Beyond the sandbox: using app groups to communicate between iOS or macOS apps

Apple’s “app group” technology allows a collection of sandboxed macOS or iOS apps from the same development team to all communicate with each other, coordinate functionality, share resources, and/or minimize redundancies. Notice that I said that I can get sandboxed apps to communicate with each other. But isn’t sandboxing a security mechanism meant to keep apps isolated from each other? Yes and no. Apple realized at some point that it would allow apps developed by the same team, with the same Team ID — built by the same people and thus hopefully less risky — to intercommunicate. I’m glad they did as the usefulness of app groups outweighs the dangers.

As long as all developers involved in creating apps meant to be part of an app group can securely share a Team ID from an Apple Developer portal account, they can write apps that can transcend sandboxing. A company may carefully decide to share its Team ID with a trusted partner company, not just with different teams within its own organizational structure.

Member apps of an app group share access to a special group container, a shared folder structure, whose root folder has the same name as the app group ID. In the macOS sample app that accompanies this tutorial, several apps share a preference stored in a .plist file that lives in the special group container. The preference is the background color to be used by app group apps’ UIView instances. If one app sets (writes) this shared background color preference to, say, the color green, other app group members read this preference and can change their view layer backgroundColor property to By starting an app group, I’ve created the beginnings of a group of related apps sharing — centralizing — settings like view color scheme, even if all the apps are sandboxed.

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