Dagger Hilt - ViewModel的依赖注入及实现原理

素颜马尾好姑娘i 2023-02-22 14:58 18阅读 0赞

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# 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][Dagger Hilt - Android] 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`

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

参考上一篇  
[Dagger Hilt - Android官方推荐的依赖注入框架][Dagger Hilt - Android]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200704144525556.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh,size_16,color_FFFFFF,t_70
[Dagger Hilt - Android]: https://blog.csdn.net/vitaviva/article/details/106957826