dagger2源码解析

上次我们对dagger2进行简单的使用，这次我们来重点看下dagger2是怎么帮助我们实现对象的注入的。
当我们完成简单的代码后，进行编译，通过apt技术帮我们在build文件夹下生成下面的几个类。

编译后生成的代码.png

下面我们来详细拆解下APT生成的这三个类:

1. CommonMainActivity_MembersInjector：Activity的@Inject后编译APT生成，专门用来注入东西到-->MainActivity的

   public final class CommonMainActivity_MembersInjector implements MembersInjector<CommonMainActivity> {
     // StudentModule_GetStudentFactory
     private final Provider<Student> studentProvider;
   
     public CommonMainActivity_MembersInjector(Provider<Student> studentProvider) {
       assert studentProvider != null;
       this.studentProvider = studentProvider;
     }
   
     // 提供一个静态创建的方法
     public static MembersInjector<CommonMainActivity> create(Provider<Student> studentProvider) {
       return new CommonMainActivity_MembersInjector(studentProvider);
     }
   
     // 注入的方法
     @Override
     public void injectMembers(CommonMainActivity instance) {
       if (instance == null) {
         throw new NullPointerException("Cannot inject members into a null reference");
       }
       instance.student = studentProvider.get();
     }
   
     public static void injectStudent(CommonMainActivity instance, Provider<Student> studentProvider) {
       instance.student = studentProvider.get();
     }
   }
   

2. StudentModule_GetStudentFactory：StudentModule后编译APT生成，专门用来提供暴露出去的 StudentModule工厂

   public final class StudentModule_GetStudentFactory implements Factory<Student> {
     private final StudentModule module;
   
     public StudentModule_GetStudentFactory(StudentModule module) {
       assert module != null;
       this.module = module;
     }
   
     @Override
     public Student get() {
       return Preconditions.checkNotNull(module.getStudent(), "Cannot return null from a non-@Nullable @Provides method");
     }
   
     // 额外生成一个方法，为后续提供
     public static Factory<Student> create(StudentModule module) {
       return new StudentModule_GetStudentFactory(module);
     }
   }
   

3. DaggerStudentComponent：StudentComponent后编译APT生成，对应StudentComponent生成的类文件

   // 这个是编译期 APT 自动生成的
   public final class DaggerStudentComponent implements StudentComponent {
     // StudentModule_GetStudentFactory ,
     private Provider<Student> getStudentProvider;
   
     // CommonMainActivity_MembersInjector
     private MembersInjector<CommonMainActivity> commonMainActivityMembersInjector;
   
     private DaggerStudentComponent(Builder builder) {
       assert builder != null;
       initialize(builder);
     }
   
     public static Builder builder() {
       return new Builder();
     }
   
     public static StudentComponent create() {
       return builder().build();
     }
   
     @SuppressWarnings("unchecked")
     private void initialize(final Builder builder) {
       this.getStudentProvider = StudentModule_GetStudentFactory.create(builder.studentModule);
       this.commonMainActivityMembersInjector = CommonMainActivity_MembersInjector.create(getStudentProvider);
     }
   
     @Override
     public void injectMainActivity(CommonMainActivity commonMainActivity) {
       commonMainActivityMembersInjector.injectMembers(commonMainActivity);
     }
   
     public static final class Builder {
       // 
       private StudentModule studentModule;
   
       private Builder() {}
   
       public StudentComponent build() {
         if (studentModule == null) {
           this.studentModule = new StudentModule();
         }
         return new DaggerStudentComponent(this);
       }
   
       public Builder studentModule(StudentModule studentModule) {
         this.studentModule = Preconditions.checkNotNull(studentModule);
         return this;
       }
     }
   }
   

具体执行流程

DaggerStudentComponent.create()
--> return builder()
    // 这里使用了构建者模式，返回了DaggerStudentComponent对象
    --> return new DaggerStudentComponent(this);
            // 在DaggerStudentComponent的构造方法中，对getStudentProvider和commonMainActivityMembersInjector进行了初始化
        --> this.getStudentProvider = StudentModule_GetStudentFactory.create(builder.studentModule);@DaggerStudentComponent
            --> return new StudentModule_GetStudentFactory(module);
                --> this.module = module;@StudentModule_GetStudentFactory

            this.commonMainActivityMembersInjector = CommonMainActivity_MembersInjector.create(getStudentProvider);
            --> return new CommonMainActivity_MembersInjector(studentProvider);
                -->  this.studentProvider = studentProvider;@CommonMainActivity_MembersInjector

   .build(); @DaggerStudentComponent
   --> if (studentModule == null) {
        this.studentModule = new StudentModule();
        --> return new Student();
      }
      return new DaggerStudentComponent(this);

.injectMainActivity(this);
    // commonMainActivityMembersInjector 是在DaggerStudentComponent构造方法中赋值的。
    // this.commonMainActivityMembersInjector = CommonMainActivity_MembersInjector.create(getStudentProvider);
--> commonMainActivityMembersInjector.injectMembers(commonMainActivity);@DaggerStudentComponent
        // studentProvider是CommonMainActivity_MembersInjector构造方法传进来的。
        // 对activity中的student对象进行赋值。这就是为什么acitivty定义的时候不能写private。
    --> instance.student = studentProvider.get();@CommonMainActivity_MembersInjector