{chains} provides validated_reactive_val(), a hybrid reactive object that acts like a shiny::reactiveVal() but with a validation expression like a shiny::reactive(). This allows for the creation of self-validating reactive values, useful for managing complex, interdependent state in ‘shiny’ applications.

In a standard Shiny app, you might have two inputs that depend on each other, like a “category” and “sub-category” selector. When the user changes the category, you need an observeEvent() to update the sub-category’s value to something valid. validated_reactive_val() and its wrappers like vrv_fct_scalar() simplify this pattern by building the validation logic directly into the reactive object itself. We also ensure that the changes happen in the correct order, and prevent observers from “seeing” the inconsistent state.

# The allowed groups depend on the selected "level".
allowed_groups <- shiny::reactive({
  if (shiny::req(input$level) == "A") {
    c("A1", "A2")
  } else {
    c("B1", "B2")
  }
})

# The group_val is validated to make sure it is always one of the allowed groups,
# returning "bad group" if not. It is also synchronized with the input$group.
group_val <- vrv_fct_scalar(
  levels = allowed_groups(),
  value = shiny::reactive(input$group),
  default = "bad group"
)

Now, any time input$level changes, group_val() will automatically re-validate its own state. When you read group_val(), you are guaranteed to get a value that is consistent with its dependencies. Also, if you set group_val to a bad value (such as group_val("C1")), it will return "bad group" anywhere that it is used.

You can install the development version of chains from GitHub with:

# install.packages("pak")
pak::pak("shinyworks/chains")

Please note that the chains project is released with a Contributor Code of Conduct. By contributing to this project, you agree to abide by its terms.