Dagger2学习笔记3

Dagger单例注入解析

最新推荐文章于 2021-12-04 15:45:31 发布

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 下面来了解@Singleton 和 @Scope

 @Scope

 @Documented

 @Retention
 (
 RUNTIME
 )

 public 
 @
 interface 
 Singleton 
 {}

 @Singleton其实就是一个@Scope，和自定义的@ActivityScope和@ApplicationScope没有区别，之所以自定义，因为@Scope是有生命周期的，他的生命周期和创建Component一样，这个后面再讲解，写看简单使用

 注入单例对象，需要Component也为单例

 @Singleton

 @Component
 (
 modules 
 = MainModule.
 class
 )

 public interface 
 MainComponent {

  
 void 
 inject
 (MainActivity activity);

}

 同样提供对象的方式也有俩种，一种是在MainModule中直接提供，还有就是构造方法中直接注入，我使用构造直接注入

 @Singleton

 public class 
 DaggerBean {

  
 private 
 String 
 name
 ;

  
 private 
 String 
 type
 ;

  
 @Inject

  
 public 
 DaggerBean
 (
 @StringName
 (
 "name"
 )String name,
 @StringName
 (
 "type"
 )String type) {

  
 this
 .
 name 
 = name;

  
 this
 .
 type 
 = type;

}

 此时在MainActivity中，多次注入将会是同一个对象

 @Inject

 DaggerBean 
 bean1
 ;

 @Inject

 DaggerBean 
 bean2
 ;

 Log.
 e
 (
 TAG
 , 
 "bean1: " 
 + 
 bean1
 );

 Log.
 e
 (
 TAG
 , 
 "bean2: " 
 + 
 bean2
 );

 E/MainActivity: bean1: com.example.daggerapp.DaggerBean@33c00a26

 E/MainActivity: bean2: 
 com.example.daggerapp.DaggerBean@33c00a26

 分析一下他的源码会发现

 this
 .
 daggerBeanProvider 
 =

  DoubleCheck.
 provider
 (DaggerBean_Factory.
 create
 (
 providerNameProvider
 , 
 providerTypeProvider
 ));

 提供daggerBean的对象使用了DoubleCheck看下他里面是使用单例的方式来提供对象的

 @SuppressWarnings
 (
 "unchecked"
 ) 
 // cast only happens when result comes from the provider

 @Override

 public 
 T 
 get
 () {

  Object result = 
 instance
 ;

  
 if 
 (result == 
 UNINITIALIZED
 ) {

  
 synchronized 
 (
 this
 ) {

  result = 
 instance
 ;

  
 if 
 (result == 
 UNINITIALIZED
 ) {

  result = 
 provider
 .
 get
 ();

 /* Get the current instance and test to see if the call to provider.get() has resulted 

 * in a recursive call. If it returns the same instance, we'll allow it, but if the

 * instances differ, throw. 

*/

 Object currentInstance = 
 instance
 ;

  
 if 
 (currentInstance != 
 UNINITIALIZED 
 && currentInstance != result) {

  
 throw new 
 IllegalStateException
 (
 "Scoped provider was invoked recursively returning "
 + 
 "different results: " 
 + currentInstance + 
 " & " 
 + result + 
 ". This is likely " 
 + 
 "due to a circular dependency."
 );

}

  
 instance 
 = result;

 /* Null out the reference to the provider. We are never going to need it again, so we

 * can make it eligible for GC. 

*/

  
 provider 
 = 
 null
 ;

}

}

}

  
 return 
 (
 T
 ) result;

}

 @Scope的生命周期
 例子如下

  E/MainActivity: bean1: com.example.daggerapp.DaggerBean@33c00a26

  E/MainActivity: bean2: com.example.daggerapp.DaggerBean@33c00a26

  E/MainActivity: bean3: com.example.daggerapp.DaggerBean@33c00a26

上面会发现，MainCompont提供注入的DaggerBean对象为同一个对象

 所有可以创建ApplicationScope，inject的时候传入Application对象，此时ApplicationScope的生命周期就是整个app的运行周期，ActivityScope同理
 