reactive: Create a reactive expression

In rstudio/shiny: Web Application Framework for R

Create a reactive expression

Description

Wraps a normal expression to create a reactive expression. Conceptually, a reactive expression is a expression whose result will change over time.

Usage

reactive(
  x,
  env = parent.frame(),
  quoted = FALSE,
  ...,
  label = NULL,
  domain = getDefaultReactiveDomain(),
  ..stacktraceon = TRUE
)

is.reactive(x)

Arguments

x	For is.reactive(), an object to test. For reactive(), an expression. When passing in a rlang::quo()sure with reactive(), remember to use rlang::inject() to distinguish that you are passing in the content of your quosure, not the expression of the quosure.
env	The parent environment for the reactive expression. By default, this is the calling environment, the same as when defining an ordinary non-reactive expression. If x is a quosure and quoted is TRUE, then env is ignored.
quoted	If it is TRUE, then the quote()ed value of x will be used when x is evaluated. If x is a quosure and you would like to use its expression as a value for x, then you must set quoted to TRUE.
...	Not used.
label	A label for the reactive expression, useful for debugging.
domain	See domains.
..stacktraceon	Advanced use only. For stack manipulation purposes; see stacktrace().

Details

Reactive expressions are expressions that can read reactive values and call other reactive expressions. Whenever a reactive value changes, any reactive expressions that depended on it are marked as "invalidated" and will automatically re-execute if necessary. If a reactive expression is marked as invalidated, any other reactive expressions that recently called it are also marked as invalidated. In this way, invalidations ripple through the expressions that depend on each other.

See the Shiny tutorial for more information about reactive expressions.

Value

a function, wrapped in a S3 class "reactive"

Examples

library(rlang)
values <- reactiveValues(A=1)

reactiveB <- reactive({
  values$A + 1
})
# View the values from the R console with isolate()
isolate(reactiveB())
# 2

# To store expressions for later conversion to reactive, use quote()
myquo <- rlang::quo(values$A + 2)
# Unexpected value! Sending a quosure directly will not work as expected.
reactiveC <- reactive(myquo)
# We'd hope for `3`, but instead we get the quosure that was supplied.
isolate(reactiveC())

# Instead, the quosure should be `rlang::inject()`ed
reactiveD <- rlang::inject(reactive(!!myquo))
isolate(reactiveD())
# 3

# (Legacy) Can use quoted expressions
expr <- quote({ values$A + 3 })
reactiveE <- reactive(expr, quoted = TRUE)
isolate(reactiveE())
# 4

rstudio/shiny documentation built on Nov. 8, 2024, 6 a.m.