Debugging Tips

Set labels to help identify reactives

valA <- reactiveVal(1, label = "A")

createReactive <- function(val, label = NULL) {
  reactive(val(), label = label)
}

rxA <- createReactive(valA)
rxB <- createReactive(valA, label = "rxB")

obs <- observe(rxA() + rxB(), label = "obs")
shiny:::flushReact()

reactlog::listDependencies(obs)
#> obs
#> +-- reactive(val())
#> | `-- A
#> `-- rxB
#>   `-- A

Wrap expressions in curly braces for source references

val <- reactiveVal(1, label = "val")

rxA <- reactive(val())  # no curly braces, no source refs
rxB <- reactive({ val() })

obs <- observe({ rxA() + rxB() }, label = "obs")
shiny:::flushReact()

val(2)
reactlog::listDependencies(obs, srcref = TRUE)
#> obs*
#> +-- reactive(val())*
#> | `-- val*
#> `-- rxB* [<text>#4]
#>   `-- val*

Use observer$onInvalidate callbacks

library(shiny)
options(shiny.reactlog = TRUE)

shinyApp(
  ui = fluidPage(
    sliderInput("A", "A", 0, 5, 0),
    sliderInput("B", "B", 0, 5, 0)
  ),
  
  server = function(input, output, session) {
    rx <- reactive({
      input$A + input$B
    })

    obs <- observe({
      rx()
    })

    obs$onInvalidate(function() {
      # slightly less intrusive than placing calls inside the observer
      reactlog::traceInvalidation(obs)
      cat("\n")
      reactlog::listDependencies(obs, invalidated = TRUE)
    })
  }
)