Mastering Shiny

Chapter 16. Escaping the Graph

Introduction

Shiny’s reactive programming framework is incredibly useful because it automatically determines the minimal set of computations needed to update all outputs when an input changes. But this framework is deliberately constraining, and sometimes you need to break free to do something risky but necessary.

In this chapter you’ll learn how you can combine reactiveValues() and observe()/observeEvent() to connect the right-hand side of the reactive graph back to the lefthand side. These techniques are powerful because they give you manual control over parts of the graph. But they’re also dangerous because they allow your app to do unnecessary work. Most importantly, you can now create infinite loops where your app gets stuck in a cycle of updates that never ends.

If you find the ideas explored in this chapter to be interesting, you might also want to look at the shinySignals and rxtools packages. These are both experimental packages designed to explore “higher order” reactivity, reactives that are created programmatically from other reactives. I wouldn’t recommend you use them in “real” apps, but reading the source code might be illuminating. To begin, we’ll load shiny:

library(shiny)

What Doesn’t the Reactive Graph Capture?

In “An Input Changes”, we discussed what happens when the user causes an input to be invalidated. There are two other important cases where you as the app author might invalidate an input:

• When you call an update function ...