signals: Reactive values, computed values and watchers

In termr: Modern Reactive Terminal Applications with Data Workflows

Reactive values, computed values and watchers

Description

A small reactive graph for application state. signal() holds a value, computed() derives a value from signals (lazily, cached), and watch() runs code whenever the signals it reads change. Widgets can be created from functions that read signals - label(function() ...) - and are re-rendered automatically.

Usage

signal(value = NULL, name = NULL)

computed(fn, name = NULL)

watch(fn, name = NULL)

dispose(x)

batch(expr)

peek(x)

untracked(expr)

update_signal(x, fn)

Arguments

value	Initial value of a signal.
name	Optional graph label used in diagnostics and cycle paths.
fn	Function applied to the current value.
x	A signal or computed.
expr	An expression.

Details

• Read a signal with x(), write it with x(value). Writing the same value (identical()) does nothing. Collections are replaced as a whole: items(c(items(), "new")) - changes made inside an object are not noticed.

• computed(fn) re-evaluates only when read after a dependency changed. A computed that depends on itself raises an error.

• watch(fn) runs fn now and again after its dependencies changed, in creation order. If it writes signals, the watchers depending on them run afterwards; a loop is stopped with an error. Use dispose() to stop it.

• batch(expr) defers watchers until expr is done, so several writes cause one run. Event handlers of an app run in a batch automatically.

• peek(x) reads a signal or computed without creating a dependency; untracked(expr) does the same for an expression.

• Watchers and bound widgets keep their signals' subscriber lists alive; call dispose() (or remove the widget) when they are no longer needed.

This graph is independent of the widget-level reactive() fields. It is experimental.

Value

signal() and computed() return functions (classes termr_signal / termr_computed); watch() returns a handle with a ⁠$dispose()⁠ method.

Examples

count <- signal(1)
double <- computed(function() count() * 2)
seen <- c()
w <- watch(function() seen <<- c(seen, double()))
count(2)
batch({ count(3); count(4) })
seen
dispose(w)

termr documentation built on Oct. 21, 2026, 5:13 p.m.