Dagger 2 is a dependency injection framework that compiles annotations to code to create dependencies without reflection
Go read about Dagger 2.
TODO: Add some links.
Dagger 2 has been turned on for SystemUI and a early first pass has been taken for converting everything in Dependency.java to use Dagger. Since a lot of SystemUI depends on Dependency, stubs have been added to Dependency to proxy any gets through to the instances provided by dagger, this will allow migration of SystemUI through a number of CLs.
For the classes that we're using in Dependency and are switching to dagger, the equivalent dagger version is using @Singleton and therefore only has one instance. To have the single instance span all of SystemUI and be easily accessible for other components, there is a single root @Component that exists that generates these. The component lives in SystemUIFactory and is called SystemUIRootComponent.
@Singleton @Component(modules = {SystemUIFactory.class, DependencyProvider.class, DependencyBinder.class, ContextHolder.class}) public interface SystemUIRootComponent { @Singleton Dependency.DependencyInjector createDependency(); }
The root component is composed of root modules, which in turn provide the global singleton dependencies across all of SystemUI.
ContextHolder is just a wrapper that provides a context.
SystemUIFactory @Provides dependencies that need to be overridden by SystemUI variants (like other form factors e.g. Car).
DependencyBinder creates the mapping from interfaces to implementation classes.
DependencyProvider provides or binds any remaining depedencies required.
Anything that depends on any @Singleton provider from SystemUIRootComponent should be declared as an @Subcomponent of the root component, this requires declaring your own interface for generating your own modules or just the object you need injected. The subcomponent also needs to be added to SystemUIRootComponent in SystemUIFactory so it can be acquired.
public interface SystemUIRootComponent { + @Singleton + Dependency.DependencyInjector createDependency(); } public class Dependency extends SystemUI { //... + @Subcomponent + public interface DependencyInjector { + Dependency createSystemUI(); + } }
For objects which extend SystemUI and require injection, you can define an injector that creates the injected object for you. This other class should be referenced in @string/config_systemUIServiceComponents.
public static class DependencyCreator implements Injector { @Override public SystemUI apply(Context context) { return SystemUIFactory.getInstance().getRootComponent() .createDependency() .createSystemUI(); } }
First tag the constructor with @Inject. Also tag it with @Singleton if only one instance should be created.
@Singleton public class SomethingController { @Inject public SomethingController(Context context, @Named(MAIN_HANDLER_NAME) Handler mainHandler) { // context and mainHandler will be automatically populated. } }
If you have an interface class and an implementation class, dagger needs to know how to map it. The simplest way to do this is to add an @Provides method to DependencyProvider. The type of the return value tells dagger which dependency it's providing.
public class DependencyProvider { //... @Singleton @Provides public SomethingController provideSomethingController(Context context, @Named(MAIN_HANDLER_NAME) Handler mainHandler) { return new SomethingControllerImpl(context, mainHandler); } }
If you need to access this from Dependency#get, then add an adapter to Dependency that maps to the instance provided by Dagger. The changes should be similar to the following diff.
public class Dependency { //... @Inject Lazy<SomethingController> mSomethingController; //... public void start() { //... mProviders.put(SomethingController.class, mSomethingController::get); } }
Fragments are created as part of the FragmentManager, so they need to be setup so the manager knows how to create them. To do that, add a method to com.android.systemui.fragments.FragmentService$FragmentCreator that returns your fragment class. Thats all thats required, once the method exists, FragmentService will automatically pick it up and use injection whenever your fragment needs to be created.
public interface FragmentCreator { + NavigationBarFragment createNavigationBar(); }
If you need to create your fragment (i.e. for the add or replace transaction), then the FragmentHostManager can do this for you.
FragmentHostManager.get(view).create(NavigationBarFragment.class);
Generally, you shouldn't need to inject for a view, as the view should be relatively self contained and logic that requires injection should be moved to a higher level construct such as a Fragment or a top-level SystemUI component, see above for how to do injection for both of which.
Still here? Yeah, ok, sysui has a lot of pre-existing views that contain a lot of code that could benefit from injection and will need to be migrated off from Dependency#get uses. Similar to how fragments are injected, the view needs to be added to the interface com.android.systemui.util.InjectionInflationController$ViewInstanceCreator.
public interface ViewInstanceCreator { + QuickStatusBarHeader createQsHeader(); }
Presumably you need to inflate that view from XML (otherwise why do you need anything special? see earlier sections about generic injection). To obtain an inflater that supports injected objects, call InjectionInflationController#injectable, which will wrap the inflater it is passed in one that can create injected objects when needed.
@Override public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) { return mInjectionInflater.injectable(inflater).inflate(R.layout.my_layout, container, false); }
There is one other important thing to note about injecting with views. SysUI already has a Context in its global dagger component, so if you simply inject a Context, you will not get the one that the view should have with proper theming. Because of this, always ensure to tag views that have @Inject with the @Named view context.
public CustomView(@Named(VIEW_CONTEXT) Context themedViewContext, AttributeSet attrs, OtherCustomDependency something) { //... }