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App extensions let apps offer custom functionality and content to users while they’re interacting with other apps or the system. Some notable ones are:
* **Custom Keyboard**: replaces the iOS system keyboard with a custom keyboard for use in all apps.
* **Share**: post to a sharing website or share content with others.
* **Today**: also called **widgets**, they offer content or perform quick tasks in the Today view of Notification Center.
For example, the user selects text in the _host app_, clicks on the "Share" button and selects one "app" or action from the list. This triggers the _app extension_ of the _containing app_. The app extension displays its view within the context of the host app and uses the items provided by the host app, the selected text in this case, to perform a specific task (post it on a social network, for example). See this picture from the [Apple App Extension Programming Guide](https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/ExtensionOverview.html#//apple\_ref/doc/uid/TP40014214-CH2-SW13) which pretty good summarizes this:
* An **app extension does never communicate directly with its containing app** (typically, it isn’t even running while the contained app extension is running).
* A **Today****widget** (and no other app extension type) can ask the system to open its containing app by calling the `openURL:completionHandler:` method of the `NSExtensionContext` class.
If you have the original source code you can search for all occurrences of `NSExtensionPointIdentifier` with Xcode (cmd+shift+f) or take a look into "Build Phases / Embed App extensions":
There you can find the names of all embedded app extensions followed by `.appex`, now you can navigate to the individual app extensions in the project.
This is important for data being shared with host apps (e.g. via Share or Action Extensions). When the user selects some data type in a host app and it matches the data types define here, the host app will offer the extension. It is worth noticing the difference between this and data sharing via `UIActivity` where we had to define the document types, also using UTIs. An app does not need to have an extension for that. It is possible to share data using only `UIActivity`.
Inspect the app extension's `Info.plist` file and search for `NSExtensionActivationRule`. That key specifies the data being supported as well as e.g. maximum of items supported. For example:
Only the data types present here and not having `0` as `MaxCount` will be supported. However, more complex filtering is possible by using a so-called predicate string that will evaluate the UTIs given. Please refer to the [Apple App Extension Programming Guide](https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/ExtensionScenarios.html#//apple\_ref/doc/uid/TP40014214-CH21-SW8) for more detailed information about this.
Remember that app extensions and their containing apps do not have direct access to each other’s containers. However, data sharing can be enabled. This is done via ["App Groups"](https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/EntitlementKeyReference/Chapters/EnablingAppSandbox.html#//apple\_ref/doc/uid/TP40011195-CH4-SW19) and the [`NSUserDefaults`](https://developer.apple.com/documentation/foundation/nsuserdefaults) API. See this figure from [Apple App Extension Programming Guide](https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/ExtensionScenarios.html#//apple\_ref/doc/uid/TP40014214-CH21-SW11):
As also mentioned in the guide, the app must set up a shared container if the app extension uses the `NSURLSession` class to perform a background upload or download, so that both the extension and its containing app can access the transferred data.
**Verifying if the App Restricts the Use of App Extensions**
It is possible to reject a specific type of app extension by using the following method:
However, it is currently only possible for "custom keyboard" app extensions (and should be verified when testing apps handling sensitive data via the keyboard like e.g. banking apps).
Following the previous example of Telegram we will now use the "Share" button on a text file (that was received from a chat) to create a note in the Notes app with it:
* This occurred under-the-hood via XPC, concretely it is implemented via a `NSXPCConnection` that uses the `libxpc.dylib` Framework.
* The UTIs included in the `NSItemProvider` are `public.plain-text` and `public.file-url`, the latter being included in `NSExtensionActivationRule` from the [`Info.plist` of the "Share Extension" of Telegram](https://github.com/TelegramMessenger/Telegram-iOS/blob/master/Telegram/Share/Info.plist).
**Identifying the App Extensions Involved**
You can also find out which app extension is taking care of your the requests and responses by hooking `NSExtension - _plugIn`:
*`com.apple.mobilenotes.SharingExtension.appex` which is receiving and will process the text file.
If you want to learn more about what's happening under-the-hood in terms of XPC, we recommend to take a look at the internal calls from "libxpc.dylib". For example you can use [`frida-trace`](https://www.frida.re/docs/frida-trace/) and then dig deeper into the methods that you find more interesting by extending the automatically generated stubs.
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