reactivity.Rmd

# (PART\*) Mastering reactivity {.unnumbered}

# Introduction {#reactivity-intro .unnumbered}

You now have a bundle of useful techniques under your belt, giving you the ability to create a wide range of useful apps.
Next we'll turn our attention to the theory of reactivity that underlies the magic of Shiny:

-   In Chapter \@ref(reactive-motivation) you'll learn why the reactivity programming model is needed, and a little bit about the history of reactive programming outside of R.

-   In Chapter \@ref(reactive-graph), you'll learn the full details of the reactive graph, which determines exactly when reactive components are updated.

-   In Chapter \@ref(reactivity-objects), you'll learn about the underlying building blocks, particularly observers and timed invalidation.

-   In Chapter \@ref(reactivity-components), you'll learn how to escape the constraints of the reactive graph using `reactiveVal()` and `observe()`.

You certainly don't need to understand all these details for routine development of Shiny apps.
But improving your understanding will help you write correct apps from the get-go, and when something behaves unexpectedly you can more quickly narrow in on the underlying issue.