Synchronize Shiny inputs across sessions

Makes Shiny inputs collaborative by synchronizing their values across all connected sessions using Automerge CRDT, without requiring an external sync server.

Usage

sync_inputs(
  session = shiny::getDefaultReactiveDomain(),
  doc_id = "default",
  include = NULL,
  exclude = NULL,
  path = NULL
)

Arguments

session: The Shiny session object. Default uses shiny::getDefaultReactiveDomain().
doc_id: Document identifier string. All sessions using the same doc_id share synchronized input state. Default is "default".
include: Optional character vector of input IDs to synchronize. If NULL (default), all eligible inputs are synchronized.
exclude: Optional character vector of input IDs to exclude from synchronization. Applied after include.
path: Optional file path for persistent state. When provided, the Automerge document is saved to this file after every change and reloaded automatically when the app restarts. Set to NULL (default) for in-memory only.

Value

A reactiveVal (logical). FALSE during normal operation, TRUE during replay. Can be ignored if replay is not used. Called primarily for side effects.

Details

Call this function once in your Shiny server logic to enable collaborative input synchronization. Scalar string, numeric, and logical input values are synchronized automatically. Complex inputs (file uploads, action buttons, data table selections, etc.) are excluded by default.

When a user changes an input, the new value is propagated to all other connected sessions. The entire app state is stored in a single Automerge document, providing automatic conflict resolution via CRDT.

Inputs with names starting with "." (Shiny internal inputs) are always excluded.

Persistence

When path is provided, the shared document is saved to disk (via automerge::am_save()) after every change. On the next app startup, the document is loaded from disk and all sessions resume from the saved state. This makes the shared input state survive R process restarts.

Note

This function synchronizes sessions within a single R process. For multi-process deployments, use an external sync server with the editor() widget instead.

Examples

if (interactive()) {
  ui <- shiny::fluidPage(
    shiny::selectInput("dist", "Distribution",
      c("Normal", "Uniform", "Exponential")),
    shiny::sliderInput("n", "Observations", 10, 500, 100),
    shiny::plotOutput("plot")
  )
  server <- function(input, output, session) {
    sync_inputs()
    output$plot <- shiny::renderPlot(
      hist(switch(input$dist,
        Normal = rnorm(input$n),
        Uniform = runif(input$n),
        Exponential = rexp(input$n)
      ))
    )
  }
  shiny::shinyApp(ui, server)
}