Ash is the “Aura Shell”, the window manager and system UI for Chrome OS. Ash uses the views UI toolkit (e.g. views::View, views::Widget, etc.) backed by the aura native widget and layer implementations.
Ash sits below chrome in the dependency graph (i.e. it cannot depend on code in //chrome). Code outside of Ash should depend solely on Ash's public interface, which is in ash/public.
Tests should be added to the ash_unittests target.
Tests can bring up most of the ash UI and simulate a login session by deriving from AshTestBase. This is often needed to test code that depends on ash::Shell and the controllers it owns.
Test support code (TestFooDelegate, FooControllerTestApi, etc.) lives in the same directory as the class under test (e.g. //ash/foo rather than //ash/test). Test code uses namespace ash; there is no special “test” namespace.
Ash supports both per-user prefs and device-wide prefs. These are called “profile prefs” and “local state” to match the naming conventions in chrome. Ash also supports “signin screen” prefs, bound to a special profile that allows users to toggle features like spoken feedback at the login screen.
Pref names are in //ash/public/cpp so that code in chrome can also use the names. Prefs are registered in the classes that use them because those classes have the best knowledge of default values.
Ash shipped on Windows for a couple years to support Windows 8 Metro mode. Windows support was removed in 2016.
The mash (some times called mus-ash or mustash) project was an effort to move ash into its own process and the browser in its own process. Communication between the two processes was done over mojo. Windowing was done using the window-service (some times called mus), which ran with Ash. Many of the mojo interfaces have been converted to pure virtual interfaces, with the implementation in ash. The mash project was stopped around 4/2019.