If so, how is it possible in A8+?
Sorry, I don't know the A8+.
The question was asked regarding the Galaxy S8+ specifically, and not all mobile phones in existence.
As you allude to, there are a multitude of devices that now implement a version of this using 'software' and the A8+ is probably one of these, as is also the Pixel 2 XL.
This hadn't been implemented on the S8+ at the time of my reply, but may now be present in some 'custom' ROM,s.
[Edit]
After doing some research on the A8+, I understand that it does actually have 2 front cameras.
It also notes that 'Live Focus' is available when taking 'selfies', ie using the '2' front cameras. In light of this, your 'trolling' of my original post makes absolutely no sense.
You're comparing apples (S8/+) to oranges (A8/+), one has a single camera on front and back = no 'Live Focus', the other has 2 on the front = 'Live Focus' when using front camera's.
Simple.