Background execution would be the best way to go about taking photos of people trying to get into your phone but Apple only allows certain UiBackgroundModes values.
AS SEEN HERE:

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Technically you could trick the iPhone into a continuous background forced state by making it do a continuous mode on loop. If you did that, you'd still have the call for the camera. The camera is not a background state option so Apple would not allow it to be published in the Apple Store sadly.

Hopefully Apple devices will soon get more access to things. As they already have been given developers access to the keyboard modifications.

Peas wrote How about something that if someone gets your password wrong it takes a selfie of them ?

SBTRKT wrote

Peas wrote How about something that if someone gets your password wrong it takes a selfie of them ?

I think theres already an app that takes selfies when the phone is reported stolen

Google has a great one called Lockwatch
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And Apple, well, my post about Background Execution explains that issues.