stackoverflow上的讨论：https://stackoverflow.com/questions/49184754/tap-vs-subscribe-to-set-a-class-property
在tap实现源代码里的注释解释了tap的用法：
This operator is useful for debugging your Observables for the correct values or performing other side effects. Note: this is different to a subscribe on the Observable. If the Observable returned by do is not subscribed, the side effects specified by the Observer will never happen. do therefore simply spies on existing execution, it does not trigger an execution to happen like subscribe does.
In general, favor the subscribe block for running side effects, use tap for debugging, but be aware that tap can do more if you need it to.
如果需要编写具有side effect改变的代码块，放在subscribe里；出于调试目的打印信息，放在tap里，但要记住，理论上subscribe能做的，tap也同样可以。
一位Angular老司机的观点：
subscribe is ran when the stream is ready for it
so that’s why you have all of the operators in rxjs
so if you have () => {} in your subscribe, then you are waiting for the “final” data
but with tap(), you can do much more work inbetween
汪子熙 认证博客专家 前端框架 Node.js SAP JerryWang，2007年从电子科技大学计算机专业硕士毕业后加入SAP成都研究院工作至今。Jerry是SAP社区导师，SAP中国技术大使。2020年5月下旬，Jerry做了脑部肿瘤的开颅切除手术，对编程和人生又有了新的感悟。