Android Custom SeekBar Preference

preference_seekbar.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:orientation="vertical"
              android:padding="15dp">

    <TextView
        android:id="@android:id/title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="2dp"
        android:textSize="15sp"/>

    <TextView
        android:id="@android:id/summary"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="5dp"
        android:textSize="13sp"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <SeekBar
            android:id="@+id/seekBar"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginStart="0dp"
            android:layout_marginEnd="5dp"
            android:layout_weight="1"/>

        <TextView
            android:id="@+id/currentValue"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="@color/colorSecondaryText"
            android:textSize="12sp"/>

    </LinearLayout>

</LinearLayout>

SeekBarPreference.kt

class SeekBarPreference(context: Context, attrs: AttributeSet) :
    Preference(context, attrs, 0) {

    private var max = 0
    private var current = 0

    init {
        widgetLayoutResource = R.layout.preference_seekbar
        layoutResource = R.layout.preference_seekbar

        val ta = context.obtainStyledAttributes(attrs, R.styleable.SeekBarPreference)
        max = ta.getInt(R.styleable.SeekBarPreference_max, 0)

        ta.recycle()
    }

    override fun onBindViewHolder(holder: PreferenceViewHolder) {
        super.onBindViewHolder(holder)
        val seekBar: SeekBar = holder.findViewById(R.id.seekBar) as SeekBar
        val currentValue: TextView = holder.findViewById(R.id.currentValue) as TextView

        seekBar.max = max
        seekBar.progress = 3
        currentValue.text = "$current/$max"

        seekBar.setOnSeekBarChangeListener(object: SeekBar.OnSeekBarChangeListener {
            override fun onProgressChanged(seekBar: SeekBar, progress: Int, fromUser: Boolean) {
                currentValue.text = "$progress/$max"
            }

            override fun onStartTrackingTouch(seekBar: SeekBar) {

            }

            override fun onStopTrackingTouch(seekBar: SeekBar) {
                persistInt(seekBar.progress)
            }
        })
    }

    override fun onGetDefaultValue(a: TypedArray, index: Int): Int {
        return a.getString(index).toInt()
    }

    override fun onSetInitialValue(restorePersistedValue: Boolean, defaultValue: Any?) {
        super.onSetInitialValue(restorePersistedValue, defaultValue)
        current = if (restorePersistedValue) getPersistedInt(0) else defaultValue as Int
    }
}