黑莓 - 创建图像查看器

Question

我需要在我的应用程序中创建图像幻灯片（即图像应定期更改，并具有淡入/淡出效果）。我尝试了一些代码，但它抛出了非法异常。

是否有任何选项可以以编程方式更改图片滚动字段中的图像？ 即来自线程或其他什么？

public class SlideTransition extends MainScreen {

    final Bitmap image000 = Bitmap.getBitmapResource("img1.jpg");
    final Bitmap image001 = Bitmap.getBitmapResource("img2.jpg");
    final Bitmap image002 = Bitmap.getBitmapResource("img3.jpg");

    final BitmapField animationField = new BitmapField(image000);
    int counter = 0;
    Timer animationTimer = new Timer();
    TimerTask animationTask;

    public SlideTransition() {
        animationTask = new TimerTask() {

            public void run() {
                if (counter == 0) {

                    animationField.setBitmap(image000);
                }

                if (counter == 1) {
                    animationField.setBitmap(image001);
                }
                if (counter == 2) {
                    animationField.setBitmap(image002);
                    counter = -1;
                }
                counter++;
            }
        };
        animationTimer.scheduleAtFixedRate(animationTask, 0, 100);
        add(animationField);
    }
}

Answer 1

发布的代码将抛出IllegalStateException，因为它试图直接从后台（计时器）线程修改 UI 元素：

animationField.setBitmap(image000);

您需要使用 UiApplication.getUiApplication().invokeLater() 或类似的东西从后台线程修改 UI。

此外，您可以让计时器不断调整位图字段的 alpha 值（不透明度）以产生淡入淡出效果。

这是一个示例，从您提供的代码开始：

public class SlideTransition extends MainScreen {

   private final Bitmap image000 = Bitmap.getBitmapResource("img1.jpg");
   private final Bitmap image001 = Bitmap.getBitmapResource("img2.jpg");
   private final Bitmap image002 = Bitmap.getBitmapResource("img3.jpg");
   private AlphaBitmapField animationField = new AlphaBitmapField(image000);
   private Timer animationTimer = new Timer();
   private TimerTask animationTask;
   private final int animationPeriodMsec = 40; // 25 Hz animation timer

   public SlideTransition() {
      animationTask = new AnimationTask();

      add(animationField);
      animationTimer.scheduleAtFixedRate(animationTask, 0, animationPeriodMsec);  
   }

   // I separated your anonymous timer task into its own class for readability
   private class AnimationTask extends TimerTask {

      private final int fadeDurationMsec = 500;
      private final int displayDurationMsec = 1500;
      private final int fadeInEndCount = fadeDurationMsec / animationPeriodMsec;
      private final int fadeOutStartCount = (fadeDurationMsec + displayDurationMsec) / animationPeriodMsec;
      private final int endCount = (2 * fadeDurationMsec + displayDurationMsec) / animationPeriodMsec;

      private int imgCounter = 0;
      private int cycleCounter = 0;

      public void run() {
         if (cycleCounter >= endCount) {
            cycleCounter = 0;
         }
         Bitmap newImage = null;
         if (cycleCounter == 0) {
            // time to switch the images
            if (imgCounter == 0) {
               newImage = image000;
            } else if (imgCounter == 1) {
               newImage = image001;
            } else if (imgCounter == 2) {
               newImage = image002;
               imgCounter = -1;
            }
            imgCounter++;
         }

         // assign final variables to use inside a UI thread Runnable
         final Bitmap currentImage = newImage;
         final int i = cycleCounter;

         UiApplication.getUiApplication().invokeLater(new Runnable() {
            public void run() {
               if (i == 0) {
                  // switch images, and start with the image invisible (alpha = 0)
                  animationField.setAlpha(0);
                  animationField.setBitmap(currentImage);
               } else if (i <= fadeInEndCount) {
                  // fade in by changing alpha
                  animationField.setAlpha(i * 255 / fadeInEndCount);
               } else if (i >= fadeOutStartCount) {
                  // fade out by changing alpha
                  animationField.setAlpha(255 + (fadeOutStartCount - i) * 255 / (endCount - fadeOutStartCount));
               }               
            }
         });

         cycleCounter++;
      }
   }

   // this class extends BitmapField to allow alpha adjustment, and automatic repainting
   private class AlphaBitmapField extends BitmapField {

      private int _alpha;

      public AlphaBitmapField(Bitmap img) {
         super(img);
      }

      public void setAlpha(int alpha) {
         if (alpha != _alpha) {
            _alpha = alpha;
            // force a repaint
            invalidate();
         }
      }

      protected void paint(Graphics graphics) {
         graphics.setGlobalAlpha(_alpha);         
         super.paint(graphics);
      }
   }
}

我从您的代码中更改了时间值，但您可以将它们修改为您喜欢的任何值。我让动画计时器以每秒 25 个周期运行。我发现这对于智能手机动画来说是一个合理的值。它的速度足够快，看起来相当平滑，但仅是人眼处理此类信息的速度的两倍左右……再快一点可能是浪费。

我定义了一些常量，您可以使用它们来控制淡入和淡出所用的时间长度 (fadeDurationMsec) 以及每个图像在淡入和淡出之间显示的时间 (displayDurationMsec)。所有值都以毫秒为单位。

有用的参考资料

BlackBerry AnimatedGIFField sample code.

BlackBerry forums thread about fade in / fade out