Android Event Listener Data Binding for ViewPager

Here is ultimate solution. Just put this class in your project.

@BindingMethods({
    @BindingMethod(type = ViewPager.class, attribute = "android:offscreenPageLimit", method = "setOffscreenPageLimit"),
    @BindingMethod(type = ViewPager.class, attribute = "android:adapter", method = "setAdapter"),
    @BindingMethod(type = ViewPager.class, attribute = "android:currentPage", method = "setCurrentItem"),
})
public final class ViewPagerBindingAdapter {

    @InverseBindingAdapter(attribute = "android:currentPage", event = "android:currentPageAttrChanged")
    public static int getCurrentPage(@NonNull final ViewPager pager) {
        return pager.getCurrentItem();
    }

    @BindingAdapter(value = {"android:onPageScrolled", "android:onPageSelected", "android:onPageScrollStateChanged",
        "android:currentPageAttrChanged"}, requireAll = false)
    public static void onSetAdapter(@NonNull final ViewPager pager, final OnPageScrolled scrolled, final OnPageSelected selected,
        final OnPageScrollStateChanged scrollStateChanged, final InverseBindingListener currentPageAttrChanged) {

        final ViewPager.OnPageChangeListener newValue;
        if (scrolled == null && selected == null && scrollStateChanged == null && currentPageAttrChanged == null) {
            newValue = null;
        } else {
            newValue = new ViewPager.OnPageChangeListener() {
                @Override
                public void onPageScrolled(final int position, final float positionOffset, final int positionOffsetPixels) {
                    if (scrolled != null) {
                        scrolled.onPageScrolled(position, positionOffset, positionOffsetPixels);
                    }
                }

                @Override
                public void onPageSelected(final int position) {
                    if (selected != null) {
                        selected.onPageSelected(position);
                    }
                    if (currentPageAttrChanged != null) {
                        currentPageAttrChanged.onChange();
                    }
                }

                @Override
                public void onPageScrollStateChanged(final int state) {
                    if (scrollStateChanged != null) {
                        scrollStateChanged.onPageScrollStateChanged(state);
                    }
                }
            };
        }
        final ViewPager.OnPageChangeListener oldValue = ListenerUtil.trackListener(pager, newValue, R.id.page_change_listener);
        if (oldValue != null) {
            pager.removeOnPageChangeListener(oldValue);
        }
        if (newValue != null) {
            pager.addOnPageChangeListener(newValue);
        }
    }

    public interface OnPageScrolled {
        void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);
    }

    public interface OnPageSelected {
        void onPageSelected(int position);
    }

    public interface OnPageScrollStateChanged {
        void onPageScrollStateChanged(int state);
    }

    private ViewPagerBindingAdapter() {
        throw new UnsupportedOperationException();
    }
}

Also add id resource in your resources.

<resources>
    <item name="page_change_listener" type="id"/>
</resources>

Then you will be able to use it in xml like:

<android.support.v4.view.ViewPager
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:currentPage="@={viewModel.currentPage}"
                android:offscreenPageLimit="@{viewModel.offscreenPageLimit}"
                android:onPageSelected="@{currentPage -> viewModel.pageSelected(currentPage)}"
                android:adapter="@{adapter}"/>

As you can see, currentPage has inverse binding, so your viewModel will be able to set current page and also get current page if user swipes.

It is possible to do this. You need to implement a custom binding adapter because there is no BindingAdapter classes predefined for View classes from Android support libraries.

For how to implement the custom adapter you may read this.

The code should be something like the below, I haven’t tested them thoroughly:

<android.support.v4.view.ViewPager
    android:id="@+id/pager"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:onPageChangeListener="@{handlers}" />

Your BindingAapter code:

@BindingAdapter("onPageChangeListener")
public static void setOnPageChangeListener(ViewPager viewPager, ViewPager.OnPageChangeListener listener) {
    Log.i("setOnPageChangeListener");

    // clear listeners first avoid adding duplicate listener upon calling notify update related code
    viewPager.clearOnPageChangeListeners();
    viewPager.addOnPageChangeListener(listener);
}

P.S. Your handler class being passed should implement ViewPager.OnPageChangeListener.

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