Create a reactive observer — observe

observe(
  x,
  env = parent.frame(),
  quoted = FALSE,
  ...,
  label = NULL,
  suspended = FALSE,
  priority = 0,
  domain = getDefaultReactiveDomain(),
  autoDestroy = TRUE,
  ..stacktraceon = TRUE
)

Arguments

x	An expression (quoted or unquoted). Any return value will be ignored.
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.
quoted	Is the 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()`.
...	Not used.
label	A label for the observer, 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. A higher value means higher priority: an observer with a higher priority value will execute before all observers with lower priority values. 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.
..stacktraceon	Advanced use only. For stack manipulation purposes; see `stacktrace()`.

Description

Creates an observer from the given expression.

Value

An observer reference class object. This object has the following methods:

suspend(): Causes this observer to stop scheduling flushes (re-executions) in response to invalidations. If the observer was invalidated prior to this call but it has not re-executed yet then that re-execution will still occur, because the flush is already scheduled.
resume(): Causes this observer to start re-executing in response to invalidations. If the observer was invalidated while suspended, then it will schedule itself for re-execution.
destroy(): Stops the observer from executing ever again, even if it is currently scheduled for re-execution.
setPriority(priority = 0): Change this observer's priority. Note that if the observer is currently invalidated, then the change in priority will not take effect until the next invalidation--unless the observer is also currently suspended, in which case the priority change will be effective upon resume.
setAutoDestroy(autoDestroy): Sets whether this observer should be automatically destroyed when its domain (if any) ends. If autoDestroy is TRUE and the domain already ended, then destroy() is called immediately."
onInvalidate(callback): Register a callback function to run when this observer is invalidated. No arguments will be provided to the callback function when it is invoked.

Details

An observer is like a reactive expression in that it can read reactive values and call reactive expressions, and will automatically re-execute when those dependencies change. But unlike reactive expressions, it doesn't yield a result and can't be used as an input to other reactive expressions. Thus, observers are only useful for their side effects (for example, performing I/O).

Another contrast between reactive expressions and observers is their execution strategy. Reactive expressions use lazy evaluation; that is, when their dependencies change, they don't re-execute right away but rather wait until they are called by someone else. Indeed, if they are not called then they will never re-execute. In contrast, observers use eager evaluation; as soon as their dependencies change, they schedule themselves to re-execute.

Starting with Shiny 0.10.0, observers are automatically destroyed by default when the domain that owns them ends (e.g. when a Shiny session ends).

Examples

values <- reactiveValues(A=1)

obsB <- observe({
  print(values$A + 1)
})

# Can use quoted expressions
obsC <- observe(quote({ print(values$A + 2) }), quoted = TRUE)

# To store expressions for later conversion to observe, use quote()
expr_q <- quote({ print(values$A + 3) })
obsD <- observe(expr_q, quoted = TRUE)

# In a normal Shiny app, the web client will trigger flush events. If you
# are at the console, you can force a flush with flushReact()
shiny:::flushReact()