浅析Dagger2依赖注入实现过程

　　Dragger2是Android应用开发中一个非常优秀的依赖注入框架。本文主要通过结合Google给出的MVP开发案例todo-mvp-dagger（GitHub连接地址：https://github.com/googlesamples/android-architecture/tree/todo-mvp-dagger/），简要分析一下Dragger2的依赖注入实现过程。

　　如果是刚入手学习Dragger2，这里推荐三篇非常不错的连载文章：http://www.jianshu.com/p/cd2c1c9f68d4，这三篇文章写得非常用心，而且思路清晰，生动有趣，容易理解。

　　todo-mvp-dagger在功能上划分为addedittask，statistics，taskdetail，tasks四个模块，由于每个模块除了功能逻辑不同之外，实现方式相差不大，所以我就只分析其中的tasks模块，其它模块可参照此模块的分析流程来分析。

　　Dragger2的依赖注入是通过Java注解的方式来实现的。Dragger2中提供了@Inject，@Component，@Module，@Provides等一系列注解，通过注解，Dragger2能够在程序编译阶段利用程序员创建的Java文件，按照编译模板自动生成对应的.class辅助文件，在这些.class辅助文件中会有相应代码来自动完成依赖对象的创建。Dragger2正是以此种看似自动化的技术手段来代替手动new对象的过程。如下图，红色框中的.class文件就是Tasks模块在编译过后自动生成的.class辅助文件。

　　其实，现在有很多框架都利用了注解的方式，通过在编译阶段生成相应的.class文件来完成依赖注入或者其它某些操作，如阿里已经开源的路由框架ARouter。

　　那Dragger2的依赖注入究竟是怎样的一个过程呢？我们先来看TasksActivity。下面是TasksActivity的部分代码。

　　TasksActivity中依赖了一个对象：mTasksPresenter。mTasksPresenter用@Inject标注，表示在TasksActivity实例化时，mTasksPresenter需要被自动创建，然后注入到TasksActivity实例当中去。此时，我们可能会想，是不是用@Inject标注一下就实现依赖注入了？其实不然。我们看到上图下方的红色框中有一段代码，这段代码很长，因为DaggerTasksComponent这个类中使用了Builder模式，只要稍微拆分一下明白了，其实这段代码就做了一件事情，就是调用了TasksComponent的inject()方法，而这里才是mTasksPresenter真正被实例化的地方。

　　我们再来看TasksComponent，这是一个很简单的接口，里面声明了一个inject(TasksActivity activity)方法：

 package com.example.android.architecture.blueprints.todoapp.tasks;

 import com.example.android.architecture.blueprints.todoapp.ToDoApplication;
 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepositoryComponent;
 import com.example.android.architecture.blueprints.todoapp.util.FragmentScoped;

 import dagger.Component;

 /**
  * This is a Dagger component. Refer to {@link ToDoApplication} for the list of Dagger components
  * used in this application.
  * <P>
  * Because this component depends on the {@link TasksRepositoryComponent}, which is a singleton, a
  * scope must be specified. All fragment components use a custom scope for this purpose.
  */
 @FragmentScoped
 @Component(dependencies = TasksRepositoryComponent.class, modules = TasksPresenterModule.class)
 public interface TasksComponent {

     void inject(TasksActivity activity);

 }

　　有接口，对应的一般就有实现类，TasksComponent的实现类在哪里呢？我们注意到TasksComponent 被@Component标注，@Component是干什么的？@Component是用来标识接口或者抽象类，被@Componen标注的接口或者抽象类，在程序编译阶段会自动生成带Dragger前缀的.class文件，例如TasksComponent 被@Component标注，就会生成DraggerTasksComponent.class文件。生成的.class文件便是被@Component标注的接口或者抽象类的实现。我们点开DraggerTasksComponent.class，其中的代码是这样子的：

 package com.example.android.architecture.blueprints.todoapp.tasks;

 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepository;
 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepositoryComponent;

 import javax.annotation.Generated;
 import javax.inject.Provider;

 import dagger.MembersInjector;
 import dagger.internal.Factory;
 import dagger.internal.Preconditions;

 @Generated(
         value = "dagger.internal.codegen.ComponentProcessor",
         comments = "https://google.github.io/dagger"
 )
 public final class DaggerTasksComponent implements TasksComponent {
     private MembersInjector<TasksPresenter> tasksPresenterMembersInjector;

     private Provider<TasksRepository> getTasksRepositoryProvider;

     private Provider<TasksContract.View> provideTasksContractViewProvider;

     private Provider<TasksPresenter> tasksPresenterProvider;

     private MembersInjector<TasksActivity> tasksActivityMembersInjector;

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

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

     @SuppressWarnings("unchecked")
     private void initialize(final Builder builder) {

         this.tasksPresenterMembersInjector = TasksPresenter_MembersInjector.create();

         this.getTasksRepositoryProvider =
                 new Factory<TasksRepository>() {
                     private final TasksRepositoryComponent tasksRepositoryComponent =
                             builder.tasksRepositoryComponent;

                     @Override
                     public TasksRepository get() {
                         return Preconditions.checkNotNull(
                                 tasksRepositoryComponent.getTasksRepository(),
                                 "Cannot return null from a non-@Nullable component method");
                     }
                 };

         this.provideTasksContractViewProvider =
                 TasksPresenterModule_ProvideTasksContractViewFactory.create(builder.tasksPresenterModule);

         this.tasksPresenterProvider =
                 TasksPresenter_Factory.create(
                         tasksPresenterMembersInjector,
                         getTasksRepositoryProvider,
                         provideTasksContractViewProvider);

         this.tasksActivityMembersInjector =
                 TasksActivity_MembersInjector.create(tasksPresenterProvider);
     }

     @Override
     public void inject(TasksActivity activity) {
         tasksActivityMembersInjector.injectMembers(activity);
     }

     public static final class Builder {
         private TasksPresenterModule tasksPresenterModule;

         private TasksRepositoryComponent tasksRepositoryComponent;

         private Builder() {
         }

         public TasksComponent build() {
             if (tasksPresenterModule == null) {
                 throw new IllegalStateException(
                         TasksPresenterModule.class.getCanonicalName() + " must be set");
             }
             if (tasksRepositoryComponent == null) {
                 throw new IllegalStateException(
                         TasksRepositoryComponent.class.getCanonicalName() + " must be set");
             }
             return new DaggerTasksComponent(this);
         }

         public Builder tasksPresenterModule(TasksPresenterModule tasksPresenterModule) {
             this.tasksPresenterModule = Preconditions.checkNotNull(tasksPresenterModule);
             return this;
         }

         public Builder tasksRepositoryComponent(TasksRepositoryComponent tasksRepositoryComponent) {
             this.tasksRepositoryComponent = Preconditions.checkNotNull(tasksRepositoryComponent);
             return this;
         }
     }
 }

　　DaggerTasksComponent采用了Builder模式进行设计，实现了inject(TasksActivity activity)方法。DaggerTasksComponent中的代码有一点点多，而且成员变量都是泛型类对象，看似稍微有点复杂，我们可以整体大概看一下，然后还是从inject(TasksActivity activity)方法的实现入手。inject(TasksActivity activity)的实现很简单，就一行代码：

 tasksActivityMembersInjector.injectMembers(activity);

　　这一行代码调用了接口MembersInjector<T>的void injectMembers(T instance)方法，好，接下来我们看看MembersInjector<T>以及其中的void injectMembers(T instance)方法是用来做什么的。MembersInjector<T>的代码如下：

 package dagger;

 /**
  * Injects dependencies into the fields and methods on instances of type {@code T}. Ignores the
  * presence or absence of an injectable constructor.
  *
  * @param <T> type to inject members of
  *
  * @author Bob Lee
  * @author Jesse Wilson
  * @since 2.0 (since 1.0 without the provision that {@link #injectMembers} cannot accept
  *      {@code null})
  */
 public interface MembersInjector<T> {

   /**
    * Injects dependencies into the fields and methods of {@code instance}. Ignores the presence or
    * absence of an injectable constructor.
    *
    * <p>Whenever the object graph creates an instance, it performs this injection automatically
    * (after first performing constructor injection), so if you're able to let the object graph
    * create all your objects for you, you'll never need to use this method.
    *
    * @param instance into which members are to be injected
    * @throws NullPointerException if {@code instance} is {@code null}
    */
   void injectMembers(T instance);
 }

　　看过注释，我们知道了，原来这个接口就是用来将依赖注入到目标实体（即依赖对象所依附的实体，显然，这里就是指TasksActivity）当中去。到这里，我们好像有点眉目了。不着急，我们继续看MembersInjector<T>的实现。刚刚我们看了DaggerTasksComponent中inject(TasksActivity activity)方法的实现，里面的tasksActivityMembersInjector对象调用了injectMembers()方法，因此可断定tasksActivityMembersInjector就是MembersInjector<T>的实现类对象，那么，tasksActivityMembersInjector是怎么得来的呢？继续看DaggerTasksComponent的代码，发现tasksActivityMembersInjector是这样被创建的：

 this.tasksActivityMembersInjector = TasksActivity_MembersInjector.create(tasksPresenterProvider);

　　TasksActivity_MembersInjector的create()方法创建了tasksActivityMembersInjector。于是，我们再来看TasksActivity_MembersInjector这个类，TasksActivity_MembersInjector代码如下：

 package com.example.android.architecture.blueprints.todoapp.tasks;

 import javax.annotation.Generated;
 import javax.inject.Provider;

 import dagger.MembersInjector;

 @Generated(
         value = "dagger.internal.codegen.ComponentProcessor",
         comments = "https://google.github.io/dagger"
 )
 public final class TasksActivity_MembersInjector implements MembersInjector<TasksActivity> {
     private final Provider<TasksPresenter> mTasksPresenterProvider;

     public TasksActivity_MembersInjector(Provider<TasksPresenter> mTasksPresenterProvider) {
         assert mTasksPresenterProvider != null;
         this.mTasksPresenterProvider = mTasksPresenterProvider;
     }

     public static MembersInjector<TasksActivity> create(
             Provider<TasksPresenter> mTasksPresenterProvider) {
         return new TasksActivity_MembersInjector(mTasksPresenterProvider);
     }

     @Override
     public void injectMembers(TasksActivity instance) {
         if (instance == null) {
             throw new NullPointerException("Cannot inject members into a null reference");
         }
         instance.mTasksPresenter = mTasksPresenterProvider.get();
     }

     public static void injectMTasksPresenter(
             TasksActivity instance, Provider<TasksPresenter> mTasksPresenterProvider) {
         instance.mTasksPresenter = mTasksPresenterProvider.get();
     }
 }

　　TasksActivity_MembersInjector正是刚刚我们所看的MembersInjector<T>的一个实现类，TasksActivity_MembersInjector实现了injectMembers()，在injectMembers()的实现中干了这么一件事情：

 instance.mTasksPresenter = mTasksPresenterProvider.get();

　　哦！原来TasksActivity中的mTasksPresenter是通过mTasksPresenterProvider.get()得来的！此时，迷雾变得逐渐清晰了！接下来再看mTasksPresenterProvider是如何来的。

　　mTasksPresenterProvider在TasksActivity_MembersInjector的构造方法中被赋值，而TasksActivity_MembersInjector的构造方法是在create()方法中被调用的，于是，我们回到DaggerTasksComponent中调用TasksActivity_MembersInjector的create()方法的地方。找到传入create()方法的参数tasksPresenterProvider，发现tasksPresenterProvider又是由TasksPresenter_Factory的create()方法创建的。接下来，我们进一步看TasksPresenter_Factory的代码：

 package com.example.android.architecture.blueprints.todoapp.tasks;

 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepository;

 import javax.annotation.Generated;
 import javax.inject.Provider;

 import dagger.MembersInjector;
 import dagger.internal.Factory;
 import dagger.internal.MembersInjectors;

 @Generated(
         value = "dagger.internal.codegen.ComponentProcessor",
         comments = "https://google.github.io/dagger"
 )
 public final class TasksPresenter_Factory implements Factory<TasksPresenter> {
     private final MembersInjector<TasksPresenter> tasksPresenterMembersInjector;

     private final Provider<TasksRepository> tasksRepositoryProvider;

     private final Provider<TasksContract.View> tasksViewProvider;

     public TasksPresenter_Factory(
             MembersInjector<TasksPresenter> tasksPresenterMembersInjector,
             Provider<TasksRepository> tasksRepositoryProvider,
             Provider<TasksContract.View> tasksViewProvider) {
         assert tasksPresenterMembersInjector != null;
         this.tasksPresenterMembersInjector = tasksPresenterMembersInjector;
         assert tasksRepositoryProvider != null;
         this.tasksRepositoryProvider = tasksRepositoryProvider;
         assert tasksViewProvider != null;
         this.tasksViewProvider = tasksViewProvider;
     }

     @Override
     public TasksPresenter get() {
         return MembersInjectors.injectMembers(
                 tasksPresenterMembersInjector,
                 new TasksPresenter(tasksRepositoryProvider.get(), tasksViewProvider.get()));
     }

     public static Factory<TasksPresenter> create(
             MembersInjector<TasksPresenter> tasksPresenterMembersInjector,
             Provider<TasksRepository> tasksRepositoryProvider,
             Provider<TasksContract.View> tasksViewProvider) {
         return new TasksPresenter_Factory(
                 tasksPresenterMembersInjector, tasksRepositoryProvider, tasksViewProvider);
     }
 }

　　我们发现，在TasksPresenter_Factory实现的get()方法中，一个匿名TasksPresenter对象被创建，这个匿名对象正是被注入到TasksActivity的依赖对象！

　　我们再回过头来看，因为这里的get()方法其实就是Provider<TasksPresenter>的具体实现，在TasksActivity_MembersInjector中的injectMembers(TasksActivity instance)方法中被TasksActivity_MembersInjector的成员变量mTasksPresenterProvider调用，mTasksPresenterProvider调用get()方法后返回的结果被赋值给TasksActivity实例中的依赖对象mTasksPresenter。

　　至此，Dragger2中一个完整的依赖注入过程分析就此结束。诚然，本文只是简单梳理了一下依赖注入的整个流程，对于更深层次的问题，如：Dragger是如何设计.class文件自动生成模板的，自动生成.class文件的具体过程是什么样的等等，本文暂时没有说明，这也是我接下来要做的事情，刚好这几天也在看ARouter的源码，发现其中的依赖注入实现和Dragger2差不多，所以，我打算从多个源码框架中来对比分析和总结Android中的依赖注入实现，待领悟再深入一个层次之后再发一篇总结性文章。