A user can capture what’s displayed on their screen by taking a screenshot. Starting in iOS 11, screenshots briefly appear in preview form at the bottom of the screen after being taken. The user can swipe a preview to the side to dismiss it (a preview auto-dismisses after a few seconds if the user does nothing) or tap a preview for quick access to Instant Markup and sharing tools. Screenshots are saved into a Screenshots album in Photos.
Don’t change your app’s interface when local screenshots are taken. The system-provided screenshot feature provides sufficient context and functionality when taking screenshots. Custom screenshot-initiated interface changes and features are repetitive, unnecessary, and potentially confusing. If appropriate for your app, it’s fine to alert users when screenshots are taken by a different user or on a different device.