Good question, the answer is a bit mysterious though. In this case, since we call doOnLayout{} and doOnNextLayout{} before the view is laid out, in both cases, it adds an OnLayoutChangeListener. This listener is called while the view is still being laid out, but after onLayout(). So, any code inside doOnLayout{..} is done during the layout phase which makes things a bit complicated.

The first doOnLayout{} gets the collapsed height, then does expandView.isVisible = true which triggers another layout pass. onDraw() never gets called because visiblity was changed during the layout phase. In the next layout phase, doOnNextLayout{}, we get the expanded height but we need to collapse it again to make it seamless to the user.

The problem here is, doing expandView.isVisible = false here doesn’t trigger a new layout pass. I tested this a bit, turns out, when you set visbility GONE to a view during layout, it doesn’t trigger a new layout. Which is why we use post{} to hide the view again which is called after layout.

While writing this answer, I tried using View.doOnPreDraw{..} instead. This works much better in this scenario because it’s called after layout is done (before onDraw). So, we don’t have to use the post{} trick anymore.