文章目录

一、前言
二、添加依赖
三、简单示例
四、作用域
五、自定义作用域
六、@Binds 和 @Provides
七、关于Dagger为什么在Android中要比Hilt复杂
八、inject(Activity act)的用法
十、参考链接

一、前言

Hilt是对Dagger的封装处理，这里对其进行下简单介绍。

二、添加依赖

    implementation 'com.google.dagger:dagger:2.41'kapt 'com.google.dagger:dagger-compiler:2.41'

三、简单示例

class UserRepository @Inject constructor(private val localDataSource: UserLocalDataSource,private val remoteDataSource: UserRemoteDataSource
) {fun printUserName(){localDataSource.printUserName()}
}// @Inject lets Dagger know how to create instances of these objects
class UserLocalDataSource @Inject constructor() {fun printUserName(){Log.e("YM--->","---获取用户姓名")}
}
class UserRemoteDataSource @Inject constructor() {  }

@Component
interface ApplicationGraph {// The return type  of functions inside the component interface is// what can be provided from the containerfun repository(): UserRepository
}

class DaggerActivity : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_dagger)// Create an instance of the application graphval applicationGraph: ApplicationGraph = DaggerApplicationGraph.create()// Grab an instance of UserRepository from the application graphval userRepository: UserRepository = applicationGraph.repository()userRepository.printUserName()}
}

需要注意的是，写完后需要重新build下才能生成DaggerApplicationGraph类。另外需要注意的是Dagger 在每次收到请求时都会创建 UserRepository 的新实例。所以以下两种实例对象不是一个对象


val applicationGraph: ApplicationGraph = DaggerApplicationGraph.create()val userRepository: UserRepository = applicationGraph.repository()
val userRepository2: UserRepository = applicationGraph.repository()assert(userRepository != userRepository2)

如果要是需要多个依赖项目共用一个实例的话，需要引入作用域的概念

四、作用域

修改代码为如下

@Singleton
class UserRepository @Inject constructor(private val localDataSource: UserLocalDataSource,private val remoteDataSource: UserRemoteDataSource
) {fun printUserName(){localDataSource.printUserName()}
}

@Singleton
@Component
interface ApplicationGraph {// The return type  of functions inside the component interface is// what can be provided from the containerfun repository(): UserRepository
}

五、自定义作用域

暂时不知道自定义作用域的目的，这里仅仅做个记录
将代码修改为以下可以实现同一目的

@Scope
@MustBeDocumented
@Retention(value = AnnotationRetention.RUNTIME)
annotation class MyCustomScope

@MyCustomScope
@Component
interface ApplicationGraph {// The return type  of functions inside the component interface is// what can be provided from the containerfun repository(): UserRepository
}

@MyCustomScope
//@Singleton
class UserRepository @Inject constructor(private val localDataSource: UserLocalDataSource,private val remoteDataSource: UserRemoteDataSource
) {fun printUserName(){localDataSource.printUserName()}
}

六、@Binds 和 @Provides

dagger的使用和Hilt的用法一直，这里不再记录，详情参考Hilt一章

七、关于Dagger为什么在Android中要比Hilt复杂

为什么Dagger在Android中会比Hilt用起来复杂？这是因为通常来说，Dagger生成对象需要通过构造函数来生成，但是由于某些 Android 框架类（如 Activity 和 Fragment）由系统实例化，因此 Dagger 无法为您创建这些类。因此生成起来就比较麻烦的多。所以对于这些类必须使用字段注入。举个例子，假设我们想在一个类里面使用Activit对象，我们是没有办法通过构造函数的方式生成这个对象的,比如如下代码


@Inject lateinit var act: Activity

虽然定义了Activity,但是却没有办法去生成它。
详情参考dagger官网

八、inject(Activity act)的用法

通常的方式是通过一个 inject()函数进行绑定，代码如下

class UserViewModel @Inject constructor(){fun test(){Log.e("YM","======")}
}

@Component
interface UserComponent{fun inject(activity: DaggerActivity)
}

class DaggerActivity : AppCompatActivity() {@Inject lateinit var useViewModel: UserViewModeloverride fun onCreate(savedInstanceState: Bundle?) {DaggerUserComponent.create().inject(this)super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)useViewModel.test()}
}

如果不使用fun inject(activity: DaggerActivity)进行绑定，那么使用useViewModel时候就会出现未初始化的问题。这里可以查看下一个生成的代码

@DaggerGenerated
@SuppressWarnings({"unchecked","rawtypes"
})
public final class DaggerUserComponent implements UserComponent {private final DaggerUserComponent userComponent = this;private DaggerUserComponent() {}public static Builder builder() {return new Builder();}public static UserComponent create() {return new Builder().build();}@Overridepublic void inject(DaggerActivity activity) {injectDaggerActivity(activity);}private DaggerActivity injectDaggerActivity(DaggerActivity instance) {DaggerActivity_MembersInjector.injectUseViewModel(instance, new UserViewModel());return instance;}public static final class Builder {private Builder() {}public UserComponent build() {return new DaggerUserComponent();}}
}

@QualifierMetadata
@DaggerGenerated
@SuppressWarnings({"unchecked","rawtypes"
})
public final class DaggerActivity_MembersInjector implements MembersInjector<DaggerActivity> {private final Provider<UserViewModel> useViewModelProvider;public DaggerActivity_MembersInjector(Provider<UserViewModel> useViewModelProvider) {this.useViewModelProvider = useViewModelProvider;}public static MembersInjector<DaggerActivity> create(Provider<UserViewModel> useViewModelProvider) {return new DaggerActivity_MembersInjector(useViewModelProvider);}@Overridepublic void injectMembers(DaggerActivity instance) {injectUseViewModel(instance, useViewModelProvider.get());}@InjectedFieldSignature("com.dagger.application.dagger.DaggerActivity.useViewModel")public static void injectUseViewModel(DaggerActivity instance, UserViewModel useViewModel) {instance.useViewModel = useViewModel;}
}

通过生成的代码可以看出是关联过程的。

九、作用域和子组件
这一章节不算复杂，这里将代码贴出作为记录

// @Inject lets Dagger know how to create instances of this object
@Singleton
class UserRepository @Inject constructor(private val localDataSource: UserLocalDataSource,private val remoteDataSource: UserRemoteDataSource
) {fun printUserName(){//        localDataSource.printUserName()remoteDataSource.printUserName()}
}// @Inject lets Dagger know how to create instances of these objects
class UserLocalDataSource @Inject constructor() {fun printUserName(){Log.e("YM--->","---获取用户姓名")}
}
class UserRemoteDataSource @Inject constructor( private val loginService: LoginRetrofitService) {fun printUserName(){Log.e("YM--->","---remote")}
}class LoginRetrofitService @Inject constructor(){}

@Module
class NetworkModule {// @Provides tell Dagger how to create instances of the type that this function// returns (i.e. LoginRetrofitService).// Function parameters are the dependencies of this type.@Providesfun provideLoginRetrofitService(): LoginRetrofitService {// Whenever Dagger needs to provide an instance of type LoginRetrofitService,// this code (the one inside the @Provides method) is run.return LoginRetrofitService()}
}// The "subcomponents" attribute in the @Module annotation tells Dagger what
// Subcomponents are children of the Component this module is included in.
@Module(subcomponents = [LoginComponent::class])
class SubcomponentsModule {}

@ActivityScope
class LoginViewModel @Inject constructor(private val userRepository: UserRepository) {fun toast(){Log.e("YM","======")}
}

@Singleton
@Component(modules = [NetworkModule::class, SubcomponentsModule::class])
interface ApplicationGraph {// The return type  of functions inside the component interface is// what can be provided from the containerfun repository(): UserRepository
//    fun inject(activity: Application)// This function exposes the LoginComponent Factory out of the graph so consumers
// can use it to obtain new instances of LoginComponentfun loginComponent(): LoginComponent.Factory
}// Definition of a custom scope called ActivityScope
@Scope
@Retention(value = AnnotationRetention.RUNTIME)
annotation class ActivityScope// Classes annotated with @ActivityScope are scoped to the graph and the same
// instance of that type is provided every time the type is requested.
@ActivityScope
@Subcomponent
interface LoginComponent {@Subcomponent.Factoryinterface Factory {fun create(): LoginComponent}fun inject(activity: DaggerActivity)//    fun activity(): DaggerActivity}

@Singleton
@Component(modules = [NetworkModule::class, SubcomponentsModule::class])
interface ApplicationGraph {// The return type  of functions inside the component interface is// what can be provided from the containerfun repository(): UserRepository
//    fun inject(activity: Application)// This function exposes the LoginComponent Factory out of the graph so consumers
// can use it to obtain new instances of LoginComponentfun loginComponent(): LoginComponent.Factory
}// Definition of a custom scope called ActivityScope
@Scope
@Retention(value = AnnotationRetention.RUNTIME)
annotation class ActivityScope// Classes annotated with @ActivityScope are scoped to the graph and the same
// instance of that type is provided every time the type is requested.
@ActivityScope
@Subcomponent
interface LoginComponent {@Subcomponent.Factoryinterface Factory {fun create(): LoginComponent}fun inject(activity: DaggerActivity)//    fun activity(): DaggerActivity}

class DaggerApp: Application() {val appComponent = DaggerApplicationGraph.create()
}

class DaggerActivity : AppCompatActivity() {@Injectlateinit var loginViewModel: LoginViewModellateinit var loginComponent: LoginComponentoverride fun onCreate(savedInstanceState: Bundle?) {val applicationGraph: ApplicationGraph = (applicationContext as DaggerApp).appComponentapplicationGraph.repository().printUserName()loginComponent = applicationGraph.loginComponent().create()// Make Dagger instantiate @Inject fields in LoginActivityloginComponent.inject(this)super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)loginViewModel.toast()}
}

十、参考链接

Dagger 基础知识

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