The concept of reactivity

Reactivity is the main concept that underlies the Shiny structure. Basically, this means that an object changes depending on the changes of another object. This concept is intimately related to the input/output relationship. In other words, the output object reacts (changes) whenever the input changes.

For example, let's think of an application that counts the number of words in the phrase that has been passed. It would look similar to this:

In this case, whenever the phrase passed to the textbox changes, the text below the textbox changes. The reaction process automatically and instantly occurs by default. This means that, unless explicitly told otherwise, the re-execution will take place immediately. For example, if the phrases change, an order is automatically triggered to re-execute the script in server.R and consequently, to change the text (that is, the output). To sum it up, the reactive process starts when the input changes and finishes when the output is...