ViewPager2 Transformer

완성짤

1. ViewPager2 구성

• 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"
    android:padding="16dp"
    tools:context=".MainActivity">

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/viewPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:clipChildren="false"
        android:clipToPadding="false"
        android:paddingEnd="50dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

• item_text.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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/black">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

• Model.kt

data class Model(val text: String)

• ViewHolder.kt

class ViewHolder(binding: ItemTextBinding) : RecyclerView.ViewHolder(binding.root) {

    private val textView = binding.textView

    fun bind(item: Model) {
        textView.text = item.text
    }
}

• Adapter.kt

class Adapter(private val itemList: List<Model>) : RecyclerView.Adapter<ViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        return ViewHolder(ItemTextBinding.inflate(LayoutInflater.from(parent.context), parent, false))
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.bind(itemList[position])
    }

    override fun getItemCount(): Int {
        return itemList.size
    }

2. Transformer

• SliderTransformer.kt

import kotlin.math.abs

class SliderTransformer(private val offscreenPageLimit: Int) : ViewPager2.PageTransformer {

    companion object {

        private const val DEFAULT_TRANSLATION_X = .0f
        private const val DEFAULT_TRANSLATION_FACTOR = 1.2f

        private const val SCALE_FACTOR = .14f
        private const val DEFAULT_SCALE = 1f

        private const val ALPHA_FACTOR = .3f
        private const val DEFAULT_ALPHA = 1f

    }

    override fun transformPage(page: View, position: Float) {
        page.apply {

            ViewCompat.setElevation(page, -abs(position))

            val scaleFactor = -SCALE_FACTOR * position + DEFAULT_SCALE
            val alphaFactor = -ALPHA_FACTOR * position + DEFAULT_ALPHA

            when {
                position <= 0f -> {
                    translationX = DEFAULT_TRANSLATION_X
                    scaleX = DEFAULT_SCALE
                    scaleY = DEFAULT_SCALE
                    alpha = DEFAULT_ALPHA + position
                }
                position <= offscreenPageLimit - 1 -> {
                    scaleX = scaleFactor
                    scaleY = scaleFactor
                    translationX = -(width / DEFAULT_TRANSLATION_FACTOR) * position
                    alpha = alphaFactor
                }
                else -> {
                    translationX = DEFAULT_TRANSLATION_X
                    scaleX = DEFAULT_SCALE
                    scaleY = DEFAULT_SCALE
                    alpha = DEFAULT_ALPHA
                }
            }
        }
    }
}

3. ViewPager2에 적용

• MainActivity.kt

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val itemList = ArrayList<Model>()
        for (i in 0..7) {
            itemList.add(Model("Text $i"))
        }

        binding.viewPager.apply {
            adapter = Adapter(itemList)
            offscreenPageLimit = 3
            setPageTransformer(SliderTransformer(3))
        }
    }
}

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땡스 투 https://medium.com/@arbelkilani/viewpager2-pagetransformer-af989976560e