Dagger Hilt
Hilt是谷歌最新发布的DI库，降低了Android中Dagger的使用成本，支持各种常见Android组件的注入，也包括我们常用的ViewModel。
Dagger Hilt - Android官方推荐的依赖注入框架
一文中介绍了ViewModel的注入方法，使用起来非常简单：
class ActivityViewModel @ViewModelInject constructor(
private val repository: Repository,
@Assisted private val savedState: SavedStateHandle
) : ViewModel() {
}@AndroidEntryPoint
class MainActivity : AppCompatActivity(R.layout.activity_main) {
private val viewModel by viewModels<ActivityViewModel>()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
}
}
ViewModle创建需要借助ViewModel.Factory、而非构造函数直接创建。上面整个注入过程并没有自定义任何factory，仅仅@ViewModelInject一个注解就搞定了，这是怎么实现的呢？
viewModels{...}
viewModels{...}是一个继承Lazy<>的ktx扩展，可以通过by关键字方便地创建ViewModel，以前我在airbnb的Mvrx中见到过类似用法，看来是被Jetpack借鉴了。
官方自然不只是简单的借(chao)鉴(xi)，里面自有其玄机
@MainThread
inline fun <reified VM : ViewModel> ComponentActivity.viewModels(
noinline factoryProducer: (() -> Factory)? = null
): Lazy<VM> {
val factoryPromise = factoryProducer ?: {
// ComponentActivity#getDefaultViewModelProviderFactory()
defaultViewModelProviderFactory
}
return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
}通过源码知道，ViewModel.Factory默认使用了defaultViewModelProviderFactory。所以只要重写这个defaultViewModelProviderFactory，就可以hook自定义实现进去
重写getDefaultViewModelProviderFactory
通过前一篇文章我们知道，Hilt会在编译期修改被注入对象的父类，
@AndroidEntryPoint
class MainActivity : AppCompatActivity(R.layout.activity_main) {MainActivity原本继承自AppCompatActivity，但经Hilt处理后父类变为了Hilt_MainActivity
@Generated("dagger.hilt.android.processor.internal.androidentrypoint.ActivityGenerator")
public abstract class Hilt_MainActivity extends AppCompatActivity implements GeneratedComponentManager<Object> {
private volatile ActivityComponentManager componentManager;
...
@Override
public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
ViewModelProvider.Factory factory = DefaultViewModelFactories.getActivityFactory(this);
if (factory != null) {
return factory;
}
return super.getDefaultViewModelProviderFactory();
}
}HIlt_MainActivity中通过DefaultViewModelFactories.getActivityFactory(this)返回ViewModel.Factory。
public final class DefaultViewModelFactories {
@Nullable
public static ViewModelProvider.Factory getActivityFactory(ComponentActivity activity) {
return getFactoryFromSet(
EntryPoints.get(activity, ActivityEntryPoint.class).getActivityViewModelFactory());
}
//...
}一句话概括：Hilt通过Dagger生成的Component获取ViewModelFactory
EntryPoints.get(activity, ActivityEntryPoint.class)创建并获取ActivityComponent，getActivityViewModelFactory()通过Component内的Module获取Factory
创建ActivityComponent
public final class EntryPoints {
@Nonnull
public static <T> T get(Object component, Class<T> entryPoint) {
if (component instanceof GeneratedComponent) {
return entryPoint.cast(component);
} else if (component instanceof GeneratedComponentManager) {
return entryPoint.cast(((GeneratedComponentManager<?>) component).generatedComponent());
}
}
}@Generated("dagger.hilt.android.processor.internal.androidentrypoint.ActivityGenerator")
public abstract class Hilt_MainActivity extends AppCompatActivity implements GeneratedComponentManager<Object> {
private volatile ActivityComponentManager componentManager;
...
@Override
public final Object generatedComponent() {
return componentManager().generatedComponent();
}
protected ActivityComponentManager createComponentManager() {
return new ActivityComponentManager(this);
}
protected final ActivityComponentManager componentManager() {
if (componentManager == null) {
synchronized (componentManagerLock) {
if (componentManager == null) {
componentManager = createComponentManager();
}
}
}
return componentManager;
}如上，Hilt_MainActivity继承自GeneratedComponentManager，ActivityComponentManager获取generatedComponent
public class ActivityComponentManager implements GeneratedComponentManager<Object> {
//...
protected Object createComponent() {
return ((ActivityComponentBuilderEntryPoint)
activityRetainedComponentManager.generatedComponent())
.activityComponentBuilder()
.activity(activity)
.build();
}
}最终通过Hilt_MainActivity的ActivityComponentManager获创建ActivityComponent，ActivityComponentManager内部通过activityRetainedComponentManager（借助ViewModel）保证了ActivityComponent的复用。
获取ViewModelFactory
获取ActivityComponent后，通过内部的Model获取ViewModelFactory。
Hilt中提供了ActivityModule，可以InstallIn到ActivityComponent。ActivityModule中定义了Factory的Provider
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
public final class ViewModelFactoryModules {
/**
* Hilt Modules for providing the activity level ViewModelFactory
*/
@Module
@InstallIn(ActivityComponent.class)
public abstract static class ActivityModule {
@NonNull
@Multibinds
abstract Map<String, ViewModelAssistedFactory<? extends ViewModel>> viewModelFactoriesMap();
@Provides
@IntoSet
@NonNull
@DefaultActivityViewModelFactory
static ViewModelProvider.Factory provideFactory(
@NonNull Activity activity,
@NonNull Application application,
@NonNull Map<String, Provider<ViewModelAssistedFactory<? extends ViewModel>>>
viewModelFactories) {
// Hilt guarantees concrete activity is a subclass of ComponentActivity.
SavedStateRegistryOwner owner = (ComponentActivity) activity;
Bundle defaultArgs = activity.getIntent() != null
? activity.getIntent().getExtras() : null;
SavedStateViewModelFactory delegate =
new SavedStateViewModelFactory(application, owner, defaultArgs);
return new HiltViewModelFactory(owner, defaultArgs, delegate, viewModelFactories)通过SavedStateViewModelFactory的defaultArgs，将ViewModel所需的参数进行注入
总结
Hilt在编译期改写Activity或者Fragment的父类，获取了自定义的ViewModel.Factory的方法，从而hook了ViewModle的创建过程，对ViewModel进行注入。整个实现过程借助@InstallIn以及@AndroidEntryPoint的注解，本身就是一个Hilt的最佳实践，值得学习和借鉴
