注解 @Injectable 的作用：
Determines which injectors will provide the injectable, by either associating it with an @NgModule or other InjectorType, or by specifying that this injectable should be provided in one of the following injectors:
‘root’ : The application-level injector in most apps.
‘platform’ : A special singleton platform injector shared by all applications on the page.
‘any’ : Provides a unique instance in each lazy loaded module while all eagerly loaded modules share one instance.
Marking a class with @Injectable ensures that the compiler will generate the necessary metadata to create the class’s dependencies when the class is injected.
@Injectable 的含义是，当被注解的 class 被注入时，编译器会生成必要的元数据，以创建类的依赖。