iOS and iPadOS allow apps to provide functionality to other apps by providing extensions. Extensions are special-purpose signed executable binaries packaged within an app. During installation, the system automatically detects extensions and makes them available to other apps using a matching system.
A system area that supports extensions is called an extension point. Each extension point provides APIs and enforces policies for that area. The system determines which extensions are available based on extension point–specific matching rules. The system automatically launches extension processes as needed and manages their lifetime. Entitlements can be used to restrict extension availability to particular system apps. For example, a Today view widget appears only in Notification Center, and a sharing extension is available only from the Sharing pane. Examples of extension points are Today widgets, Share, Actions, Photo Editing, File Provider, and Custom Keyboard.
How extensions communicate
Extensions run in their own address space. Communication between the extension and the app from which it was activated uses interprocess communications mediated by the system framework. They don’t have access to each other’s files or memory spaces. Extensions are designed to be isolated from each other, from their containing apps, and from the apps that use them. They are sandboxed like any other third-party app and have a container separate from the containing app’s container. However, they share the same access to privacy controls as the container app. So if a user grants Contacts access to an app, this grant is extended to the extensions that are embedded within the app, but not to the extensions activated by the app.
How custom keyboards are used
Custom keyboards are a special type of extension since they’re enabled by the user for the entire system. Once enabled, a keyboard extension is used for any text field except the passcode input and any secure text view. To restrict the transfer of user data, custom keyboards run by default in a very restrictive sandbox that blocks access to the network, to services that perform network operations on behalf of a process, and to APIs that would allow the extension to exfiltrate typing data. Developers of custom keyboards can request that their extension have Open Access, which lets the system run the extension in the default sandbox after getting consent from the user.
MDM and extensions
For devices enrolled in a mobile device management (MDM) solution, document and keyboard extensions obey Managed Open In rules. For example, the MDM solution can prevent a user from exporting a document from a managed app to an unmanaged Document Provider, or using an unmanaged keyboard with a managed app. Additionally, app developers can prevent the use of third-party keyboard extensions within their app.