Dagger2 (三) 总结篇

一.Dagger2注入原理

Dagger2以自动生成代码的形式，帮助我们构建依赖图，在使用依赖的时候方便清晰，这里说明一点，在我们使用Dagger2的时候，绝大多数错误都是编译器就会暴漏出来，这也就决定了这套框架的稳定性会更高。

关于生成的源码，我们一起看一下。我们就以之前提到的例子来看：

@Generated("dagger.internal.codegen.ComponentProcessor")
public final class DaggerApplicationComponent implements ApplicationComponent {
  private Provider<Application> applicationProvider;
  private Provider<Context> contextProvider;

  private DaggerApplicationComponent(Builder builder) {  
    assert builder != null;
    initialize(builder);
  }

  public static Builder builder() {  
    return new Builder();
  }

  private void initialize(final Builder builder) {  
    this.applicationProvider = ApplicationModule_ApplicationFactory.create(builder.applicationModule);
    this.contextProvider = ApplicationModule_ContextFactory.create(builder.applicationModule);
  }

  @Override
  public Application application() {  
    return applicationProvider.get();
  }

  @Override
  public Context context() {  
    return contextProvider.get();
  }

  public static final class Builder {
    private ApplicationModule applicationModule;
    private GsonModule gsonModule;

    private Builder() {  
    }

    public ApplicationComponent build() {  
      if (applicationModule == null) {
        throw new IllegalStateException("applicationModule must be set");
      }
      if (gsonModule == null) {
        this.gsonModule = new GsonModule();
      }
      return new DaggerApplicationComponent(this);
    }

    public Builder applicationModule(ApplicationModule applicationModule) {  
      if (applicationModule == null) {
        throw new NullPointerException("applicationModule");
      }
      this.applicationModule = applicationModule;
      return this;
    }

    public Builder gsonModule(GsonModule gsonModule) {  
      if (gsonModule == null) {
        throw new NullPointerException("gsonModule");
      }
      this.gsonModule = gsonModule;
      return this;
    }
  }
}

可以看到DaggerApplicationComponent中，有一个建造者模式来构建一个ApplicationComponent对象，这也帮我们初始化了两个Module中的依赖，但是注意，这里并没有直接初始化所有模块内的依赖，而只是初始化了组件对象而已。

可以看到initialize方法中有两个工厂方法。

@Generated("dagger.internal.codegen.ComponentProcessor")
public final class ApplicationModule_ContextFactory implements Factory<Context> {
  private final ApplicationModule module;

  public ApplicationModule_ContextFactory(ApplicationModule module) {  
    assert module != null;
    this.module = module;
  }

  @Override
  public Context get() {  
    Context provided = module.context();
    if (provided == null) {
      throw new NullPointerException("Cannot return null from a non-@Nullable @Provides method");
    }
    return provided;
  }

  public static Factory<Context> create(ApplicationModule module) {  
    return new ApplicationModule_ContextFactory(module);
  }
}

可以看出这个工厂中一直保留着ApplicationModule，当我们每次获取依赖，则会重新调用Module的context()方法，即如果里面是new的形式提供依赖 ，则会重新创建对象。

而反观我们用PerActivity标注的ToasterProvider：

@Generated("dagger.internal.codegen.ComponentProcessor")
public final class DaggerActivityComponent implements ActivityComponent {
  private Provider<Context> contextProvider;
  private MembersInjector<BaseActivity> baseActivityMembersInjector;
  private Provider<Toaster> provideToasterProvider;
  private Provider<Toaster> provideTheToasterProvider;

  private DaggerActivityComponent(Builder builder) {  
    assert builder != null;
    initialize(builder);
  }

  public static Builder builder() {  
    return new Builder();
  }

  private void initialize(final Builder builder) {  
    this.contextProvider = new Factory<Context>() {
      private final ApplicationComponent applicationComponent = builder.applicationComponent;
      @Override public Context get() {
        Context provided = applicationComponent.context();
        if (provided == null) {
          throw new NullPointerException("Cannot return null from a non-@Nullable component method");
        }
        return provided;
      }
    };
    this.baseActivityMembersInjector = BaseActivity_MembersInjector.create((MembersInjector) MembersInjectors.noOp(), contextProvider);
    this.provideToasterProvider = ScopedProvider.create(ActivityUtilModule_ProvideToasterFactory.create(builder.activityUtilModule));
    this.provideTheToasterProvider = ActivityUtilModule_ProvideTheToasterFactory.create(builder.activityUtilModule);
  }

  @Override
  public void inject(BaseActivity activity) {  
    baseActivityMembersInjector.injectMembers(activity);
  }

  @Override
  public Toaster theToaster() {  
    return provideToasterProvider.get();
  }

  @Override
  public Toaster toaster() {  
    return provideTheToasterProvider.get();
  }

  public static final class Builder {
    private ActivityUtilModule activityUtilModule;
    private ApplicationComponent applicationComponent;

    private Builder() {  
    }

    public ActivityComponent build() {  
      if (activityUtilModule == null) {
        throw new IllegalStateException("activityUtilModule must be set");
      }
      if (applicationComponent == null) {
        throw new IllegalStateException("applicationComponent must be set");
      }
      return new DaggerActivityComponent(this);
    }

    public Builder activityUtilModule(ActivityUtilModule activityUtilModule) {  
      if (activityUtilModule == null) {
        throw new NullPointerException("activityUtilModule");
      }
      this.activityUtilModule = activityUtilModule;
      return this;
    }

    public Builder applicationComponent(ApplicationComponent applicationComponent) {  
      if (applicationComponent == null) {
        throw new NullPointerException("applicationComponent");
      }
      this.applicationComponent = applicationComponent;
      return this;
    }
  }
}

provideToasterProvider则和其他初始化方式不同，可以看到后面ProviderToaster的工厂很普通：

@Generated("dagger.internal.codegen.ComponentProcessor")
public final class ActivityUtilModule_ProvideToasterFactory implements Factory<Toaster> {
  private final ActivityUtilModule module;

  public ActivityUtilModule_ProvideToasterFactory(ActivityUtilModule module) {  
    assert module != null;
    this.module = module;
  }

  @Override
  public Toaster get() {  
    Toaster provided = module.provideToaster();
    if (provided == null) {
      throw new NullPointerException("Cannot return null from a non-@Nullable @Provides method");
    }
    return provided;
  }

  public static Factory<Toaster> create(ActivityUtilModule module) {  
    return new ActivityUtilModule_ProvideToasterFactory(module);
  }
}

那么ScopedProvider.create包住的工厂有什么特别的呢？

public final class ScopedProvider<T> implements Provider<T> {
  private static final Object UNINITIALIZED = new Object();

  private final Factory<T> factory;
  private volatile Object instance = UNINITIALIZED;

  private ScopedProvider(Factory<T> factory) {
    assert factory != null;
    this.factory = factory;
  }

  @SuppressWarnings("unchecked") // cast only happens when result comes from the factory
  @Override
  public T get() {
    // double-check idiom from EJ2: Item 71
    Object result = instance;
    if (result == UNINITIALIZED) {
      synchronized (this) {
        result = instance;
        if (result == UNINITIALIZED) {
          instance = result = factory.get();
        }
      }
    }
    return (T) result;
  }

  /** Returns a new scoped provider for the given factory. */
  public static <T> Provider<T> create(Factory<T> factory) {
    if (factory == null) {
      throw new NullPointerException();
    }
    return new ScopedProvider<T>(factory);
  }
}

我们看下源码，这里的create是创建了一个ScopeProvider的对象，并将工厂传入，当这个ScopeProvider去get的时候，内部有个单例来维持这个对象，这就是为什么我们自定义注解是个单例的秘密。

接下来看看神器的注解是怎么起作用的。

public final class BaseActivity_MembersInjector implements MembersInjector<BaseActivity> {
    private final MembersInjector<FragmentActivity> supertypeInjector;
    private final Provider<Context> contextProvider;

    public BaseActivity_MembersInjector(MembersInjector<FragmentActivity> supertypeInjector, Provider<Context> contextProvider) {
        assert supertypeInjector != null;

        this.supertypeInjector = supertypeInjector;

        assert contextProvider != null;

        this.contextProvider = contextProvider;
    }

    public void injectMembers(BaseActivity instance) {
        if(instance == null) {
            throw new NullPointerException("Cannot inject members into a null reference");
        } else {
            this.supertypeInjector.injectMembers(instance);
            instance.context = (Context)this.contextProvider.get();
        }
    }

    public static MembersInjector<BaseActivity> create(MembersInjector<FragmentActivity> supertypeInjector, Provider<Context> contextProvider) {
        return new BaseActivity_MembersInjector(supertypeInjector, contextProvider);
    }
}

在我们注入这个对象后，injectMembers方法中，写明了BaseActivity中的context对象是从ContextProvider的get方法中得到，根据代码也可以看到，这部分依赖是从ApplicationComponent中的provideContext方法取得。

再看看Lazy Load：

public final class DoubleCheckLazy<T> implements Lazy<T> {
  private static final Object UNINITIALIZED = new Object();

  private final Provider<T> provider;
  private volatile Object instance = UNINITIALIZED;

  private DoubleCheckLazy(Provider<T> provider) {
    assert provider != null;
    this.provider = provider;
  }

  @SuppressWarnings("unchecked") // cast only happens when result comes from the factory
  @Override
  public T get() {
    // to suppress it.
    Object result = instance;
    if (result == UNINITIALIZED) {
      synchronized (this) {
        result = instance;
        if (result == UNINITIALIZED) {
          instance = result = provider.get();
        }
      }
    }
    return (T) result;
  }

  public static <T> Lazy<T> create(Provider<T> provider) {
    if (provider == null) {
      throw new NullPointerException();
    }
    return new DoubleCheckLazy<T>(provider);
  }
}

toasterLazy加载的时候是使用DoubleCheckLazy.create(this.toasterLazyProvider)来进行初始化。而内部是个单例，只有在get时才会初始化。

至此，Dagger2的主要源码就差不多理解了。

二.使用分析

Dagger2很优雅，优雅到你可以特别轻松的调试它，以为他和你手写的代码几乎一模一样，又优雅到几乎所有的错误都从编译器暴露出。

与RoboGuice的反射不同，生成代码必然会导致方法数的增加。但是，我们可以看到生成的代码数量并不多，而且在实际应用过程中也可以看出，确实影响不大，这个可以大家在以后的使用中慢慢体会。

性能上Dagger2会优于RoboGuice，尤其是天生支持懒加载，但是在易用性上，RoboGuice更容易上手和理解，并且针对Android做了很多通用依赖，为项目开发提高便利。

综上，如果是一个小而美的应用，使用RoboGuice可以快速帮你完成开发工作，而Dagger2在长期来看，性能和效率更佳。

至此，Dagger2入坑系列结束。