1# Starting SystemUI CoreStartables
2
3## Overview
4
5A [CoreStartable](/packages/SystemUI/src/com/android/systemui/CoreStartable.java) class represents
6a chunk of SystemUI functionality that is initialized at startup time, independent of the rest of
7the system. Which CoreStartables are included and run can be customized per-build via Dagger.
8
9The base class contains nominal functionality, making it lightweight and inexpensive to construct.
10Unlike Activities, Services, and similar Android constructs, CoreStartables do not have  unique
11context, and have no lifecycle methods except for a singular `#start` method that is called once.
12
13## How to Define a CoreStartable
14
151) Subclass `CoreStartable`. Put any initialization logic in its `#start` method. Preferably, put it
16   in its own source package (with related code) to keep it organizationally distinct from other
17   code in SystemUI.
18
192) Mark its class with `@SysUISingleton` and its constructor with `@Inject`.
20
213) Define a corresponding Dagger module in the same package. The name of the module should follow
22   the pattern: “Start<Feature>Module” where <Feature> is replaced with the name of the
23   CoreStartable.
24
254) Put the following definition inside your new module:
26
27```java
28     @Binds
29     @IntoMap
30     @ClassKey(Feature.class)
31     abstract CoreStartable bindFeature(Feature impl);
32```
33
345) Include the new module in any clients that may need it. For AOSP, this is the
35   SystemUICoreStartableModule.
36
37## Tips and Tricks
38
39**CoreStartables should be single-feature focused.** If you need something run at startup time
40that doesn't have a clear initialization path in existing code, strongly consider defining a _new_
41CoreStartable instead of inserting into a random place in an existing one.
42
43**CoreStartables should be order independent.** They currently are started in an arbitrary but
44deterministic order. We do not promise that this order won't change in the future, however. We do
45not provide a mechanism for changing the order. If you need some other part of the system to
46come online first, consider adding a listener to that part of the system.