1. providedIn property of the @Injectable() to "root".
2. Include the service in the AppModule or in a module that is only imported by the AppModule
https://angular.io/guide/singleton-services
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1.
2.
3. 在於Service，設定參數a = Math.random(); ，就可知是否為Single Service
在登入、選單裡的WebService是為同一個，因a參數 random相同。
