1. 在 gradle 中添加依赖

在主项目的 build.gradle 中添加 Dagger2 库的依赖

dependencies {
    compile 'com.google.dagger:dagger:2.16'
    annotationProcessor 'com.google.dagger:dagger-compiler:2.16'
}

2. Dagger2 最简单用法, 手工赋值

Demo 中只有两个简单的类 SimpleActivity 和 UserServer, SimpleActivity 有个 UserServer 类型的 server 变量, 通过添加 @Inject 注解为其赋值:

public class SimpleActivity extends AppCompatActivity {

    @Inject
    UserServer server;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

public class UserServer {

    @Inject
    public UserServer() {
    }
}

Android Studio 编译后 Dagger2 库在 DaggerTest/app/build/generated/source/apt/debug 路径下生成了两个辅助类

import dagger.MembersInjector;
import javax.inject.Provider;

public final class SimpleActivity_MembersInjector implements MembersInjector<SimpleActivity> {
  private final Provider<UserServer> serverProvider;

  public SimpleActivity_MembersInjector(Provider<UserServer> serverProvider) {
    this.serverProvider = serverProvider;
  }

  public static MembersInjector<SimpleActivity> create(Provider<UserServer> serverProvider) {
    return new SimpleActivity_MembersInjector(serverProvider);
  }

  @Override
  public void injectMembers(SimpleActivity instance) {
    injectServer(instance, serverProvider.get());
  }

  public static void injectServer(SimpleActivity instance, UserServer server) {
    instance.server = server;
  }
}

import dagger.internal.Factory;

public final class UserServer_Factory implements Factory<UserServer> {
  private static final UserServer_Factory INSTANCE = new UserServer_Factory();

  @Override
  public UserServer get() {
    return provideInstance();
  }

  public static UserServer provideInstance() {
    return new UserServer();
  }

  public static UserServer_Factory create() {
    return INSTANCE;
  }

  public static UserServer newUserServer() {
    return new UserServer();
  }
}

SimpleActivity_MembersInjector 类中的 injectMembers 方法实现了为 server 变量赋值. UserServer_Factory 类正如其名, 是 UserServer 的一个工厂类, 它实现了 Factory 接口, 查看源码发现 Factory 接口是一个空接口, 它继承了 Provider 接口:

public interface Factory<T> extends Provider<T> {
}

public interface Provider<T> {
    T get();
}

然后在 SimpleActivity 类中就可以调用自动生成的两个类去给 UserServer 变量赋值:

public class SimpleActivity extends AppCompatActivity {

    @Inject
    UserServer server;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Provider<UserServer> serverProvider = UserServer_Factory.create();
        SimpleActivity_MembersInjector.create(serverProvider).injectMembers(this);
    }
}

由于写的 UserServer 类很简单, 只有一个无参的构造方法, 所以自动生成的 UserServer_Factory 类也很简单, 给 UserServer 类赋值也很简单.
下面还有更简单的方法.

3. 引入 @Component 注解

我们再定义一个接口, 接口实现很简单, 重点是在类定义上的@Component注解

import dagger.Component;

@Component
public interface SimpleActivityComponent {
    void inject(SimpleActivity activity);
}

编译后依然在 DaggerTest/app/build/generated/source/apt/debug 目录下会再生成一个辅助类:

public final class DaggerSimpleActivityComponent implements SimpleActivityComponent {
  private DaggerSimpleActivityComponent(Builder builder) {}

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

  public static SimpleActivityComponent create() {
    return new Builder().build();
  }

  @Override
  public void inject(SimpleActivity activity) {
    injectSimpleActivity(activity);
  }

  private SimpleActivity injectSimpleActivity(SimpleActivity instance) {
    SimpleActivity_MembersInjector.injectServer(instance, new UserServer());
    return instance;
  }

  public static final class Builder {
    private Builder() {}

    public SimpleActivityComponent build() {
      return new DaggerSimpleActivityComponent(this);
    }
  }
}

DaggerSimpleActivityComponent 的 injectSimpleActivity 方法调用了 SimpleActivity_MembersInjector 类的 injectServer 方法, 直接 new 了一个 UserServer 对象, 没有用到前面提到的 UserServer_Factory 类.

这时在 SimpleActivity 类中再给 UserServer 变量赋值就可以改成如下:

public class SimpleActivity extends AppCompatActivity {

    @Inject
    UserServer server;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 手工注入
        Provider<UserServer> serverProvider = UserServer_Factory.create();
        SimpleActivity_MembersInjector.create(serverProvider).injectMembers(this);

        // 通过 @Component 注解生成 DaggerSimpleActivityComponent 类来注入
        DaggerSimpleActivityComponent.create().inject(this);
    }
}

如果 SimpleActivity 中有多个变量需要注入, 通过这种方法也只需要一行, 不用和各个需要注入的类打交道.