Quero exibir o conteúdo da tela no imageview no android

Eu não estou entendendo o imageview.draw (canvas);

Heres meu código:

public class Matrix extends Activity {
    public Bitmap mybitmap,newbmp,bitmap,bmp;
    ImageView imageview;

    Paint paint;
    @Override
    public void onCreate(final Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        imageview=(ImageView)findViewById(R.id.ImageView01);
        imageview.setDrawingCacheEnabled(true);
    }

    protected void onDraw(Canvas canvas)
    {
        imageview.draw(canvas);
        mybitmap=BitmapFactory.decodeResource(getResources(), R.drawable.image);
        canvas.drawBitmap(mybitmap, 0, 0, paint);
    }

}