

TestMyFrameLayout myFrameLayout = new TestMyFrameLayout(this);
LayoutParams myFrameLayoutParams = new LayoutParams(300,300);
myFrameLayout.setLayoutParams(myFrameLayoutParams);
setContentView(myFrameLayout);

Ma in realtà la funzione TestMyFrameLayout.onDraw (Canvas canvas) non viene chiamata, perché?

Risolto. Aggiungi this.setWillNotDraw(false); costruttore