Event handler — observeEvent

observeEvent(
  eventExpr,
  handlerExpr,
  event.env = parent.frame(),
  event.quoted = FALSE,
  handler.env = parent.frame(),
  handler.quoted = FALSE,
  ...,
  label = NULL,
  suspended = FALSE,
  priority = 0,
  domain = getDefaultReactiveDomain(),
  autoDestroy = TRUE,
  ignoreNULL = TRUE,
  ignoreInit = FALSE,
  once = FALSE
)

eventReactive(
  eventExpr,
  valueExpr,
  event.env = parent.frame(),
  event.quoted = FALSE,
  value.env = parent.frame(),
  value.quoted = FALSE,
  ...,
  label = NULL,
  domain = getDefaultReactiveDomain(),
  ignoreNULL = TRUE,
  ignoreInit = FALSE
)

Arguments

eventExpr	A (quoted or unquoted) expression that represents the event; this can be a simple reactive value like input$click, a call to a reactive expression like dataset(), or even a complex expression inside curly braces
handlerExpr	The expression to call whenever eventExpr is invalidated. This should be a side-effect-producing action (the return value will be ignored). It will be executed within an isolate() scope.
event.env	The parent environment for eventExpr. By default, this is the calling environment.
event.quoted	Is the eventExpr expression quoted? By default, this is FALSE. This is useful when you want to use an expression that is stored in a variable; to do so, it must be quoted with quote().
handler.env	The parent environment for handlerExpr. By default, this is the calling environment.
handler.quoted	Is the handlerExpr expression quoted? By default, this is FALSE. This is useful when you want to use an expression that is stored in a variable; to do so, it must be quoted with quote().
...	Currently not used.
label	A label for the observer or reactive, useful for debugging.
suspended	If TRUE, start the observer in a suspended state. If FALSE (the default), start in a non-suspended state.
priority	An integer or numeric that controls the priority with which this observer should be executed. An observer with a given priority level will always execute sooner than all observers with a lower priority level. Positive, negative, and zero values are allowed.
domain	See domains.
autoDestroy	If TRUE (the default), the observer will be automatically destroyed when its domain (if any) ends.
ignoreNULL	Whether the action should be triggered (or value calculated, in the case of eventReactive) when the input is NULL. See Details.
ignoreInit	If TRUE, then, when this observeEvent is first created/initialized, ignore the handlerExpr (the second argument), whether it is otherwise supposed to run or not. The default is FALSE. See Details.
once	Whether this observeEvent should be immediately destroyed after the first time that the code in handlerExpr is run. This pattern is useful when you want to subscribe to a event that should only happen once.
valueExpr	The expression that produces the return value of the eventReactive. It will be executed within an isolate() scope.
value.env	The parent environment for valueExpr. By default, this is the calling environment.
value.quoted	Is the valueExpr expression quoted? By default, this is FALSE. This is useful when you want to use an expression that is stored in a variable; to do so, it must be quoted with quote().

Description

Respond to "event-like" reactive inputs, values, and expressions.

Value

observeEvent returns an observer reference class object (see observe()). eventReactive returns a reactive expression object (see reactive()).

Details

Shiny's reactive programming framework is primarily designed for calculated values (reactive expressions) and side-effect-causing actions (observers) that respond to any of their inputs changing. That's often what is desired in Shiny apps, but not always: sometimes you want to wait for a specific action to be taken from the user, like clicking an actionButton(), before calculating an expression or taking an action. A reactive value or expression that is used to trigger other calculations in this way is called an event.

These situations demand a more imperative, "event handling" style of programming that is possible--but not particularly intuitive--using the reactive programming primitives observe() and isolate(). observeEvent and eventReactive provide straightforward APIs for event handling that wrap observe and isolate.

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.) The first argument is the event you want to respond to, and the second argument is a function that should be called whenever the event occurs.

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; it only invalidates in response to the given event.

ignoreNULL and ignoreInit

Both observeEvent and eventReactive take an ignoreNULL parameter that affects behavior when the eventExpr evaluates to NULL (or in the special case of an actionButton(), 0). In these cases, if ignoreNULL is TRUE, then an observeEvent will not execute and an eventReactive will raise a silent validation error. This is useful behavior if you don't want to do the action or calculation when your app first starts, but wait for the user to initiate the action first (like a "Submit" button); whereas ignoreNULL=FALSE is desirable if you want to initially perform the action/calculation and just let the user re-initiate it (like a "Recalculate" button).

Likewise, both observeEvent and eventReactive also take in an ignoreInit argument. By default, both of these will run right when they are created (except if, at that moment, eventExpr evaluates to NULL and ignoreNULL is TRUE). But when responding to a click of an action button, it may often be useful to set ignoreInit to TRUE. For example, if you're setting up an observeEvent for a dynamically created button, then ignoreInit = TRUE will guarantee that the action (in handlerExpr) will only be triggered when the button is actually clicked, instead of also being triggered when it is created/initialized. Similarly, if you're setting up an eventReactive that responds to a dynamically created button used to refresh some data (then returned by that eventReactive), then you should use eventReactive([...], ignoreInit = TRUE) if you want to let the user decide if/when they want to refresh the data (since, depending on the app, this may be a computationally expensive operation).

Even though ignoreNULL and ignoreInit can be used for similar purposes they are independent from one another. Here's the result of combining these:

ignoreNULL = TRUE and ignoreInit = FALSE

This is the default. This combination means that handlerExpr/ valueExpr will run every time that eventExpr is not NULL. If, at the time of the creation of the observeEvent/eventReactive, eventExpr happens to not be NULL, then the code runs.

ignoreNULL = FALSE and ignoreInit = FALSE

This combination means that handlerExpr/valueExpr will run every time no matter what.

ignoreNULL = FALSE and ignoreInit = TRUE

This combination means that handlerExpr/valueExpr will not run when the observeEvent/eventReactive is created (because ignoreInit = TRUE), but it will run every other time.

ignoreNULL = TRUE and ignoreInit = TRUE

This combination means that handlerExpr/valueExpr will not run when the observeEvent/eventReactive is created (because ignoreInit = TRUE). After that, handlerExpr/valueExpr will run every time that eventExpr is not NULL.

See also

actionButton()

Examples

## Only run this example in interactive R sessions
if (interactive()) {

  ## App 1: Sample usage
  shinyApp(
    ui = fluidPage(
      column(4,
        numericInput("x", "Value", 5),
        br(),
        actionButton("button", "Show")
      ),
      column(8, tableOutput("table"))
    ),
    server = function(input, output) {
      # Take an action every time button is pressed;
      # here, we just print a message to the console
      observeEvent(input$button, {
        cat("Showing", input$x, "rows\n")
      })
      # Take a reactive dependency on input$button, but
      # not on any of the stuff inside the function
      df <- eventReactive(input$button, {
        head(cars, input$x)
      })
      output$table <- renderTable({
        df()
      })
    }
  )

  ## App 2: Using `once`
  shinyApp(
    ui = basicPage( actionButton("go", "Go")),
    server = function(input, output, session) {
      observeEvent(input$go, {
        print(paste("This will only be printed once; all",
              "subsequent button clicks won't do anything"))
      }, once = TRUE)
    }
  )

  ## App 3: Using `ignoreInit` and `once`
  shinyApp(
    ui = basicPage(actionButton("go", "Go")),
    server = function(input, output, session) {
      observeEvent(input$go, {
        insertUI("#go", "afterEnd",
                 actionButton("dynamic", "click to remove"))

        # set up an observer that depends on the dynamic
        # input, so that it doesn't run when the input is
        # created, and only runs once after that (since
        # the side effect is remove the input from the DOM)
        observeEvent(input$dynamic, {
          removeUI("#dynamic")
        }, ignoreInit = TRUE, once = TRUE)
      })
    }
  )
}