Layout manager that will simply render everything inside in requested size and then scale down it to fit the parent size.

ScaleToFitLayout.kt

import android.content.Context
import android.util.AttributeSet
import android.view.View
import android.widget.LinearLayout
import androidx.core.view.children
import kotlin.math.roundToInt

/**
 * Layout manager that will simply render everything inside in requested size
 * and then scale down it to fit the parent size.
 *
 * Current implementation is the simplest possible and evaluate just width
 * of all children and scale them down if width is bigger than parent view itself.
 *
 * If all children fit parent it works as LinerLayout.
 *
 * Works  only with vertical Linear Layout orientation.
 *
 * @author Tomáš Procházka
 */
class ScaleToFitLayout @JvmOverloads constructor(
    context: Context,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0,
    defStyleRes: Int = 0,
) : LinearLayout(
    context,
    attrs,
    defStyleAttr,
    defStyleRes
) {

    override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {
        super.onLayout(changed, left, top, right, bottom)
        children.forEach { child: View ->
            if (child.width > width) {
                // Only if child is bigger than parent
                val scale = width.toFloat() / (child.width * 1f)
                child.scaleX *= scale
                child.scaleY *= scale
                val nleft = ((width - child.width) / 2f).roundToInt()
                // layout is evalueated before scale down, so we need to render more than visible area to be able to scale it down
                child.layout(nleft, child.top, child.width + nleft, child.bottom)
            }
        }
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
        children.forEach {
            // we will ignore parent size and allow to use as much space as possible
            it.measure(MeasureSpec.UNSPECIFIED, MeasureSpec.UNSPECIFIED)
        }
    }
}