R Shiny: Reuse lengthy computation for different output controls

Question

I have the following code in server.R:

library(shiny)

source("helpers.R")

shinyServer(function(input, output) {
    output$txtOutput1 <- renderText({ 
        someLengthyComputation(input$txtInput)[1]
    })
    output$txtOutput2 <- renderText({ 
        someLengthyComputation(input$txtInput)[2]
    })
    output$txtOutput3 <- renderText({ 
        someLengthyComputation(input$txtInput)[3]
    })
})

helpers.R contains the method someLengthyComputation which returns a vector of size 3. How can I get around calling it three times every time txtInput changes and only call it once while updating all three text output controls?

Answer 1

You can simply place someLengthyComputation inside a reactive expression:

shinyServer(function(input, output) {
    someExpensiveValue <- reactive({
        someLengthyComputation(input$txtInput)
    })

    output$txtOutput1 <- renderText({ 
        someExpensiveValue()[1]
    })

    output$txtOutput2 <- renderText({ 
        someExpensiveValue()[2]
    })

    output$txtOutput3 <- renderText({ 
        someExpensiveValue()[3]
    })
})

someLengthyComputation will be triggered only when input$txtInput changes and the first of the outputs is rendered otherwise someExpensiveValue will return a cached value.

It is also possible, although execution strategy is a little bit different, to use a combination of reactiveValues and observe.

If someLengthyComputation is really expensive you should consider adding an action button or a submit button and triggering the computations only when it is clicked, especially when you use textInput.