Android: Swipe Image Viewer

EDIT: I have a much simpler implementation with ViewPager, and the github project has been updated. See post here.

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I have a GrideView of images, which enlarges to full screen when you tap on any image. When I showed it to my friend he swiped to get to the next image, but that did not do anything because I was just using a plain old ImageView. I looked around for a standard widget to do that, and couldn't find any. So I unwillingly rolled my own solution: http://github.com/chiuki/android-swipe-image-viewer.

I show the image in an ImageSwitcher. It contains two views, one displays the current image, and the other one holds the image to swap in. I lifted the gesture listener code from the Android Gallery app:

private class SwipeListener extends SimpleOnGestureListener {
  private static final int SWIPE_MIN_DISTANCE = 75;
  private static final int SWIPE_MAX_OFF_PATH = 250;
  private static final int SWIPE_THRESHOLD_VELOCITY = 200;

  @Override
  public boolean onFling(MotionEvent e1, MotionEvent e2, 
      float velocityX, float velocityY) {
    try {
      if (Math.abs(e1.getY() - e2.getY()) > SWIPE_MAX_OFF_PATH)
        return false;
      // right to left swipe
      if (e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE
          && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
        moveNextOrPrevious(1);
      } else if (e2.getX() - e1.getX() > SWIPE_MIN_DISTANCE
          && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
        moveNextOrPrevious(-1);
      }
    } catch (Exception e) {
      // nothing
    }
    return false;
  }
}

moveNextOrPrevious() swaps in the previous or next image with the appropriate animation:

private void moveNextOrPrevious(int delta) {
  int nextImagePos = mCurrentPosition + delta;
  if (nextImagePos < 0) {
    mOverscrollLeft.setVisibility(View.VISIBLE);
    mOverscrollLeft.startAnimation(mOverscrollLeftFadeOut);
    return;
  }
  if (nextImagePos >= mImages.length) {
    mOverscrollRight.setVisibility(View.VISIBLE);
    mOverscrollRight.startAnimation(mOverscrollRightFadeOut);
    return;
  }

  mImageSwitcher.setInAnimation(
    delta > 0 ? mSlideInRight : mSlideInLeft);
  mImageSwitcher.setOutAnimation(
    delta > 0 ? mSlideOutLeft : mSlideOutRight);

  mCurrentPosition = nextImagePos;
  mImageSwitcher.setImageResource(mImages[mCurrentPosition]);
}

Notice how I show an overscroll grow when the user reaches the start or the end of the list of images. I make it visible immediately, and then use an animation to fade it away.

Right now all the logic is in MainActivity. I thought about encapsulating the viewer code into a custom view and bundling it into a jar file, but I am not sure how many people will find it useful, so laziness got the better of me. If you want to get a jar file to reuse this code, please let me know.

Until then, here is the source code: http://github.com/chiuki/android-swipe-image-viewer.