Add docs for CoreStartable.

Bug: 205725937
Test: n/a
Change-Id: I651b240f2bc92ee8d6331263deb2d4c9058bea85

1 files changed, 46 insertions, 0 deletions

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+# Starting SystemUI CoreStartables
+
+## Overview
+
+A [CoreStartable](/packages/SystemUI/src/com/android/systemui/CoreStartable.java) class represents
+a chunk of SystemUI functionality that is initialized at startup time, independent of the rest of
+the system. Which CoreStartables are included and run can be customized per-build via Dagger.
+
+The base class contains nominal functionality, making it lightweight and inexpensive to construct.
+Unlike Activities, Services, and similar Android constructs, CoreStartables do not have  unique
+context, and have no lifecycle methods except for a singular `#start` method that is called once.
+
+## How to Define a CoreStartable
+
+1) Subclass `CoreStartable`. Put any initialization logic in its `#start` method. Preferably, put it
+   in its own source package (with related code) to keep it organizationally distinct from other
+   code in SystemUI.
+
+2) Mark its class with `@SysUISingleton` and its constructor with `@Inject`.
+
+3) Define a corresponding Dagger module in the same package. The name of the module should follow
+   the pattern: “Start<Feature>Module” where <Feature> is replaced with the name of the
+   CoreStartable.
+
+4) Put the following definition inside your new module:
+
+```java
+     @Binds
+     @IntoMap
+     @ClassKey(Feature.class)
+     abstract CoreStartable bindFeature(Feature impl);
+```
+
+5) Include the new module in any clients that may need it. For AOSP, this is the
+   SystemUICoreStartableModule.
+
+## Tips and Tricks
+
+**CoreStartables should be single-feature focused.** If you need something run at startup time
+that doesn't have a clear initialization path in existing code, strongly consider defining a _new_
+CoreStartable instead of inserting into a random place in an existing one.
+
+**CoreStartables should be order independent.** They currently are started in an arbitrary but
+deterministic order. We do not promise that this order won't change in the future, however. We do
+not provide a mechanism for changing the order. If you need some other part of the system to
+come online first, consider adding a listener to that part of the system.
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