Mention how to freeze inputs

[hadley]

rstudio/shiny#3055

[hadley]

Probably somewhere in https://mastering-shiny.org/action-dynamic.html#updating-inputs

[hadley]

cf https://engineering-shiny.org/common-app-caveats.html

cc @malcolmbarrett

[malcolmbarrett]

Oh, that would be great to add. I didn't know about this.

[hadley]

In

ui <- fluidPage(
  dateInput("date", "choose a date"), 
  selectInput("year", "Choose a year", choices = 2010:2030)
)

server <- function(input, output, session) {
  observeEvent(input$date, {
    year <- format(input$date, "%Y")
    message("Changing year to ", year)
    updateSelectInput(inputId = "year", selected = year)
  })
  
  observeEvent(input$year, {
    date <- as.Date(ISOdate(input$year, 1, 1))
    message("Changing date to ", date)
    updateDateInput(inputId = "date", value = date)
  })
}

The key problem is that all updates processed by the browser in a single batch, after all observers have completed. This means that it since it starts in an inconsistent state it rapidly toggles back and forward. The easiest fix is to just set selected = 2021.