Extensions (iOS)

On an iPhone, every app is normally a closed world. You open it, use it, and close it again. An extension is a small add-on program that belongs to such an app but appears outside of it. It shows up wherever you happen to need it: as its own keyboard, as a small tile on the home screen, or as an entry in the menu that opens when sharing a photo. In doing so, the user never actually leaves the app they’re currently in. Apple introduced this principle in 2014 and has continued to expand it ever since.

Why Apple opened up its system with this

For a long time, the iPhone was considered particularly locked down. Apps could barely talk to one another, and any exchange of data had to go through roundabout ways. That was secure, but inconvenient. Anyone who wanted to edit a photo had to export it, open a second app, and feed the result back in. Extensions eliminated this detour without giving up the lockdown entirely.

For software providers, extensions are therefore valuable. A password manager that appears directly in a website’s login field gets used far more often than one that has to be opened separately. The same applies to translation services or note-taking apps. Visibility within the system is a tangible competitive advantage.

This has now become relevant for AI providers as well. A language model that can be reached in any text field via a keyboard extension competes directly with the manufacturer’s built-in feature. Competition authorities in the EU are watching closely how far Apple opens up such access — or doesn’t.

Separate processes and shared containers

Technically, an extension is not part of the main app but a standalone program. It gets installed together with the app, but starts separately and is only loaded by the operating system when needed. When you close the share menu, iOS often shuts the extension down again immediately. You can think of it like a branch office: it belongs to the company, but has its own opening hours and its own staff.

This separation has consequences. An extension gets less working memory than a normal app and is shut down more quickly if it uses too much. Extensive computations are therefore problematic. Anyone wanting to run a large AI model directly inside a keyboard quickly hits this limit and offloads the computation to a server instead.

The app and the extension exchange data via a shared storage area known as the shared container. Both are allowed to write to it and read from it, but other apps are not. This is how, for example, an article saved in the extension later ends up in the main app’s reading list. A common misconception is that an extension is simply a second window of the same app — it shares neither working memory nor running state.

Keyboards, widgets, and the share menu

Extensions are most visible in the share menu. It opens when you tap the icon with the arrow in Photos, Safari, or Mail. Every entry in it that doesn’t come from Apple itself is an extension. The same applies to widgets on the home screen and to alternative keyboards like Gboard or SwiftKey.

Less conspicuous but widespread are extensions for ad blockers in Safari, for password managers during login, and for camera filters. Siri Shortcuts also run via this mechanism. On Android, a comparable principle exists under a different name — there, the building blocks are called intents and services.

In news coverage, the term usually comes up in the context of competition or data protection. A keyboard extension, for instance, is only allowed to access the internet if the user explicitly permits it — otherwise it could record every single input. Apple sets these rules, and changes to them help determine what features third-party providers can even offer on the iPhone in the first place.