Managing the Extension Lifecycle

The techniques described in previous pages allow a WebExtension API to be loaded and instantiated only when an extension that uses the API is activated. But there are a few other events in the extension lifecycle that an API may need to respond to.

Extension Shutdown

APIs that allocate any resources (e.g., adding elements to the browser’s user interface, setting up internal event listeners, etc.) must free these resources when the extension for which they are allocated is shut down. An API does this by using the callOnClose() method on an Extension object.

Extension Uninstall and Update

In addition to resources allocated within an individual browser session, some APIs make durable changes such as setting preferences or storing data in the user’s profile. These changes are typically not reverted when an extension is shut down, but when the extension is completely uninstalled (or stops using the API). To handle this, extensions can be notified when an extension is uninstalled or updated. Extension updates are a subtle case – consider an API that makes some durable change based on the presence of a manifest property. If an extension uses the manifest key in one version and then is updated to a new version that no longer uses the manifest key, the onManifestEntry() method for the API is no longer called, but an API can examine the new manifest after an update to detect that the key has been removed.

Handling lifecycle events

To be notified of update and uninstall events, an extension lists these events in the API manifest:

"myapi": {
  "schema": "...",
  "url": "...",
  "events": ["update", "uninstall"]
}

If these properties are present, the onUpdate() and onUninstall() methods will be called for the relevant ExtensionAPI instances when an extension that uses the API is updated or uninstalled.

Note that these events can be triggered on extensions that are inactive. For that reason, these events can only be handled by extension APIs that are built into the browser. Or, in other words, these events cannot be handled by APIs that are implemented in WebExtension experiments. If the implementation of an API relies on these events for correctness, the API must be built into the browser and not delievered via an experiment.