To support multiple simultaneous browsing sessions the architecture of Focus has been refactored to strictly separate sessions (and associated data) from the UI that displays this data.

Sessions are managed by a global `SessionManager` instance. UI components display data hold by `Session` objects.

The new multisession architecture makes use of Android's [Architecture Components](https://developer.android.com/topic/libraries/architecture/index.html). The list of sessions in the `SessionManager` and the data inside a `Session` is returned as [LiveData](https://developer.android.com/topic/libraries/architecture/livedata.html) objects. UI components can observe those LiveData objects for changes and update the UI state if needed.

![](http://i.imgur.com/CUb5ZLW.png)

* _BrowserFragment_ is the fragment that displays the browser chrome and web content. A _BrowserFragment_ is always connected to a _Session_ (1:1).

* _IWebView_ is a generic interface for the view that renders web content. At runtime it's implemented by either [WebView](https://developer.android.com/reference/android/webkit/WebView.html) or GeckoView. State changes will be reported to a class that implements _IWebView.Callback_.

* With the multisession architecture _IWebView.Callback_ is implemented by _SessionCallbackProxy_. The purpose of this class is to update the _Session_ object for this browsing session. Currently _SessionCallbackProxy_ will still delegate some callback methods to the BrowserFragment. This is expected to be removed in a follow-up refactoring eventually.

* _BrowserFragment_ displays data hold by the _Session_ object. Data that changes periodically (e.g. URL, progress, ..) are represented as [LiveData](https://developer.android.com/topic/libraries/architecture/livedata.html) objects and can be observed so that the UI can be updated whenever the state changes.

* Sessions are switched by displaying a new _BrowserFragment_ for a different _Session_ object.
