Icon-only Tab Bar in iOS [duplicate]

This question already has an answer here:

• Remove tab bar item text, show only image 3 answers

I want my tab bar to have icon only. However, when I set the tabbar label to "", it just shows no text in the place where the text originally should be, so it looks weird, like the following.

One can see that problem is the offset. I want the icon to be vertically aligned in the center, not like this in the above image. How to implement it?

PS: I'm using Swift.