HostBindings

https://www.thecodecampus.de/blog/angular-2-use-hostbindings-set-class/
And I played a little bit with it in the sandbox,  it seems it can not only bind to class (class itself is just an attr) but also can bind a boolean value to a specific class as a switch.  And it can bind to any customized attribute.
https://stackblitz.com/edit/angular-playground-325-fn6rkg?file=app%2Fbox%2Fbox.component.ts

in case forget about the syntax of using ampersand (&) in pseudo-class :host in sass/scss
here is the reference:
https://css-tricks.com/the-sass-ampersand/

Angular observable tricks

https://medium.com/@luukgruijs/understanding-hot-vs-cold-observables-62d04cf92e03

这篇文章举例说明了hot and cold  observables的区别，简单来说，observable is the bridge between producer and observer.

producer outside is hot,

producer inside is cold.

另外，当一个component 中有多个observable 的不同订阅时，如果订阅回调里涉及到UI  binding, 有时这些binding有依赖性。这时可以依靠delay()来控制先后顺序。