Dagger2使用攻略

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Dagger2使用攻略

Dagger 2 是 Square 的 Dagger 分支,是一种依赖注入框架。目前由 Google 接手进行开发,Dagger2是使用代码自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%,并且同样功能强大。

1.Gradle配置

(1)需要配置apt 插件:(在project根目录build.gradle文件中添加如下代码)

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

(2)添加依赖:(在modulebuild.gradle文件中添加如下代码)

apply plugin: 'com.neenbedankt.android-apt'// 注释处理

    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
    }

● 当前最新版本是2.0.2

● 添加2、3条依赖的原因参考:Dagger2入坑指南

● 如果在项目中同时用了Butterknife,在Build时会报注释冲突。
这里写图片描述

解决方法:(在modulebuild.gradle文件中添加如下代码)

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

(3)最后点击Build-->Make Project就可以开始使用Dagger2了。

2.Dagger2 常用注解

写了一个简单的Demo,下面根据Demo进行介绍。Dagger2要理解必须看Dagger 2自动生成的代码,Build后代码在项目-->app-->build-->generated-->source-->apt-->debug目录下。

1.Inject

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

1.构造器注入:首先举一个简单的例子,无参构造方法。

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();
     }
 }

这里我们可以查看生成的代码:

@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);
  }
}

同时我们也可以了解到@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马上被调用,所以这表示我们可以传入完全被构造的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在哪里可以找到依赖。

@Module
public class StorageModule {

    private final MyApplication application;

    public StorageModule(MyApplication application) {
        this.application = application;
    }

    @Provides
    @Singleton
    SharedPreferences provideSharedPreferences(){
        return PreferenceManager.getDefaultSharedPreferences(application);
    }

}

@Provides下面说到,@Singleton 单例,使用@Singleton注解之后,对象只会被初始化一次,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 我们需要构造实例并提供依赖。

为什么要使用@Provides,因为默认情况下,Dagger满足依赖关系是通过调用构造方法得到的实例,比如上面的Person类使用。但是有时因为@Inject 的局限性,我们不能使用@Inject。比如构造方法有多个、三方库中的类我们不知道他是怎么实现的等等。例如下面代码中的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:

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

● 如果@Provides方法有参数,这个参数也要保证能够被Dagger得到(例如通过其他的@Provides方法或者@Inject注解的构造方法。)

4.Component

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

@Singleton
@Component(modules = {
        StorageModule.class ,
        ScheduleModule.class
})
public interface StorageComponent {

    Storage execute();
    void inject(MainActivity mMainActivity);
}

Dagger会按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:(因为是单例,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();//执行储存操作

    }

    @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();
                //上面是示例获取mPreferences,实际中将SharedPreferences操作都可以封装进Storage中,如下
                //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 执行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;
    }
  }
}

上面代码最后执行到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 后执行mStorageComponent.inject(this); 方法注入MainActivity,最终回调到上面代码中的injectMembers方法中,可以看出MainActivity中的成员变量全部初始完成。之后就可以直接使用了。

5.Lazy 类

Lazy类是实现懒加载,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。例如:

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,通过自定义注解限定对象的作用范围。它是JSR-330标准的一部分,其实@Singleton就是一种@Scope。在Dagger 2中,@Scope被用于标记自定义的scope注解。简单说它们可以类似单例地标记依赖。被作注解的依赖会变成单例,但是这会与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;
    }
  }
}

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。源码如下:

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。如果我们没有去区分,会报错:xxx cannot be provided without an @Provides-annotated method。例如上面的Person类,我们现在准备返回两个: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,可以使用它,下面是父组件使用方法。另一种是注解属性添加dependencies。

@Singleton
@Component(
        modules ={ AppModule.class
    }
)
public interface AppComponent {

    Context getAppContext();
    UserComponent createUserComponent(UserModule userModule);
}

这种复用组件其实是在在父组件中创建了子组件的内部类。如下:

@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;
        }
    }
}

具体生成的代码,大家下载Demo后自行查看。

通过自动生成的代码可以看出Dagger 2主要用到了Builder模式、Factory模式,代码不难理解。同时因为Dagger 2没有使用反射,虽然效率提高了,但是缺乏灵活性。这也是为了提高效率的代价。

3.Dagger2 练习Demo

Demo下载链接:Dagger2Example

这里写图片描述

4.参考

1. Dagger 2 API文档

2. Android Dagger2学习

3. 使用Dagger 2依赖注入 - API

4. Dependency injection with Dagger 2 - Custom scopes


PS:最后说一下,关于Dagger2的学习成本还是挺高的。我自己也是从零开始接触,利用业余时间前前后后用了近一周时间去学习,一开始看的也是云里雾里。其实对照着自动生成的代码多看看就比较好理解了。有什么错误地方,希望多多交流。就这样。。。

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