Shiny in R: How to properly use observe? - r

Question

I have a problem with my code. I have 2 input files which I want to read with click of button and a numeric input which contains a filter value for the output of the table being created from the 2 files (after manipulating the data). The whole process (read files + create table + filter) right now is executed every time the user click the button. I want to do only the filter action if the input files doesn't change, because the process takes long time.
After the first click I want to do only the filtering command when the numeric input changes, unless the input files is also changed by the user.
The following code reproduces my problem:
library(shiny)
library(data.table)
server <- function(input, output, session) {
output$table1 <- renderDataTable({
input$gobtn
isolate({
infile1 <<- input$f1
infile2 <<- input$f2
if (is.null(infile1) || is.null(infile1)) {
return (NULL)
}
else {
calc()
}
})
})
calc <- function() {
inf1 <<- fread(infile1$datapath)
inf2 <<- fread(infile2$datapath)
# do some process with files data.....
my_table <- as.data.table(rbind(inf1, inf2))
setnames(my_table, c('name', 'rank'))
result <- my_table[rank > input$rank]
return(result)
}
}
ui <- basicPage(
fileInput("f1", "f1"),
fileInput("f2", "f2"),
numericInput("rank", "show rank only above :", value = 6),
actionButton("gobtn", "show"),
dataTableOutput('table1')
)
shinyApp(ui = ui, server = server)

Answer 1

The way to use reactivity is to break things into parts, so that you only need to update what is necessary. The first step in your pipeline is reading and processing the files. This seems like a good reactive: if they don't change, nothing happens, but when they change, everything that needs to be recalculated is recalculated. The next step is filtering, when the filter variable changes we want to refilter the data. Then we can just put that in the output.
server <- function(input, output, session) {
processedData <- reactive({
req(input$f1,input$f2)
inf1 <- fread(input$f1$datapath)
inf2 <- fread(input$f2$datapath)
# do some process with files data.....
my_table <- as.data.table(rbind(inf1, inf2))
setnames(my_table, c('name', 'rank'))
my_table
}
filteredData <- reactive({
req(input$rank)
processedData()[processedData()$rank > input$rank]
})
output$table1 <- renderDataTable({
input$gobtn
isolate({
filteredData()
})
})
}