This directory contains the code for building a Window Service implementation on top of an existing Aura hierarchy.

Each client is managed by an instance of WindowTree. In this directory, a client generally means a unique connection to the WindowService. More specifically a client is an implementation of mojom::WindowTreeClient. WindowTree implements the mojom::WindowTree implementation that is passed to the client. WindowTree creates a ClientRoot for the window the client is embedded in, as well as a ClientRoot for all top-level window requests.

Clients establish a connection to the WindowService by configuring Aura with a mode of MUS. See aura::Env::Mode for details.

The WindowService provides a way for one client to embed another client in a specific window (application composition). Embedding establishes a connection to a new client and provides the embedded client with a window to use. See the mojom for more details.

For example, on Chrome OS, Ash uses the WindowService to enable separate processes, such as the tap_visualizer, to connect to the WindowService. The tap_visualizer is a client of the WindowService. The tap_visualizer uses the WindowService to create and manage windows, receive events, and ultimately draw to the screen (using Viz). This is mostly seamless to the tap_visualizer. The tap_visualizer configures Views to use Mus, which results in Views and Aura, using the WindowService.