Reusable ViewStubBehavior within Scrolling Layout.

ViewStubBehavior.kt

import android.view.ViewStub
import android.widget.FrameLayout
import androidx.databinding.ViewStubProxy

object ViewStubBehavior {

    fun setup(
        stubWrapper: FrameLayout,
        stubProxy: ViewStubProxy,
        scrollContainer: FrameLayout,
        onInflateListener: ViewStub.OnInflateListener? = null
    ) {
        stubProxy.setOnInflateListener(onInflateListener)
        scrollContainer.viewTreeObserver.addOnScrollChangedListener {
            // skip if stub is already inflated
            if (stubProxy.isInflated) return@addOnScrollChangedListener

            val wrapperEdge = stubWrapper.top
            val scrollEdge = scrollContainer.height + scrollContainer.scrollY
            val diff = wrapperEdge - scrollEdge

            // inflate if wrapper edge's and scroll edge's collided. Called once only.
            if (diff <= 0) stubProxy.viewStub?.inflate()
        }
    }

}