Design Overview

Component generation and module/entry point installation

Hilt generates components by finding all of the modules and entry points in the transitive classpath. The @InstallIn annotation on every module and entry point generates a small metadata class in a defined package. The special package is inspected when processing @HiltAndroidApp to find all of the aggregated items that need to be installed in the components. The same strategy is used for other helper classes like @DefineComponent and @AliasOf.

Since the Android Application is generated at the same time, the generated Application has a direct reference to the root generated component which is the SingletonComponent.

Since the HiltTestApplication must support multiple tests, unlike in the production application, reflection is used to find the generated components. This is helpful because it allows the test application to be decoupled from building with the tests which allows Hilt to provide a convenient default instead of requiring each project to code generate a test application. Reflection is not used in production because it provides less value and reflection may have more costs.

Aggregating all of the modules in the classpath works well for tests because it means tests can easily add bindings by just nesting classes in the test class (or even better using @BindValue which generates the module). Similarly, the module detection also allows classes to embed @Module classes as inner classes. This can be used to ensure the class cannot be used without the associated Dagger bindings and makes its usage less error prone (e.g. pairing a class with an @BindsOptionalOf it consumes or an @Binds to an interface).

@AndroidEntryPoint injection

@AndroidEntryPoint works by generating a base class that the user code extends either directly or indirectly via a transform in the Gradle plugin. This base class is responsible for retrieving the parent component (via Hilt interfaces on the parent), creating the component, injecting the class, and exposing the component to children via Hilt interfaces.

For example, to inject the activity the generated code essentially does the following (simplified for readability):

@Override public void onCreate(Bundled savedInstanceState) {
  // This gets the parent component from the Application (in reality there is
  // actually the activity retained component as the parent).
  Object parentComponent =
      ((GeneratedComponentManager) getApplication()).generatedComponent();
  // This creates the activity component. This involves an unsafe cast
  // to know the parent component has the methods to build the activity component.
  Object activityComponent = ((ActivityComponentBuilderEntryPoint) parentComponent)
  // This injects the activity. It also involves an unsafe cast to get access
  // to the activity inject method. Like the other unsafe casts, these casts
  // break build dependencies and are safe because they are code generated and
  // guaranteed via the classpath discovery of modules/interfaces.
  (MyActivity_GeneratedInjector) activityComponent).inject(this).

The generation of all of this glue code makes breaking dependencies with unsafe casts safe and easy. Also, the automatic discovery combined with the fact that the interfaces are generated with the activity that uses them makes it so that including or removing an @AndroidEntryPoint adds/takes all of its dependencies with it. This allows apps built with Hilt to be modular.

Most of the time the parent component is easy to get, but in the case of views and fragments it isn’t so easy because views get the activity context. To support views with fragment bindings, the generated base class for fragments override getLayoutInflater to wrap the Context in a ContextWrapper that holds the Dagger component for the view to get.

By standardizing all of these design decisions in Hilt, integrating libraries with activities/fragments/views should be much easier.