FragmentContainer that allow to use fitsSystemWindow in fragments that added using fragment transaction

FragmentContainer.kt

package com.imslp.app.presentation.base

import android.content.Context
import android.graphics.Rect
import android.os.Parcel
import android.os.Parcelable
import android.util.AttributeSet
import android.view.View
import android.view.ViewGroup
import android.widget.FrameLayout

/**
 * A FrameLayout subclass that dispatches WindowInsets to its children instead of adjusting its padding.
 * Useful for Fragment containers.
 *
 * See details at https://medium.com/google-developers/why-would-i-want-to-fitssystemwindows-4e26d9ce1eec
 * and https://stackoverflow.com/questions/31190612/fitssystemwindows-effect-gone-for-fragments-added-via-fragmenttransaction/34345286
 */

class FragmentContainer @JvmOverloads constructor(
        context: Context,
        attrs: AttributeSet? = null,
        defStyleAttr: Int = 0) : FrameLayout(context, attrs, defStyleAttr) {

    private var windowInsets = Rect()

    override fun fitSystemWindows(insets: Rect): Boolean {
        windowInsets.set(insets)
        super.fitSystemWindows(insets)
        return false
    }

    override fun addView(child: View, index: Int, params: ViewGroup.LayoutParams) {
        super.addView(child, index, params)
        super.fitSystemWindows(windowInsets)
    }

    public override fun onSaveInstanceState(): Parcelable? {
        val superState = super.onSaveInstanceState()
        val savedState = SavedState(superState)
        savedState.stateToSave = this.windowInsets
        return savedState
    }

    public override fun onRestoreInstanceState(state: Parcelable) {
        if (state !is SavedState) {
            super.onRestoreInstanceState(state)
            return
        }

        super.onRestoreInstanceState(state.superState)
        this.windowInsets = state.stateToSave
        super.fitSystemWindows(windowInsets)
    }

    internal class SavedState : View.BaseSavedState {
        var stateToSave: Rect = Rect()

        constructor(superState: Parcelable) : super(superState)

        private constructor(parcel: Parcel) : super(parcel) {
            this.stateToSave = parcel.readParcelable(Rect::class.java.classLoader)
        }

        override fun writeToParcel(out: Parcel, flags: Int) {
            super.writeToParcel(out, flags)
            out.writeParcelable(this.stateToSave, Parcelable.PARCELABLE_WRITE_RETURN_VALUE)
        }

        companion object CREATOR : Parcelable.Creator<SavedState> {
            override fun createFromParcel(parcel: Parcel): SavedState {
                return SavedState(parcel)
            }

            override fun newArray(size: Int): Array<SavedState?> {
                return arrayOfNulls(size)
            }
        }
    }

}