Event handler
Respond to "event-like" reactive inputs, values, and expressions.
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 )
eventExpr |
A (quoted or unquoted) expression that represents the event;
this can be a simple reactive value like |
handlerExpr |
The expression to call whenever |
event.env |
The parent environment for |
event.quoted |
Is the |
handler.env |
The parent environment for |
handler.quoted |
Is the |
... |
Currently not used. |
label |
A label for the observer or reactive, useful for debugging. |
suspended |
If |
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 |
ignoreNULL |
Whether the action should be triggered (or value
calculated, in the case of |
ignoreInit |
If |
once |
Whether this |
valueExpr |
The expression that produces the return value of the
|
value.env |
The parent environment for |
value.quoted |
Is the |
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.
observeEvent returns an observer reference class object (see
observe()). eventReactive returns a reactive expression
object (see reactive()).
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.
## 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)
})
}
)
}Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.