Here’s an open source component called FoldingTabBar submitted by Yalantis that allows you to easily create expanding tab bars.
FoldingTabBar includes a custom tab bar controller that expands when pressed on the button in the middle, includes a spot for two side buttons that automatically move out-of-the-way when the tab bar expands, and has a tab bar item component for image items.
This image from the readme shows FoldingTabBar in action:
You can find FoldingTabBar on Github here.
You can read more about FoldingTabBar on the Yalantis blog.