# Dagger 2 in SystemUI
*Dagger 2 is a dependency injection framework that compiles annotations to code
to create dependencies without reflection*

## Recommended reading

Go read about Dagger 2.

TODO: Add some links.

## State of the world

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.

### How it works in SystemUI

For the classes that we're using in Dependency and are switching to dagger, the
equivalent dagger version is using @Singleton and only having 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.

```java
@Singleton
@Component(modules = {SystemUIFactory.class, DependencyProvider.class, ContextHolder.class})
public interface SystemUIRootComponent {
    @Singleton
    Dependency.DependencyInjector createDependency();
}
```

The root modules are what provides the global singleton dependencies across
SystemUI. ContextHolder is just a wrapper that provides a context.
SystemUIFactory @Provide dependencies that need to be overridden by SystemUI
variants (like other form factors). DependencyProvider provides or binds any
remaining depedencies required.

### Adding injection to a new SystemUI object

Anything that depends on any @Singleton provider from SystemUIRootComponent
should be declared as a 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.

```java
public interface SystemUIRootComponent {
+    @Singleton
+    Dependency.DependencyInjector createDependency();
}

public class Dependency extends SystemUI {
  ...
+  @Subcomponent
+  public interface DependencyInjector {
+      Dependency createSystemUI();
+  }
}
```

For objects that 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.

```java
public static class DependencyCreator implements Injector {
    @Override
    public SystemUI apply(Context context) {
        return SystemUIFactory.getInstance().getRootComponent()
                .createDependency()
                .createSystemUI();
    }
}
```

### Adding a new injectable object

First tag the constructor with @Inject. Also tag it with @Singleton if only one
instance should be created.

```java
@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 a provides method to
DependencyProvider.

```java
public class DependencyProvider {
  ...
  @Singleton
  @Provide
  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.

```java
public class Dependency {
  ...
  @Inject Lazy<SomethingController> mSomethingController;
  ...
  public void start() {
    ...
    mProviders.put(SomethingController.class, mSomethingController::get);
  }
}
```

### Using injection with Fragments

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.

```java
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.

```java
FragmentHostManager.get(view).create(NavigationBarFragment.class);
```

## TODO List

 - Eliminate usages of Depndency#get
 - Add support for Fragments to handle injection automatically
   - (this could be through dagger2-android or something custom)
 - Reduce number of things with @Provide in DependencyProvider (many can be
   @Inject instead)
 - Migrate as many remaining DependencyProvider instances to @Bind
 - Add links in above TODO