Android 安卓与绘制星图

我在画布中单击的位置绘制图片。这是代码：

public class CanvasdrawActivity extends Activity implements OnTouchListener {
  ImageView imageView;
  Bitmap bitmap;
  Bitmap bitmap2;
  Canvas canvas;
  Paint paint;

  float downx = 0,downy = 0,upx = 0,upy = 0;

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    imageView = (ImageView) this.findViewById(R.id.imageView1);
    Display currentDisplay = getWindowManager().getDefaultDisplay();
    float dw = currentDisplay.getWidth();
    float dh = currentDisplay.getHeight();

    bitmap = Bitmap.createBitmap((int) dw, (int) dh,
        Bitmap.Config.ARGB_8888);
    bitmap2=BitmapFactory.decodeResource(getResources(),
            R.drawable.star_bez_nog);

    canvas = new Canvas(bitmap);

    imageView.setImageBitmap(bitmap);
    imageView.setOnTouchListener(this);
  }

  public boolean onTouch(View v, MotionEvent event) {
    int action = event.getAction();
    switch (action) {
    case MotionEvent.ACTION_DOWN:
      downx = event.getX();
      downy = event.getY();
      canvas.drawBitmap(bitmap2, downx, downy, null);
      imageView.invalidate();
      break;

    }
    return true;
  }
}

我怎么能只画一次这幅画。当我第一次点击时，星星被画出来了，当我在其他地方点击时，什么也没发生。只能在第一次单击时绘制。

设置布尔值

Canvas canvas;
Paint paint;
Boolean drawOnlyOnce = true;

public boolean onTouch(View v, MotionEvent event) {
...
if (drawOnlyOnce) {
  canvas.drawBitmap(bitmap2, downx, downy, null);
  imageView.invalidate();
  drawOnlyOnce = false;
}
...