withStyledAttributes extension function tutorial - https://kennay-kermani.medium.com/android-kotlin-resolve-custom-views-attributes-with-style-f29d5084e9fb

KTXwayTutorialCustomView.kt

class TutorialCustomView(context: Context, attrs: AttributeSet): FrameLayout(context, attrs) {

    init {
        inflate(context, R.layout.tutorial_custom_view, this)
        val mainText : TextView = findViewById(R.id.main_text)

        context.withStyledAttributes(attrs, R.styleable.TutorialCustomView) {
            mainText.apply {
                setBackgroundColor(getColorOrThrow(R.styleable.TutorialCustomView_mainColor))
                text = getString(R.styleable.TutorialCustomView_mainText)
            }
        }
    }
}

OldwayTutorialCustomView.kt

class TutorialCustomView(context: Context, attrs: AttributeSet): FrameLayout(context, attrs) {

    init {
        inflate(context, R.layout.tutorial_custom_view, this)
        val mainText : TextView = findViewById(R.id.main_text)
        val attributesTypedArray = context.obtainStyledAttributes(attrs, R.styleable.TutorialCustomView)

        mainText.apply {
            setBackgroundColor(attributesTypedArray.getColorOrThrow(R.styleable.TutorialCustomView_mainColor))
            text = attributesTypedArray.getString(R.styleable.TutorialCustomView_mainText)
        }

        attributesTypedArray.recycle()
    }
}

tutorial_custom_view.xml

<?xml version="1.0" encoding="utf-8"?>
<merge
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/main_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_margin="16dp"/>
</merge>

TutorialCustomView.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="TutorialCustomView">
        <attr name="mainText" format="string"/>
        <attr name="mainColor" format="color"/>
    </declare-styleable>
</resources>