Canvas 在 运行 模式下启动应用程序时对象为空，在调试模式下 运行 时对象具有值

Question

Im using canvas object to set drawbitmap to set wallpaper change it based > on user selected interval. Canvas object is null when app is launched in run mode and in debug mode its not null. below is my code very strange behaviour

    public MyWallpaperEngine() {
        mImagesArray = new int[] {R.drawable.one,R.drawable.two,R.drawable.three,
                R.drawable.four,R.drawable.five,R.drawable.six,
                R.drawable.seven,R.drawable.eight,R.drawable.nine,
                R.drawable.ten};

        myTimertask = new TimerTask() {
            @Override
            public void run() {
                System.out.println("TIMER SCHEDULED INSIDE RUN");
                drawFrame();
                incrementCounter();
            }
        };
        myTimer.schedule(myTimertask,startInterval,WALLPAPER_DURATION);
    }

    private void incrementCounter() {
        mImagesArrayIndex++;

        if (mImagesArrayIndex >= mImagesArray.length) {
            mImagesArrayIndex = 0;
        }
    }

    private void drawFrame() {
        System.out.println("inside draw frame");
         SurfaceHolder holder = getSurfaceHolder();
        System.out.println("holder Object "+holder);
        Canvas canvas = null;

        try {
            canvas = holder.lockCanvas();
            System.out.println("Canvas Object "+canvas);
            if (canvas != null) {
                System.out.println("inside draw image");
                drawImage(canvas);
            }
        } finally {
            if (canvas != null) {
                holder.unlockCanvasAndPost(canvas);
            }
        }
    }

    private void drawImage(Canvas canvas)
    {
        Bitmap image = BitmapFactory.decodeResource(getResources(),
                mImagesArray[mImagesArrayIndex]);
        Bitmap b=Bitmap.createScaledBitmap(image, canvas.getWidth(), canvas.getHeight(), true);
        canvas.drawBitmap(b, 0,0, null);
    }

Answer 1

你的 drawImage 方法是什么时候调用的？ （来自 activity/fragment 生命周期）。

您无法在 运行 时间内收到的原因，可能是因为您的视图尚未完全绘制在屏幕上，您 canvas 对象所在的位置。

您 canvas 在调试模式下不为空，因为调试模式速度较慢并且有足够的时间进行绘制。

确保您遵循 activity/ 片段生命周期。