Create a reactive observer

Creates an observer from the given expression.

observe(x, env = parent.frame(), quoted = FALSE, label = NULL, suspended = FALSE, priority = 0, domain = getDefaultReactiveDomain(), autoDestroy = TRUE, ..stacktraceon = TRUE)
An expression (quoted or unquoted). Any return value will be ignored.
The parent environment for the reactive expression. By default, this is the calling environment, the same as when defining an ordinary non-reactive expression.
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().
A label for the observer, useful for debugging.
If TRUE, start the observer in a suspended state. If FALSE (the default), start in a non-suspended state.
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.
See domains.
If TRUE (the default), the observer will be automatically destroyed when its domain (if any) ends.
Advanced use only. For stack manipulation purposes; see stacktrace.

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).


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

  • observe
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()
Documentation reproduced from package shiny, version 0.14.2, License: GPL-3 | file LICENSE

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