Android Event Listener Data Binding for ViewPager

Question

Is it possible to bind a handler for setOnPageChangeListener to a ViewPager in XML file with the Android Binding functionality?

The demos show onClick events but I am curious as to how much event functionality I can implement with it. Any links on the capabilities of Data Binding would be great as well. Thanks.

Hypothetical example:

example_activity.xml

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">

<data>
   <variable name="handlers" type="com.example.Handlers"/>
</data>

<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"

    android:onPageChangeListener="@{handlers.pageChanged}" />
</layout>

Handler.java

package com.example.viewmodels;

import android.view.View;

public class Handlers {
    public void pageChanged(View view){}
}

The compilation error is:

Error:(62) No resource identifier found for attribute 'onPageChangeListener' in package 'android'

Answer 1

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.

Answer 2

Instead of OnPageChangeListener, you can use SimpleOnPageChangeListener and only override the method you're interested in:

@BindingAdapter("onPageChanged")
@JvmStatic
fun addPageChangedListener(viewPager: ViewPager, listener: OnPageChanged) {
    viewPager.addOnPageChangeListener(object : ViewPager.SimpleOnPageChangeListener() {
        override fun onPageSelected(position: Int) {
            listener.onPageChanged(position)
        }
    })
}

interface OnPageChanged {
    fun onPageChanged(position: Int)
}

Layout:

<androidx.viewpager.widget.ViewPager
    app:onPageChanged="@{viewModel::handlePageChanged}">

Handler:

fun handlePageChanged(position: Int) {
    println("Page changed: $position")
}

As others have pointed out, you may want to be careful not to attach duplicate listeners.