OnPageChangeListener alpha crossfading

Question

There are a lot of questions with regard to crossfading in Android, but they all include animations. My question is about crossfading using the OnPageChangeListener of a Vie

Answer 1

ViewPager.PageTransformer is your friend. I'm going to take a different approach to what you tried, but it results in what I understand to bed your desired result - swiping left/right swipes the content, but fades between two background images that don't move.

Each Fragment in the ViewPager will have a layout like so:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <ImageView
        android:id="@+id/image_view"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:scaleType="center" />
    <LinearLayout
        android:id="@+id/content_area"
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <!-- content goes here -->
    </LinearLayout>
</FrameLayout>

And you will create a PageTransformer that manipulates the layout depending the position it has been swiped:

public class CustomPageTransformer implements ViewPager.PageTransformer {
    public void transformPage(View view, float position) {
        int pageWidth = view.getWidth();
        View imageView = view.findViewById(R.id.image_view);
        View contentView = view.findViewById(R.id.content_area);

        if (position < -1) { // [-Infinity,-1)
            // This page is way off-screen to the left
        } else if (position <= 0) { // [-1,0]
            // This page is moving out to the left

            // Counteract the default swipe
            view.setTranslationX(pageWidth * -position);
            if (contentView != null) {
                // But swipe the contentView
                contentView.setTranslationX(pageWidth * position);
            }
            if (imageView != null) {
                // Fade the image in
                imageView.setAlpha(1 + position);
            }

        } else if (position <= 1) { // (0,1]
            // This page is moving in from the right

            // Counteract the default swipe
            view.setTranslationX(pageWidth * -position);
            if (contentView != null) {
                // But swipe the contentView
                contentView.setTranslationX(pageWidth * position);
            }
            if (imageView != null) {
                // Fade the image out
                imageView.setAlpha(1 - position);
            }
        } else { // (1,+Infinity]
            // This page is way off-screen to the right
        }
    }
}

And finally hook this PageTransformer up to your ViewPager:

mViewPager.setPageTransformer(true, new CustomPageTransformer());

I've tested it in an existing app and it works well as long as the fragment layouts have a transparent background.