Android 如何从mainactivity调用ondraw方法

我无法成功调用keypaint中的ondraw函数。我对编程技能很陌生。希望有人能帮助我。多谢各位

public class MyActivity extends Activity {

Paint p;
com.example.lean.myapplication.keypaint keypain;

@Override

protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.list_fragment);
    Canvas canvas = null;
    keypain = new com.example.lean.myapplication.keypaint(this);
    keypain.ondraw(canvas);

}

---

}

系统调用onDraw。。。。并且view.invalidate（）应该强制系统调用它。。。但是你的代码没有意义。。。你会得到NPE。。。而且onDraw是受保护的，不公开，我甚至不明白。你介意提供代码吗

public class keypaint extends View {
Paint p;

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    p=new Paint();
    Bitmap b= BitmapFactory.decodeResource(getResources(), R.drawable.icon);
    p.setColor(Color.RED);
    canvas.drawBitmap(b, 0, 0, p);
}




public keypaint(Context context) {
    super(context);
}