(4) Dagger2 @Module analysis 2

Code example

class Food {
    
    
    // @Inject:该注解用于类构造方法上表示该类可以被Dagger2实例化对象后供注射器使用.
    @Inject
    public Food() {
    
    
    }
}
class Tiger {
    
    
    // 假如老虎需要食物
    Food food;
    public Tiger(Food food) {
    
    
        this.food = food;
    }
    public void sleep() {
    
    
        System.out.println("Tiger sleeping");
    }
}
// 为注射器提供工厂类
// @Module: 表示Dagger2可以将该类当作对象工厂.
@Module
class ZooModule {
    
    
    // @Provides: 表示Dagger2可以调用providerTiger(Food food)方法为注射器提供对象.
    @Provides
    public Tiger providerTiger(Food food) {
    
    
        return new Tiger(food);
    }
}
// 对象使用者
public class Zoo {
    
    
    // @Inject: 该注解用于成员变量上代表注射器可以为该成员变量赋值对象.
    @Inject
    Tiger tiger;

    @Test
    public void 案例四() {
    
    
        DaggerZooComponent.create().inject(this);
        tiger.sleep();
    }
}
// 注射器
// @Component: 该注解是用来创造对象提供方和对象使用间的桥梁,如何将对象赋值给成员变量都是由它来做的.
// (modules = {ZooModule.class}): 表示该注射器可以使用ZooModule对象工厂提供方法.
@Component(modules = {
    
    ZooModule.class})
interface ZooComponent {
    
    
    Zoo inject(Zoo zoo);
}

Dagger2 generated code reading

Mainly analyze the dependency relationship between the method in Module and an object, add them to the same injector, and the injector will automatically handle the dependency between the Module factory method and an object according to the needs of the object user.

Looking at the code generated by Degger2 based on the above case, the generated code is in the build\generated\sources\annotationProcessor\..folder.

1. DaggerZooComponent.create()
create()Used to generate DaggerZooComponentobjects, DaggerZooComponentthe parent class is the ZooComponentinterface, which overrides the inject()methods in the interface .

final class DaggerZooComponent implements ZooComponent {
    
    
    private final ZooModule zooModule;
    // 由Builder.build()创建DaggerZooComponent对象.
    private DaggerZooComponent(ZooModule zooModuleParam) {
    
    
      this.zooModule = zooModuleParam;
    }
    // create()方法创建DaggerZooComponent()对象
    public static ZooComponent create() {
    
    
      return new Builder().build();
    }
    static final class Builder {
    
    
        private ZooModule zooModule;
        private Builder() {
    
    
        }
        // build()方法返回DaggerZooComponent对象.  
        public ZooComponent build() {
    
    
            // 如果zooModule为null,那么就创建该对象.ZooModule代表工厂.
            if (zooModule == null) {
    
    
              this.zooModule = new ZooModule();
            }
            // 将ZooModule工厂对象存储在DaggerZooComponent对象中
            return new DaggerZooComponent(zooModule);
        }
    }
}

2. .inject(this)Object injection
   Use DaggerZooComponentobjects to inject the Dagger2created Tigerobjects into Zooit.

final class DaggerZooComponent implements ZooComponent {
    
    
    // 接口中的方法,由DaggerZooComponent实现,该方法就是用来向Zoo对象中注入Tiger对象的.
    @Override
    public Zoo inject(Zoo zoo) {
    
    
        // zoo就是inject(this)中的this了,代表Zoo对象.
        return injectZoo(zoo);
    }
    private Zoo injectZoo(Zoo instance) {
    
    
        // 1. getTiger(): 通过ZooModule工厂获取Zoo需要的Tiger对象.
        // 2. Zoo_MembersInjector.injectTiger(instance, getTiger()): 这里就是为Zoo对象中所需字段赋值.
        Zoo_MembersInjector.injectTiger(instance, getTiger());
        return instance;
    }
}

3. getTiger()Get the Tigerobject.

final class DaggerZooComponent implements ZooComponent {
    
    
        private final ZooModule zooModule;// 提供对象的工厂
        private DaggerZooComponent(ZooModule zooModuleParam) {
    
    
            this.zooModule = zooModuleParam;
        }
        private Tiger getTiger() {
    
    
            // 接着向下看
            // zooModule：提供对象的工厂
            // new Food()： 因为Food类的构造方法中标注了@Inject。
            // 从这里可以看出，只要将Module或者对象加入到注射器中，Module和对象间的依赖注射器都会自动处理好。
            return ZooModule_ProviderTigerFactory.providerTiger(zooModule, new Food());
        } 
}
public final class ZooModule_ProviderTigerFactory implements Factory<Tiger> {
    
    
    public static Tiger providerTiger(ZooModule instance, Object food) {
    
    
        // instance: 为ZooModule类，它的providerTiger(Food food)作用是返回Tiger对象。
        // food： 为上一步中由注射器创建的Food对象，Food类的构造被方法标注了@Inject。
        return Preconditions.checkNotNull(instance.providerTiger((Food) food), "Cannot return null from a non-@Nullable @Provides method");
    }
}

4. Zoo_MembersInjector.injectTiger(instance, getTiger())
   This method Zooassigns values ​​to the required fields in instancethe Zooobject, getTiger()creates a tiger object for the object.

public final class Zoo_MembersInjector implements MembersInjector<Zoo> {
    
    
    @InjectedFieldSignature("com.yey.dagger2.Zoo.tiger")
    public static void injectTiger(Zoo instance, Object tiger) {
    
    
        // 此处就是为zoo.tiger字段赋值的操作.
        instance.tiger = (Tiger) tiger;
    }
}