渲染SurfaceView - 触摸时画布清除

Question

我是Android游戏开发的新手。这就是我想要做的事情

• 使用项目渲染背景（绿色区域）（项目位于背景后面，但我现在已将它们显示在顶部）
• 触摸时绘制附加图像（挖洞）
• 进行碰撞测试以查看用户是否找到了该项目

我坚持第二步，因为我根本无法让它发挥作用。一旦调用触摸事件方法，就会出现空白屏幕。在空白屏幕上仅绘制挖孔图像。我已通过文件阅读但无法弄清楚这里出了什么问题。

这是我的代码

class DrawSurface extends android.view.SurfaceView implements SurfaceHolder.Callback, View.OnTouchListener {
Bitmap mBMPField;
Bitmap mBMPHole;
SurfaceHolder surfaceHolder;
float x = 0;
float y = 0;
boolean touched = false;

public DrawSurface(Context context) {
    super(context);
    getHolder().addCallback(this);
    setOnTouchListener(this);
}

public DrawSurface(Context context, AttributeSet attrSet) {
    super(context, attrSet);
    getHolder().addCallback(this);
    setOnTouchListener(this);
}

public DrawSurface(Context context, AttributeSet attrSet, int styleAttr) {
    super(context, attrSet, styleAttr);
    getHolder().addCallback(this);
    setOnTouchListener(this);
}

@Override
public void surfaceCreated(SurfaceHolder holder) {
    Log.i("a", "Inside surfaceCreated");
    setWillNotDraw(false);
    mBMPField = BitmapFactory.decodeResource(getResources(),R.drawable.field);
    mBMPHole = BitmapFactory.decodeResource(getResources(),R.drawable.hole);
    surfaceHolder = holder;
    Canvas canvas = null;
    try {
        canvas = surfaceHolder.lockCanvas();
        Log.d(this.getClass().getName(), String.valueOf(canvas.hashCode()));
        canvas.drawBitmap(mBMPField, 0, 0, null);
        Paint paintText = new Paint();
        paintText.setColor(Color.WHITE);
        paintText.setStyle(Paint.Style.FILL);
        paintText.setAntiAlias(true);
        paintText.setTextSize(20);
        ArrayList<Item> mItems = loadItems();
        for (Item item : mItems) {
            item.setX((int) (Math.random() * this.getWidth()));
            item.setY((int) (Math.random() * this.getHeight()));
            canvas.drawText(item.getText(), item.getX(), item.getY(), paintText);
        }
    }
    finally
    {
        if(canvas!=null) {
            surfaceHolder.unlockCanvasAndPost(canvas);
        }
    }
}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {

}

@Override
public boolean onTouch(View v, MotionEvent event) {
    Log.i("a", "Inside onTouch");
    if(event.getAction() == MotionEvent.ACTION_DOWN) {
        if (surfaceHolder.getSurface().isValid()) {
            touched = true;
            x = event.getX();
            y = event.getY();
            Canvas canvas = null;
            try {
                canvas = surfaceHolder.lockCanvas();
                Log.d(this.getClass().getName(), String.valueOf(canvas.hashCode()));
                canvas.drawBitmap(mBMPHole, x - mBMPHole.getWidth() / 2, y - mBMPHole.getHeight() / 2, null);
            }
            finally {
                if(canvas!=null) {
                    surfaceHolder.unlockCanvasAndPost(canvas);
                }
            }
        }
    }
    return false;
}

private class Item {
    private int x;
    private int y;
    private String text;
    private boolean found;

    public boolean isFound() {
        return found;
    }

    public void setFound(boolean found) {
        this.found = found;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

}

private ArrayList<Item> loadItems()
{
    InputStream input = getResources().openRawResource(R.raw.items);
    BufferedReader reader = null;
    ArrayList<Item> items = new ArrayList<Item>();
    String line;

    try
    {
        reader = new BufferedReader(new InputStreamReader(input));
        while((line = reader.readLine()) != null)
        {
            Item item = new Item();
            item.setText(line);
            items.add(item);
        }
    }
    catch (Exception e)
    {
        Log.e("MainActivity", "Reading list of Items failed!", e);
    }
    finally {
        try
        {
            if (reader != null) reader.close();
        }
        catch (Exception e)
        {
            Log.e("MainActivity", "Error closing file reader.", e);
        }
    }
    return items;
}

}

任何建议都会对此有所帮助