R Shiny reactivity

Reactivity is one of the most important and confusing concepts of R Shiny Programming. This article is going to explain the definition and different between:

1. Reactive : define reactive expression that is impacted by input
2. Observe : similar with print function, which allow the user to observe what is happening inside the app
3. EventReactive: define reactive expression that is impacted by specific input
4. ObserveEvent: similar with Eventreactive, while it will not return any value, nor the value could be accessed out side of…