anchor.diff:      This rule makes the component safe to construct under test without fear that it will do something dramatic like talk to the server.
       That's why you call the `HeroService` from within the `ngOnInit` rather than the constructor.
       
  -    强烈推荐简单的构造函数。它们应该***只***用来初始化变量。这个规则会帮助我们在测试环境中放心的构造组件，以免在构造它们时，无意做了一些非常戏剧化的动作(比如连接服务)。
  +    让构造函数保持简单。它们应该***只***用来初始化变量。这个规则会帮助我们在测试环境中放心的构造组件，以免在构造它们时，无意做了一些非常戏剧化的动作(比如连接服务)。
       这就是为什么我们要在`ngOnInit`里面调用`HeroService`，而不是在构造函数中。
   
   :marked
  
positive.code: :marked
    Angular asks the injector for the service associated with the `LoggerService`
    and assigns the returned value to the `logger` parameter.
    Angular向注入器请求与`LoggerService`对应的服务，并将返回值赋给`logger`参数。
    Where did the injector get that value?
    It may already have that value in its internal container.
    If it doesn't, it may be able to make one with the help of a ***provider***.
  
negative.code: a#class-interface-parent
  :marked
    ### Find a parent by its class-interface
    
    ### 通过类-接口找到父组件
    
    You can find a parent component with a [class-interface](#class-interface).
  
    可以通过[类-接口](#class-interface)找到一个父组件。
  