CenterZoomLayoutManager.kt

package com.hfh.wellbe.smartwatch.utils

import android.content.Context
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import kotlin.math.abs
import kotlin.math.min


class CenterZoomLayoutManager : LinearLayoutManager {

    private val mShrinkAmount = 0.36f
    private val mShrinkDistance = 1f
    private val factor = 2f

    constructor(context: Context) : super(context)

    constructor(context: Context, orientation: Int, reverseLayout: Boolean) : super(
        context,
        orientation,
        reverseLayout
    )

    override fun scrollVerticallyBy(
        dy: Int,
        recycler: RecyclerView.Recycler?,
        state: RecyclerView.State?
    ): Int {
        val orientation = orientation
        if (orientation == VERTICAL) {
            val scrolled = super.scrollVerticallyBy(dy, recycler, state)
            val midpoint = height / factor
            val d0 = 0f
            val d1 = mShrinkDistance * midpoint
            val s0 = 1f
            val s1 = 1f - mShrinkAmount
            for (i in 0 until childCount) {
                val child = getChildAt(i)
                val childMidpoint = (getDecoratedBottom(child!!) + getDecoratedTop(child)) / factor
                val d = min(d1, abs(midpoint - childMidpoint))
                val scale = s0 + (s1 - s0) * (d - d0) / (d1 - d0)
                child.scaleX = scale
                child.scaleY = scale
            }
            return scrolled
        } else {
            return 0
        }
    }

    override fun scrollHorizontallyBy(
        dx: Int,
        recycler: RecyclerView.Recycler?,
        state: RecyclerView.State?
    ): Int {
        val orientation = orientation
        if (orientation == HORIZONTAL) {
            val scrolled = super.scrollHorizontallyBy(dx, recycler, state)

            val midpoint = width / factor
            val d0 = 0f
            val d1 = mShrinkDistance * midpoint
            val s0 = 1f
            val s1 = 1f - mShrinkAmount
            for (i in 0 until childCount) {
                val child = getChildAt(i)
                val childMidpoint = (getDecoratedRight(child!!) + getDecoratedLeft(child)) / factor
                val d = min(d1, abs(midpoint - childMidpoint))
                val scale = s0 + (s1 - s0) * (d - d0) / (d1 - d0)
                child.scaleX = scale
                child.scaleY = scale
            }
            return scrolled
        } else {
            return 0
        }

    }

}