Understand How an App Extension Works

An app extension is not an app. It implements a specific, well scoped task that adheres to the policies defined by a particular extension point.

An App Extension’s Life Cycle

Because an app extension is not an app, its life cycle and environment are different. In most cases, an extension launches when a user chooses it from an app’s UI or from a presented activity view controller. An app that a user employs to choose an app extension is called a host app. A host app defines the context provided to the extension and kicks off the extension life cycle when it sends a request in response to a user action. An extension typically terminates soon after it completes the request it received from the host app.

For example, imagine that a user selects some text in an OS X host app, activates the Share button, and chooses an app extension from the sharing list to help them post the text to a social sharing website. The host app responds to the user’s choice by issuing to the extension a request that contains the selected text. A generalized version of this situation is pictured in step 1 of Figure 2-1.

In step 2 of Figure 2-1, the system instantiates the app extension identified in the host app’s request and sets up a communication channel between them. The extension displays its view within the context of the host app and then uses the items it received in the host app’s request to perform its task (in this example, the extension receives the selected text).

In step 3 of Figure 2-1, the user performs or cancels the task in the app extension and dismisses it. In response to this action, the extension completes the host app’s request by immediately performing the user’s task or, if necessary, initiating a background process to perform it. The host app tears down the extension’s view and the user returns to their previous context within the host app. When the extension’s task is finished, whether immediately or later, a result may be returned to the host app.

Shortly after the app extension performs its task (or starts a background session to perform it), the system terminates the extension, as shown in step 4.

How an App Extension Communicates

An app extension communicates primarily with its host app, and does so in terms reminiscent of transaction processing: There is a request from the host and a response from the extension. Figure 2-2 shows a simplified view of the relationship between a running extension, the host app that launched it, and the containing app.