ObservableWebView.kt

open class ObservableWebView @JvmOverloads constructor(
        context: Context, 
        attrs: AttributeSet? = null, 
        defStyleAttr: Int = 0
) : WebView(context, attrs, defStyleAttr), ScrollableViewCompat {

    private lateinit var listener : OnScrollChangeObserver

    override fun addOnScrollChangeListener(l: OnScrollChangeObserver) {
        listener = l
    }

    override fun getAttachedActivity() = context as AppCompatActivity

    override fun onScrollChanged(l: Int, t: Int, oldl: Int, oldt: Int) {
        super.onScrollChanged(l, t, oldl, oldt)
        listener.onScrollChange(this, l, t, oldl, oldt)
    }
}