“Everything you see in Android that's not an app”
SystemUI is a persistent process that provides UI for the system but outside of the system_server process.
Inputs directed at sysui (as opposed to general listeners) generally come in through IStatusBar. Outputs from sysui are through a variety of private APIs to the android platform all over.
When SystemUIApplication starts up, it instantiates a Dagger graph from which various pieces of the application are built.
To support customization, SystemUIApplication relies on the AndroidManifest.xml having an android.app.AppComponentFactory specified. Specifically, it relies on an AppComponentFactory that subclases SystemUIAppComponentFactoryBase. Implementations of this abstract base class must override #createSystemUIInitializer(Context) which returns a SystemUIInitializer. SystemUIInitializer primary job in turn is to intialize and return the Dagger root component back to the SystemUIApplication.
Writing a custom SystemUIAppComponentFactoryBase and SystemUIInitializer, should be enough for most implementations to stand up a customized Dagger graph, and launch a custom version of SystemUI.
See dagger.md and https://dagger.dev/.
The starting point for most of SystemUI code is a list of classes that implement CoreStartable that are started up by SystemUIApplication. CoreStartables are like miniature services. They have their #start method called after being instantiated, and a reference to them is stored inside SystemUIApplication. They are in charge of their own behavior beyond this, registering and unregistering with the rest of the system as needed.
CoreStartable also receives a callback for #onBootCompleted since these objects may be started before the device has finished booting.
CoreStartable is an ideal place to add new features and functionality that does not belong directly under the umbrella of an existing feature. It is better to define a new CoreStartable than to stick unrelated initialization code together in catch-all methods.
CoreStartables are tied to application startup via Dagger:
class FeatureStartable @Inject constructor( /* ... */ ) : CoreStartable { override fun start() { // ... } } @Module abstract class FeatureModule { @Binds @IntoMap @ClassKey(FeatureStartable::class) abstract fun bind(impl: FeatureStartable): CoreStartable }
Including FeatureModule in the Dagger graph such as this will ensure that FeatureStartable gets constructed and that its #start method is called.
CommandQueue is the object that receives all of the incoming events from the system_server. It extends IStatusBar and dispatches those callbacks back any number of listeners. The system_server gets a hold of the IStatusBar when StatusBar calls IStatusBarService#registerStatusBar, so if StatusBar is not included in the XML service list, it will not be registered with the OS.
CommandQueue posts all incoming callbacks to a handler and then dispatches those messages to each callback that is currently registered. CommandQueue also tracks the current value of disable flags and will call #disable immediately for any callbacks added.
There are a few places where CommandQueue is used as a bus to communicate across sysui. Such as when StatusBar calls CommandQueue#recomputeDisableFlags. This is generally used a shortcut to directly trigger CommandQueue rather than calling StatusManager and waiting for the call to come back to IStatusBar.
Creates/initializes the channels sysui uses when posting notifications.
Manages keyguard view state.
Recents tracks all the data needed for recents and starts/stops the recents activity. It provides this cached data to RecentsActivity when it is started.
Registers all the callbacks/listeners required to show the Volume dialog when it should be shown.
This shows the UI for the status bar and the notification shade it contains. It also contains a significant amount of other UI that interacts with these surfaces (keyguard, AOD, etc.). CentralSurfaces also contains a notification listener to receive notification callbacks.
Tracks USB status and sends notifications for it.
Tracks power status and sends notifications for low battery/power saver.
Plays ringtones.
Shows UI for keyboard shortcuts (triggered by keyboard shortcut).
Dispatches shortcut to System UI components.
Component allowing the vendor/OEM to inject a custom component.
Tracks large objects in sysui to see if there are leaks.
Class that only runs on debuggable builds that listens to broadcasts that simulate actions in the system that are used for testing the latency.
Shows the global actions dialog (long-press power).
Draws decorations about the screen in software (e.g. rounded corners, cutouts).
Biometric UI.
Delegates SysUI events to WM Shell controllers.