Is there a way of getting a notification when you are using data?

Often when I am using my iPhone I find that I am using mobile data without realising it. If I am engrossed in an activity, it seems unlikely that I would look up to check whether I am using 4G. So is there a way of getting a notification when, unbeknownst to me, I start using 4G rather than Wi-Fi.

I’m not against using 4G; I’d just like to know when this is happening.

Can I enable this directly or with a third party app?

There's no way in iOS that would send a notification if you start using mobile data rather than Wi-Fi.

However, you can either disable mobile-data entirely; disable Wi-Fi assist, which is a feature baked into iOS that lets the phone switch automatically to mobile data if the Wi-Fi signal is weak; or disable mobile data on a per app basis.

If you are concerned about a data hungry app such as video streaming or social networking apps, you can selectively disable mobile data for them by going to Settings app → Mobile Data and turn off the switch shown corresponding to the desired apps.