directive: 目的是给DOM元素添加自定义行为，可以和Component class一样，实现life cycle hooks：
css selector：
在template中唯一标识一个Directive，并且触发Directive实例的初始化。
you cant use input/output outside component / directive tag
正确做法？
in notification-message.component.html, it’s directly used in div tag
when I ctrl + click on line 3 above, it goes to line 12 in focus.directive.ts.
so it seems the name of selector and input are kept the same ON PURPOSE?
I think for small directives (single responsibility and single input) its simplified usage
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版权声明：本文为CSDN博主「汪子熙」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：
https://blog.csdn.net/i042416/article/details/113334525