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。