ZoomPageTransformer

gistfile1.txt

class ZoomPageTransformer: ViewPager.PageTransformer {
    companion object {
        private const val MIN_SCALE = 0.85f
        private const val MIN_ALPHA = 0.5f
    }

    override fun transformPage(view: View, position: Float) {
        val pageWidth = view.width
        val pageHeight = view.height

        when {
            position < -1 -> view.alpha = 0f
            position <= 1 -> {
                val scaleFactor = Math.max(MIN_SCALE, 1 - Math.abs(position))
                val vertMargin = pageHeight * (1 - scaleFactor) / 2
                val horzMargin = pageWidth * (1 - scaleFactor) / 2
                view.apply {
                    if (position < 0) {
                        translationX = horzMargin - vertMargin / 2
                    } else {
                        translationX = -horzMargin + vertMargin / 2
                    }

                    scaleX = scaleFactor
                    scaleY = scaleFactor

                    alpha = MIN_ALPHA + (scaleFactor - MIN_SCALE) / (1 - MIN_SCALE) * (1 - MIN_ALPHA)
                }
            }
            else -> view.alpha = 0f
        }
    }
}