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：
1 package com.example.android.architecture.blueprints.todoapp.tasks;
2
3 import com.example.android.architecture.blueprints.todoapp.ToDoApplication;
4 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepositoryComponent;
5 import com.example.android.architecture.blueprints.todoapp.util.FragmentScoped;
6
7 import dagger.Component;
8
9 /**
10 * This is a Dagger component. Refer to {@link ToDoApplication} for the list of Dagger components
11 * used in this application.
12 * <P>
13 * Because this component depends on the {@link TasksRepositoryComponent}, which is a singleton, a
14 * scope must be specified. All fragment components use a custom scope for this purpose.
15 */
16 @FragmentScoped
17 @Component(dependencies = TasksRepositoryComponent.class, modules = TasksPresenterModule.class)
18 public interface TasksComponent {
19
20 void inject(TasksActivity activity);
21
22 }
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：
1 package com.example.android.architecture.blueprints.todoapp.tasks;
2
3 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepository;
4 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepositoryComponent;
5
6 import javax.annotation.Generated;
7 import javax.inject.Provider;
8
9 import dagger.MembersInjector;
10 import dagger.internal.Factory;
11 import dagger.internal.Preconditions;
12
13 @Generated(
14 value = "dagger.internal.codegen.ComponentProcessor",
15 comments = "https://google.github.io/dagger"
16 )
17 public final class DaggerTasksComponent implements TasksComponent {
18 private MembersInjector<TasksPresenter> tasksPresenterMembersInjector;
19
20 private Provider<TasksRepository> getTasksRepositoryProvider;
21
22 private Provider<TasksContract.View> provideTasksContractViewProvider;
23
24 private Provider<TasksPresenter> tasksPresenterProvider;
25
26 private MembersInjector<TasksActivity> tasksActivityMembersInjector;
27
28 private DaggerTasksComponent(Builder builder) {
29 assert builder != null;
30 initialize(builder);
31 }
32
33 public static Builder builder() {
34 return new Builder();
35 }
36
37 @SuppressWarnings("unchecked")
38 private void initialize(final Builder builder) {
39
40 this.tasksPresenterMembersInjector = TasksPresenter_MembersInjector.create();
41
42 this.getTasksRepositoryProvider =
43 new Factory<TasksRepository>() {
44 private final TasksRepositoryComponent tasksRepositoryComponent =
45 builder.tasksRepositoryComponent;
46
47 @Override
48 public TasksRepository get() {
49 return Preconditions.checkNotNull(
50 tasksRepositoryComponent.getTasksRepository(),
51 "Cannot return null from a [email protected] component method");
52 }
53 };
54
55 this.provideTasksContractViewProvider =
56 TasksPresenterModule_ProvideTasksContractViewFactory.create(builder.tasksPresenterModule);
57
58 this.tasksPresenterProvider =
59 TasksPresenter_Factory.create(
60 tasksPresenterMembersInjector,
61 getTasksRepositoryProvider,
62 provideTasksContractViewProvider);
63
64 this.tasksActivityMembersInjector =
65 TasksActivity_MembersInjector.create(tasksPresenterProvider);
66 }
67
68 @Override
69 public void inject(TasksActivity activity) {
70 tasksActivityMembersInjector.injectMembers(activity);
71 }
72
73 public static final class Builder {
74 private TasksPresenterModule tasksPresenterModule;
75
76 private TasksRepositoryComponent tasksRepositoryComponent;
77
78 private Builder() {
79 }
80
81 public TasksComponent build() {
82 if (tasksPresenterModule == null) {
83 throw new IllegalStateException(
84 TasksPresenterModule.class.getCanonicalName() + " must be set");
85 }
86 if (tasksRepositoryComponent == null) {
87 throw new IllegalStateException(
88 TasksRepositoryComponent.class.getCanonicalName() + " must be set");
89 }
90 return new DaggerTasksComponent(this);
91 }
92
93 public Builder tasksPresenterModule(TasksPresenterModule tasksPresenterModule) {
94 this.tasksPresenterModule = Preconditions.checkNotNull(tasksPresenterModule);
95 return this;
96 }
97
98 public Builder tasksRepositoryComponent(TasksRepositoryComponent tasksRepositoryComponent) {
99 this.tasksRepositoryComponent = Preconditions.checkNotNull(tasksRepositoryComponent);
100 return this;
101 }
102 }
103 }
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：
1 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：
1 package dagger;
2
3 /**
4 * Injects dependencies into the fields and methods on instances of type {@code T}. Ignores the
5 * presence or absence of an injectable constructor.
6 *
7 * @param <T> type to inject members of
8 *
9 * @author Bob Lee
10 * @author Jesse Wilson
11 * @since 2.0 (since 1.0 without the provision that {@link #injectMembers} cannot accept
12 * {@code null})
13 */
14 public interface MembersInjector<T> {
15
16 /**
17 * Injects dependencies into the fields and methods of {@code instance}. Ignores the presence or
18 * absence of an injectable constructor.
19 *
20 * <p>Whenever the object graph creates an instance, it performs this injection automatically
21 * (after first performing constructor injection), so if you‘re able to let the object graph
22 * create all your objects for you, you‘ll never need to use this method.
23 *
24 * @param instance into which members are to be injected
25 * @throws NullPointerException if {@code instance} is {@code null}
26 */
27 void injectMembers(T instance);
28 }
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：
1 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：
1 package com.example.android.architecture.blueprints.todoapp.tasks;
2
3 import javax.annotation.Generated;
4 import javax.inject.Provider;
5
6 import dagger.MembersInjector;
7
8 @Generated(
9 value = "dagger.internal.codegen.ComponentProcessor",
10 comments = "https://google.github.io/dagger"
11 )
12 public final class TasksActivity_MembersInjector implements MembersInjector<TasksActivity> {
13 private final Provider<TasksPresenter> mTasksPresenterProvider;
14
15 public TasksActivity_MembersInjector(Provider<TasksPresenter> mTasksPresenterProvider) {
16 assert mTasksPresenterProvider != null;
17 this.mTasksPresenterProvider = mTasksPresenterProvider;
18 }
19
20 public static MembersInjector<TasksActivity> create(
21 Provider<TasksPresenter> mTasksPresenterProvider) {
22 return new TasksActivity_MembersInjector(mTasksPresenterProvider);
23 }
24
25 @Override
26 public void injectMembers(TasksActivity instance) {
27 if (instance == null) {
28 throw new NullPointerException("Cannot inject members into a null reference");
29 }
30 instance.mTasksPresenter = mTasksPresenterProvider.get();
31 }
32
33 public static void injectMTasksPresenter(
34 TasksActivity instance, Provider<TasksPresenter> mTasksPresenterProvider) {
35 instance.mTasksPresenter = mTasksPresenterProvider.get();
36 }
37 }
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：
1 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：
1 package com.example.android.architecture.blueprints.todoapp.tasks;
2
3 import com.example.android.architecture.blueprints.todoapp.data.source.TasksRepository;
4
5 import javax.annotation.Generated;
6 import javax.inject.Provider;
7
8 import dagger.MembersInjector;
9 import dagger.internal.Factory;
10 import dagger.internal.MembersInjectors;
11
12 @Generated(
13 value = "dagger.internal.codegen.ComponentProcessor",
14 comments = "https://google.github.io/dagger"
15 )
16 public final class TasksPresenter_Factory implements Factory<TasksPresenter> {
17 private final MembersInjector<TasksPresenter> tasksPresenterMembersInjector;
18
19 private final Provider<TasksRepository> tasksRepositoryProvider;
20
21 private final Provider<TasksContract.View> tasksViewProvider;
22
23 public TasksPresenter_Factory(
24 MembersInjector<TasksPresenter> tasksPresenterMembersInjector,
25 Provider<TasksRepository> tasksRepositoryProvider,
26 Provider<TasksContract.View> tasksViewProvider) {
27 assert tasksPresenterMembersInjector != null;
28 this.tasksPresenterMembersInjector = tasksPresenterMembersInjector;
29 assert tasksRepositoryProvider != null;
30 this.tasksRepositoryProvider = tasksRepositoryProvider;
31 assert tasksViewProvider != null;
32 this.tasksViewProvider = tasksViewProvider;
33 }
34
35 @Override
36 public TasksPresenter get() {
37 return MembersInjectors.injectMembers(
38 tasksPresenterMembersInjector,
39 new TasksPresenter(tasksRepositoryProvider.get(), tasksViewProvider.get()));
40 }
41
42 public static Factory<TasksPresenter> create(
43 MembersInjector<TasksPresenter> tasksPresenterMembersInjector,
44 Provider<TasksRepository> tasksRepositoryProvider,
45 Provider<TasksContract.View> tasksViewProvider) {
46 return new TasksPresenter_Factory(
47 tasksPresenterMembersInjector, tasksRepositoryProvider, tasksViewProvider);
48 }
49 }
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。