You need implement your custom tabbar - with 2 views:
1-st view - content view. You will add current selected controller to it as subview.
2-nd view - tabbar view. You will have collection view here. Each cell will be your button. When cell is selected you add view controller for this cell to content view.
UPDATE:
Take a look on this example: https://github.com/antrix1989/CustomTabBar/tree/master/CustomTabBar