public void OnOffsetChanged(AppBarLayout appBarLayout, int verticalOffset)
{
// fab should scroll out down in sync with the appBarLayout scrolling out up.
// let's see how far along the way the appBarLayout is
// (if displacementFraction == 0.0f then no displacement, appBar is fully expanded;
// if displacementFraction == 1.0f then full displacement, appBar is totally collapsed)
float displacementFraction = -verticalOffset / (float)appBarLayout.Height;
// need to separate translationY on the fab that comes from this behavior
// and one that comes from other sources
// translationY from this behavior is stored in a tag on the fab
float translationYFromThis = (float)(float?)_fab.GetTag(Resource.Id.fab_translationY_from_AppBarBoundFabBehavior);
// top position, accounting for translation not coming from this behavior
float topUntranslatedFromThis = _fab.Top + _fab.TranslationY - translationYFromThis;
// total length to displace by (from position uninfluenced by this behavior) for a full appBar collapse
float fullDisplacement = _parent.Bottom - topUntranslatedFromThis;
// calculate and store new value for displacement coming from this behavior
float newTranslationYFromThis = fullDisplacement * displacementFraction;
_fab.SetTag(Resource.Id.fab_translationY_from_AppBarBoundFabBehavior, newTranslationYFromThis);
// update translation value by difference found in this step
_fab.TranslationY = newTranslationYFromThis - translationYFromThis + _fab.TranslationY;
}
