anchor.diff:  :marked
     The `@Host()` function decorating the  `heroCache` property ensures that 
     you get a reference to the cache service from the parent `HeroBioComponent`.
  -  Angular throws an errorif the parent lacks that service, even if a component higher in the component tree happens to have it.
  +  Angular throws an error if the parent lacks that service, even if a component higher in the component tree happens to have it.
     
     `@Host()`函数是`heroCache`属性的装饰器，确保从其父组件`HeroBioComponent`得到一个缓存服务。如果该父组件不存在这个服务，Angular就会抛出错误，即使组件树里的再上级有某个组件拥有这个服务，Angular也会抛出错误。
     
  
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 `provideParent()` helper function](#provideparent)
    
        [`provideParent()`助手函数](#provideparent)
  
    * [Break circularities with a forward class reference (*forwardRef*)](#forwardref)
    
  