Shiny: what is the difference between observeEvent and eventReactive?

Question

I read the Shiny documentation about reactive programming a few times now, but I can't properly understand the difference between observeEvent and eventReactive.

The documentation says:

Use observeEvent whenever you want to perform an action in response to an event. (Note that "recalculate a value" does not generally count as performing an action–see eventReactive for that.)

....

Use eventReactive to create a calculated value that only updates in response to an event. This is just like a normal reactive expression except it ignores all the usual invalidations that come from its reactive dependencies;

In all the situation I tried I saw no difference between using observeEvent and eventReactive (the code works just fine regardless the function I use, with no apparent impact in performances).

Can you help me figure out what is the real difference between the two? Ideally I would like a few examples showing when they are interchangeable, one when observeEvent would work but not eventReactive and vice versa.

Answer 1

It's like the difference between observe and reactive. One is intended to be run when some reactive variable is "triggered" and is meant to have side effects (observeEvent), and the other returns a reactive value and is meant to be used as a variable (eventReactive). Even in the documentation for those functions, the former is shown without being assigned to a variable (because it is intended to just produce a side effect), and the latter is shown to be assigned into a variable and used later on.