FAB is the main action on the current screen. The default positioning to the bottom right enforces this thinking with users. But what if you have an action that you want to be reachable from anywhere from your top level screens?
One option would be to repeat the button on each screen. In these cases you should make sure to keep the FAB constantly visible to indicate that it has not changed.
But there’s another option. Place the FAB on the bottom navigation bar.
The bottom navigation pattern is also very well defined in the Material Design guidelines. It’s worth taking a look before diving in.
The bottom navigation is used to navigate between your top-level content. Your main action might logically feel to be on the same level than the other top-level content. However, making one of the navigation items an action does not feel right. All content accessible from the bottom navigation should be on the same level in your app hierarchy.
FAB is already a well-established pattern and users can be expected to understand it. FAB triggers an action. Combining FAB visuals with bottom navigation placement should give you best of both worlds. It’s clear that you’re triggering an action and not a navigation action when hitting the FAB.
If you want to try how the FAB in the nav bar feels like give a try to Spendee app.