How to use PageTransformer with ViewPager in Kotlin

In this android kotlin source code example, we are going to use PageTransformer with ViewPager in Kotlin.

You can copy and adopt this source code example to your Kotlin android project without reinventing the wheel.

Below is a step by step source code to use PageTransformer with ViewPager in Kotlin.

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ViewPager.ViewPagerActivity16">

    <androidx.viewpager.widget.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.kt

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.fragment.app.Fragment
import androidx.fragment.app.FragmentManager
import androidx.fragment.app.FragmentPagerAdapter
import androidx.viewpager.widget.ViewPager
import com.bluapp.kotlinview.R

class ViewPagerActivity16 : AppCompatActivity() {
    private var viewPager: ViewPager? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_view_pager16)
        viewPager = findViewById(R.id.viewpager) as ViewPager
        viewPager!!.adapter = MyAdapter(supportFragmentManager)
        viewPager!!.offscreenPageLimit = 3
        val pageMargin = 30f
        val pageOffset = 30f
        viewPager!!.setPageTransformer(false, ViewPager.PageTransformer { page, position ->
            val mOffset = position * -(2 * pageOffset + pageMargin)
            if(position < -1){
                page.translationX=-mOffset
            }else if(position <= 1){
                val scaleFactor = Math.max(0.7f, 1 - Math.abs(position - 0.14285715f))
                page.translationX = mOffset
                page.scaleY = scaleFactor
                page.alpha = scaleFactor
            }else{
                page.alpha = 0f
                page.translationX = mOffset

            }
        })
    }

    private inner class MyAdapter(fm: FragmentManager?) : FragmentPagerAdapter(fm!!, FragmentPagerAdapter.BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
        private val int_items = 5

        override fun getItem(position: Int): Fragment {
            var fragment: Fragment? = null
            when (position) {
                0 -> fragment = ViewPagerFragment()
                1 -> fragment = ViewPagerFragment()
                2 -> fragment = ViewPagerFragment()
                3 -> fragment = ViewPagerFragment()
                4 -> fragment = ViewPagerFragment()
            }
            return fragment!!
        }

        override fun getCount(): Int {
            return int_items
        }
    }
}

fragment_view_pager.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#000000"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".ViewPager.ViewPagerFragment">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="ViewPagerFragment"
        android:textSize="20sp"
        android:textColor="@color/white"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>

ViewPagerFragment.kt

import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import com.bluapp.kotlinview.R

class ViewPagerFragment : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_view_pager, container, false)
    }
}

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