domains: Reactive domains

Description

Reactive domains are a mechanism for establishing ownership over reactive primitives (like reactive expressions and observers), even if the set of reactive primitives is dynamically created. This is useful for lifetime management (i.e. destroying observers when the Shiny session that created them ends) and error handling.

Usage

getDefaultReactiveDomain()
withReactiveDomain(domain, expr)
onReactiveDomainEnded(domain, callback, failIfNull = FALSE)

Arguments

domain

A valid domain object (for example, a Shiny session), or NULL

expr

An expression to evaluate under domain

callback

A callback function to be invoked

failIfNull

If TRUE then an error is given if the domain is NULL

Details

At any given time, there can be either a single "default" reactive domain object, or none (i.e. the reactive domain object is NULL). You can access the current default reactive domain by calling getDefaultReactiveDomain.

Unless you specify otherwise, newly created observers and reactive expressions will be assigned to the current default domain (if any). You can override this assignment by providing an explicit domain argument to reactive or observe.

For advanced usage, it's possible to override the default domain using withReactiveDomain. The domain argument will be made the default domain while expr is evaluated.

Implementers of new reactive primitives can use onReactiveDomainEnded as a convenience function for registering callbacks. If the reactive domain is NULL and failIfNull is FALSE, then the callback will never be invoked.