Dagger2使用攻略

Dagger 2 是 Square 的 Dagger 分支，是一种依赖注入框架。眼下由 Google 接手进行开发，Dagger2是使用代码自己主动生成和手写代码来实现依赖注入。据说在 Dagger 的基础上效率又提升了13%。而且相同功能强大。

1.Gradle配置

（1）须要配置apt 插件：（在project根文件夹build.gradle文件里加入例如以下代码）

dependencies {

        ...

        classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'

    }

（2）加入依赖：（在modulebuild.gradle文件里加入例如以下代码）

apply plugin: 'com.neenbedankt.android-apt'// 凝视处理

    dependencies {

        ...

        compile 'com.google.dagger:dagger:2.0.2'

        apt 'com.google.dagger:dagger-compiler:2.0.2'

        compile 'org.glassfish:javax.annotation:10.0-b28' // Java标注

    }

● 当前最新版本号是2.0.2

● 加入2、3条依赖的原因參考：Dagger2入坑指南

● 假设在项目中同一时候用了Butterknife，在Build时会报凝视冲突。

解决方法：（在modulebuild.gradle文件里加入例如以下代码）

 packagingOptions {

        exclude 'META-INF/services/javax.annotation.processing.Processor'

 }

（3）最后点击Build-->Make Project就能够開始使用Dagger2了。

2.Dagger2 经常使用注解

写了一个简单的Demo，以下依据Demo进行介绍。

Dagger2要理解必须看Dagger 2自己主动生成的代码。Build后代码在项目-->app-->build-->generated-->source-->apt-->debug文件夹下。

1.Inject

@Inject：在须要依赖的地方使用这个注解。告诉Dagger这个类或者字段须要依赖注入，这样Dagger会构造一个这个类实例来满足依赖。

1.构造器注入：首先举一个简单的样例。无參构造方法。

public class Person {

    private String name;

    private int age;

    @Inject

    public Person() {

    }

    public String getName() {

        return "Jack";

    }

    public int getAge() {

        return 15;

    }

}

这个的局限性是我们不能给这个类中的多个构造器作@Inject注解。

2.注解成员变量：

接着上面我们要使用这个实例化类。

public class MainActivity extends AppCompatActivity {

    @Inject

    Person mPerson;

    StorageComponent mStorageComponent;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        mStorageComponent = ((MyApplication)this.getApplication()).getStorageComponent();

        mStorageComponent.inject(this);//注入MainActivity

        Toast.makeText(this,mPerson.getName() + "----" +mPerson.getAge(),Toast.LENGTH_SHORT).show();

     }

 }

这里我们能够查看生成的代码：

@Generated("dagger.internal.codegen.ComponentProcessor")

public final class MainActivity_MembersInjector implements MembersInjector<MainActivity> {

  private final MembersInjector<AppCompatActivity> supertypeInjector;

  private final Provider<Person> mPersonProvider;

  public MainActivity_MembersInjector(MembersInjector<AppCompatActivity> supertypeInjector, Provider<Person> mPersonProvider) {  

    assert supertypeInjector != null;

    this.supertypeInjector = supertypeInjector;

    assert mPersonProvider != null;

    this.mPersonProvider = mPersonProvider;

  }

  @Override

  public void injectMembers(MainActivity instance) {

    if (instance == null) {

      throw new NullPointerException("Cannot inject members into a null reference");

    }

    supertypeInjector.injectMembers(instance);

    instance.mPerson = mPersonProvider.get();//这里mPersonProvider替我们实例化了Person

  }

  public static MembersInjector<MainActivity> create(MembersInjector<AppCompatActivity> supertypeInjector, Provider<Person> mPersonProvider) {

      return new MainActivity_MembersInjector(supertypeInjector, mPersonProvider);

  }

}

同一时候我们也能够了解到@Inject Person mPerson; 为什么不能使用private 。上面代码中的injectMembers 方法调用后面会说到。

3.方法注入

public class LoginActivityPresenter {

    private LoginActivity loginActivity;

    @Inject //构造方法注入

    public LoginActivityPresenter(LoginActivity loginActivity) {

        this.loginActivity = loginActivity;

    }

    @Inject //方法注入

    public void enableWatches(Watches watches) {

        watches.register(this);

    }

}

如当我们希望传入类的当前实例（this引用）到被注入的依赖中。方法注入会在构造器调用后立即被调用，所以这表示我们能够传入全然被构造的this。

2.Module

@Module：用来修饰类，表示此类的方法是用来提供依赖的，它告诉Dagger在哪里能够找到依赖。

@Module

public class StorageModule {

    private final MyApplication application;

    public StorageModule(MyApplication application) {

        this.application = application;

    }

    @Provides

    @Singleton

    SharedPreferences provideSharedPreferences(){

        return PreferenceManager.getDefaultSharedPreferences(application);

    }

}

@Provides以下说到，@Singleton 单例，使用@Singleton注解之后。对象仅仅会被初始化一次。之后的每次都会被直接注入相同的对象。@Singleton就是一个内置的作用域。

3.Provides

@Provides：在@Module 中使用。我们定义的方法用这个注解，用于告诉 Dagger 我们须要构造实例并提供依赖。

为什么要使用@Provides，由于默认情况下，Dagger满足依赖关系是通过调用构造方法得到的实例，比方上面的Person类使用。可是有时由于@Inject 的局限性。我们不能使用@Inject。比方构造方法有多个、三方库中的类我们不知道他是怎么实现的等等。比如以下代码中的SharedPreferences。我们使用@Provides 返回一个创建好的实例，这样做也显得灵活不是吗？

    @Provides

    @Singleton

    SharedPreferences provideSharedPreferences(){

        return PreferenceManager.getDefaultSharedPreferences(application);

    }

注意：

● 依照习惯 @Providers方法都会用provide作为前缀，@Module类都用Module作为后缀。

● 假设@Provides方法有參数。这个參数也要保证能够被Dagger得到（比如通过其它的@Provides方法或者@Inject注解的构造方法。）

4.Component

@Component：是@Inject和@Module的桥梁,须要列出所有的@Modules以组成该组件。

@Singleton

@Component(modules = {

        StorageModule.class ,

        ScheduleModule.class

})

public interface StorageComponent {

    Storage execute();

    void inject(MainActivity mMainActivity);

}

Dagger会依照上面接口生成一个实现类。生成类以Dagger为前缀。提供builder()来生成实例。调用方法：（由于是单例，所以这里放到了MyApplication）

public class MyApplication extends Application {

    private StorageComponent component;

    @Override

    public void onCreate() {

        super.onCreate();

        component = DaggerStorageComponent

                .builder()//调用构建类

                .storageModule(new StorageModule(this)) //传入Module

                .build();//生成实例

    }

    public StorageComponent getStorageComponent() {

        return component;

    }

}

MainActivity代码：

public class MainActivity extends AppCompatActivity {

    @Bind(R.id.button1)

    Button mButton1;

    @Bind(R.id.button2)

    Button mButton2;

    @Inject

    SharedPreferences mPreferences;//全局的SharedPreferences

    @Inject

    Person mPerson;

    StorageComponent mStorageComponent;

    private final String KEY = "Dagger 2";

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        ButterKnife.bind(this);

        mStorageComponent = ((MyApplication)this.getApplication()).getStorageComponent();

        mStorageComponent.inject(this);//注入MainActivity

        mStorageComponent.execute().storage();//运行储存操作

    }

    @OnClick({R.id.button1,R.id.button2})

    void onButtonClicked(View v) {

        switch (v.getId()) {

            case R.id.button1:

                Toast.makeText(this,mPreferences.getString(KEY,"---"),Toast.LENGTH_SHORT).show();

                //上面是演示样例获取mPreferences，实际中将SharedPreferences操作都能够封装进Storage中。例如以下

                //Toast.makeText(this,

                //mStorageComponent.execute().getStorage(),Toast.LENGTH_SHORT).show();

                break;

            case R.id.button2:

                Toast.makeText(this,

                mPerson.getName() + "----" + mPerson.getAge(),Toast.LENGTH_SHORT).show();

                break;

        }

    }

}

下来把整个流程走一遍：首先进入MyApplication 运行DaggerStorageComponent.builder().storageModule(new StorageModule(this)).build(); 方法获取实例化StorageComponent。那我我们查看DaggerStorageComponent 类：

@Generated("dagger.internal.codegen.ComponentProcessor")

public final class DaggerStorageComponent implements StorageComponent {

  private Provider<SharedPreferences> provideSharedPreferencesProvider;

  private Provider<ScheduleImpl> provideScheduleProvider;

  private Provider<Storage> storageProvider;

  private MembersInjector<MainActivity> mainActivityMembersInjector;

  private DaggerStorageComponent(Builder builder) {

    assert builder != null;

    initialize(builder);

  }

  public static Builder builder() {

    return new Builder();

  }

  private void initialize(final Builder builder) {

    this.provideSharedPreferencesProvider = ScopedProvider.create(StorageModule_ProvideSharedPreferencesFactory.create(builder.storageModule));

    this.provideScheduleProvider = ScopedProvider.create(ScheduleModule_ProvideScheduleFactory.create(builder.scheduleModule));

    this.storageProvider = Storage_Factory.create(provideSharedPreferencesProvider, provideScheduleProvider);

    this.mainActivityMembersInjector = MainActivity_MembersInjector.create((MembersInjector) MembersInjectors.noOp(), provideSharedPreferencesProvider, Person_Factory.create());//实例化到这里结束

  }

  @Override

  public Storage execute() {

    return storageProvider.get();

  }

  @Override

  public void inject(MainActivity mMainActivity) {

    mainActivityMembersInjector.injectMembers(mMainActivity);

  }

  public static final class Builder {

    private StorageModule storageModule;

    private ScheduleModule scheduleModule;

    private Builder() {

    }

    public StorageComponent build() {

      if (storageModule == null) {

        throw new IllegalStateException("storageModule must be set");

      }

      if (scheduleModule == null) {

        this.scheduleModule = new ScheduleModule();

      }

      return new DaggerStorageComponent(this);

    }

    public Builder storageModule(StorageModule storageModule) {

      if (storageModule == null) {

        throw new NullPointerException("storageModule");

      }

      this.storageModule = storageModule;

      return this;

    }

    public Builder scheduleModule(ScheduleModule scheduleModule) {

      if (scheduleModule == null) {

        throw new NullPointerException("scheduleModule");

      }

      this.scheduleModule = scheduleModule;

      return this;

    }

  }

}

上面代码最后运行到MainActivity_MembersInjector.create（…）查看MainActivity_MembersInjector类：

@Generated("dagger.internal.codegen.ComponentProcessor")

public final class MainActivity_MembersInjector implements MembersInjector<MainActivity> {

  private final MembersInjector<AppCompatActivity> supertypeInjector;

  private final Provider<SharedPreferences> mPreferencesProvider;

  private final Provider<Person> mPersonProvider;

  public MainActivity_MembersInjector(MembersInjector<AppCompatActivity> supertypeInjector, Provider<SharedPreferences> mPreferencesProvider, Provider<Person> mPersonProvider) {

    assert supertypeInjector != null;

    this.supertypeInjector = supertypeInjector;

    assert mPreferencesProvider != null;

    this.mPreferencesProvider = mPreferencesProvider;

    assert mPersonProvider != null;

    this.mPersonProvider = mPersonProvider;

  }

  @Override

  public void injectMembers(MainActivity instance) {

    if (instance == null) {

      throw new NullPointerException("Cannot inject members into a null reference");

    }

    supertypeInjector.injectMembers(instance);

    instance.mPreferences = mPreferencesProvider.get();//赋值

    instance.mPerson = mPersonProvider.get();

  }

  public static MembersInjector<MainActivity> create(MembersInjector<AppCompatActivity> supertypeInjector, Provider<SharedPreferences> mPreferencesProvider, Provider<Person> mPersonProvider) {

      return new MainActivity_MembersInjector(supertypeInjector, mPreferencesProvider, mPersonProvider);

  }

}

下来进入到了MainActivity 。在通过((MyApplication)this.getApplication()).getStorageComponent() 获取到component 后运行mStorageComponent.inject(this); 方法注入MainActivity，终于回调到上面代码中的injectMembers方法中。能够看出MainActivity中的成员变量所有初始完毕。之后就能够直接使用了。

5.Lazy 类

Lazy类是实现懒载入，调用的时候才创建实例，通过Lazy对象实现，得到对象的实例使用get()方法。比如：

public class Storage {

    private SharedPreferences mPreferences;

    private Lazy<ScheduleImpl> mScheduleImpl;//Lazy 类

    private final String KEY = "Dagger 2";

    @Inject

    public Storage(SharedPreferences mPreferences ,Lazy<ScheduleImpl> mScheduleImpl) {

        this.mPreferences = mPreferences;

        this.mScheduleImpl = mScheduleImpl;

    }

    public void storage() {

        mScheduleImpl.get().start();//get()方法

        mPreferences.edit().putString(KEY, "Dagger 2 -- Example").commit();

        mScheduleImpl.get().end();

    }

}

6.Scope

@Scope：注解作用域，通过自己定义注解限定对象的作用范围。它是JSR-330标准的一部分，事实上@Singleton就是一种@Scope。

在Dagger 2中，@Scope被用于标记自己定义的scope注解。简单说它们能够相似单例地标记依赖。

被作注解的依赖会变成单例，可是这会与component的生命周期（不是整个应用）关联。

首先创建一个LoginScope：

@Scope

@Retention(RetentionPolicy.RUNTIME)

public @interface LoginScope {

}

Module：

@Module

public class LoginModule {

    @Provides

    @LoginScope  //<---这里为单例

    Person providePerson() {

        Person mPerson = new Person();

        mPerson.setAge(23);

        mPerson.setName("WeiLu");

        return mPerson;

    }

    @Provides

    Login provideLogin() {

        Login mLogin = new Login();

        mLogin.setPassword("######");

        mLogin.setName("小关");

        return mLogin;

    }

}

Component：

@LoginScope

@Component(modules = {

        LoginModule.class

})

public interface LoginComponent {

    void inject(MyApplication myApplication);

}

调用:

mLoginComponent = DaggerLoginComponent.builder()

                .loginModule(new LoginModule())

                .build();

mLoginComponent.inject(this);

这里我们看一下生成代码:

@Generated("dagger.internal.codegen.ComponentProcessor")

public final class DaggerLoginComponent implements LoginComponent {

  private Provider<Person> providePersonProvider;

  private Provider<Login> provideLoginProvider;

  private MembersInjector<MyApplication> myApplicationMembersInjector;

  private DaggerLoginComponent(Builder builder) {

    assert builder != null;

    initialize(builder);

  }

  public static Builder builder() {

    return new Builder();

  }

  public static LoginComponent create() {

    return builder().build();

  }

  private void initialize(final Builder builder) {

    this.providePersonProvider = ScopedProvider.create(LoginModule_ProvidePersonFactory.create(builder.loginModule));

    this.provideLoginProvider = LoginModule_ProvideLoginFactory.create(builder.loginModule);

    this.myApplicationMembersInjector = MyApplication_MembersInjector.create((MembersInjector) MembersInjectors.noOp(), providePersonProvider, provideLoginProvider);

  }

  @Override

  public void inject(MyApplication myApplication) {

    myApplicationMembersInjector.injectMembers(myApplication);

  }

  public static final class Builder {

    private LoginModule loginModule;

    private Builder() {

    }

    public LoginComponent build() {

      if (loginModule == null) {

        this.loginModule = new LoginModule();

      }

      return new DaggerLoginComponent(this);

    }

    public Builder loginModule(LoginModule loginModule) {

      if (loginModule == null) {

        throw new NullPointerException("loginModule");

      }

      this.loginModule = loginModule;

      return this;

    }

  }

}

看initialize方法：没有加@LoginScope 的Login类。那么他的创建是就是利用工厂模式new了一个Login。

相反加了@LoginScope 的Person类，是利用ScopedProvider 储存了起来。源代码例如以下:

public final class ScopedProvider<T> implements Provider<T> {

    private static final Object UNINITIALIZED = new Object();

    private final Factory<T> factory;

    private volatile Object instance;

    private ScopedProvider(Factory<T> factory) {

        this.instance = UNINITIALIZED;

        assert factory != null;

        this.factory = factory;

    }

    public T get() {

        Object result = this.instance;

        if(result == UNINITIALIZED) {

            synchronized(this) {

                result = this.instance;

                if(result == UNINITIALIZED) {

                    this.instance = result = this.factory.get();

                }

            }

        }

        return result;

    }

    public static <T> Provider<T> create(Factory<T> factory) {

        if(factory == null) {

            throw new NullPointerException();

        } else {

            return new ScopedProvider(factory);

        }

    }

}

7.Qualifier

@Qualifier：限定符，当类的类型不足以鉴别一个依赖的时候会使用到。假设我们没有去区分，会报错：xxx cannot be provided without an @Provides-annotated method。比如上面的Person类，我们如今准备返回两个：小明与小关。返回的都是Person类，怎么区分依赖？

首先自己定义一个@Qualifier：

@Qualifier

@Retention(RetentionPolicy.RUNTIME)

public @interface User {

}

下来是Module：

@Module

public class UserModule {

    @Provides

    @User//加上这个自己定义注解用于区分

    Login provideXiaoMingUser() {

        Login xiaomin = new Login();

        xiaomin.setPassword("******");

        xiaomin.setName("小明");

        return xiaomin;

    }

    @Provides

    Login provideXiaoGuanUser() {

        Login xiaoguan = new Login();

        xiaoguan.setPassword("######");

        xiaoguan.setName("小关");

        return xiaoguan;

    }

}

Component：

@Subcomponent(modules = {

        UserModule.class

})

public interface UserComponent {

    void inject(SecondActivity mSecondActivity);

}

这里用到了@Subcomponent，我们想复用组件时。能够使用它。以下是父组件用法。还有一种是注解属性加入dependencies。

@Singleton

@Component(

        modules ={ AppModule.class

    }

)

public interface AppComponent {

    Context getAppContext();

    UserComponent createUserComponent(UserModule userModule);

}

这样的复用组件事实上是在在父组件中创建了子组件的内部类。例如以下：

@Generated("dagger.internal.codegen.ComponentProcessor")

public final class DaggerAppComponent implements AppComponent {

  private Provider<Context> provideContextProvider;

  private DaggerAppComponent(Builder builder) {

    assert builder != null;

    initialize(builder);

  }

  public static Builder builder() {

    return new Builder();

  }

  private void initialize(final Builder builder) {

    this.provideContextProvider = ScopedProvider.create(AppModule_ProvideContextFactory.create(builder.appModule));

  }

  @Override

  public Context getAppContext() {

    return provideContextProvider.get();

  }

  @Override

  public UserComponent createUserComponent(UserModule userModule) {

    return new UserComponentImpl(userModule);

  }

  public static final class Builder {

    private AppModule appModule;

    private Builder() {

    }

    public AppComponent build() {

      if (appModule == null) {

        throw new IllegalStateException("appModule must be set");

      }

      return new DaggerAppComponent(this);

    }

    public Builder appModule(AppModule appModule) {

      if (appModule == null) {

        throw new NullPointerException("appModule");

      }

      this.appModule = appModule;

      return this;

    }

  }

  private final class UserComponentImpl implements UserComponent {//内部类

    private final UserModule userModule;

    private Provider<Login> provideXiaoMingUserProvider;

    private Provider<Login> provideXiaoGuanUserProvider;

    private MembersInjector<SecondActivity> secondActivityMembersInjector;

    private UserComponentImpl(UserModule userModule) {

      if (userModule == null) {

        throw new NullPointerException();

      }

      this.userModule = userModule;

      initialize();

    }

    private void initialize() {

      this.provideXiaoMingUserProvider = UserModule_ProvideXiaoMingUserFactory.create(userModule);

      this.provideXiaoGuanUserProvider = UserModule_ProvideXiaoGuanUserFactory.create(userModule);

      this.secondActivityMembersInjector = SecondActivity_MembersInjector.create((MembersInjector) MembersInjectors.noOp(), provideXiaoMingUserProvider, provideXiaoGuanUserProvider);

    }

    @Override

    public void inject(SecondActivity mSecondActivity) {

      secondActivityMembersInjector.injectMembers(mSecondActivity);

    }

  }

}

初始化：（MyApplication中）

mAppComponent = DaggerAppComponent.builder()

                .appModule(new AppModule(this))

                .build();

        mUserComponent = mAppComponent.createUserComponent(new UserModule());

调用：

public class SecondActivity extends AppCompatActivity {

    UserComponent userComponent;

    @Inject

    @User

    Login xiaoming;

    @Inject

    Login xiaoguan;

    @Bind(R.id.button4)

    Button mButton4;

    @Bind(R.id.button5)

    Button mButton5;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_second);

        ButterKnife.bind(this);

        userComponent = ((MyApplication)this.getApplication()).getUserComponent();

        userComponent.inject(this);

    }

    @OnClick({

            R.id.button4,

            R.id.button5,

    })

    void onButtonClicked(View v) {

        switch (v.getId()) {

            case R.id.button4:

                Toast.makeText(this,

                xiaoming.getName() + "----" + xiaoming.getPassword(),Toast.LENGTH_SHORT).show();

                break;

            case R.id.button5:

                Toast.makeText(this,

                xiaoguan.getName() + "----" + xiaoguan.getPassword(),Toast.LENGTH_SHORT).show();

                break;

        }

    }

}