Subsetting a reactive object in Shiny - r

Question

I am in the process of learning Shiny and developing a simple app. The start of the program will allow a user to import a CSV file and then apply a filter variable(s) if needed. They will only be able to use factors as filter variables at this stage. I apply the filters on an iterative basis. So, one can apply a filter based on a factor level and then apply another factor level and so on until completed.
The best application I could find of being able to subset a reactive data frame was to apply the data frame as a reactive value. This seems to work, but I am having a couple issues that I can't figure out how to resolve.
1) Given the filtering is an iterative process, I would like to keep track and print out each variable and level applied during the filtering process. The best way I could figure out was creating a global variable (<<-) and using renderText to print out the contents after hitting the apply filter button. The issue is renderText just flashes on the screen and quickly disappears. I included a print to console statement that verifies the text is being saved correctly. I believe this is happening from the filter being applied to the reactive data frame and the updating process, but I can't figure out how to stop the text from disappearing on the screen?
2) When I try to save out the reactive data frame at the end of the shiny code, I get the following error "Warning: Error in $: $ operator is invalid for atomic vectors". I tried a couple things, but don't really understand what is going on here because the object "file$dfSource" is not like a normal reactive data frame dfSource()?
The shiny app below uses iris data so its easier to use/test. I don't know if applying the data frame to a reactive value is the best way to program this or if there is an easier way to do all this - just trying to learn best approach here.
library(shiny)
allfilters <- c()
ui <- (fluidPage(
# Application title
titlePanel("Filter Data"),
# Input Forms
sidebarLayout(
sidebarPanel(
h3("Data"),
checkboxInput("selectFilter", label = "Apply Filter Variable", value = FALSE),
uiOutput("selectFilterVar"),
uiOutput("selectFilterGroup"),
helpText("Apply filter to data"),
uiOutput("selectFilterButton"),
helpText("Reset data to total"),
uiOutput("selectResetButton"),
h3("Download Data"),
helpText("Download Data"),
downloadButton("downloadData", "Download File")
),
# Output Forms
mainPanel(
tabsetPanel(
tabPanel("Summary",
h2("Output Summary"),
textOutput("ncases"),
textOutput("selectedfilters")))
)
)
))
server <- (function(input, output, session) {
data <- iris
file <- reactiveValues(dfSource = data)
## Select Filter Variable
output$selectFilterVar <- renderUI({
req(file$dfSource)
if (input$selectFilter){
selectInput("filterVar", "Select Filter Variable", multiple = FALSE, choices = sort(names(file$dfSource[, sapply(file$dfSource, is.factor), drop = FALSE])))
}
})
# Select Filter Group(s)
output$selectFilterGroup <- renderUI({
req(file$dfSource)
req(input$filterVar)
if (input$selectFilter){
selectInput("filterGroup", "Select Filter Group", multiple = TRUE, choices = sort(unique(file$dfSource[,input$filterVar])))
}
})
# Apply Filter Button
output$selectFilterButton <- renderUI({
req(file$dfSource)
if (input$selectFilter) {
actionButton("filterButton", "Apply Filter")
}
})
# Apply filter group to data
observeEvent(input$filterButton, {
temp <- file$dfSource[(file$dfSource[,input$filterVar] %in% c(input$filterGroup)),]
file$dfSource <- temp
})
# Reset Total Sample Button
output$selectResetButton <- renderUI({
req(file$dfSource)
if (input$selectFilter) {
actionButton("resetButton", "Reset Total")
}
})
# Reset data to total sample
observeEvent(input$resetButton, {
file$dfSource <- data
updateCheckboxInput(session, "selectFilter", value = FALSE)
allfilters <- NULL
})
## Summary number of cases
output$ncases <- renderText({
req(file$dfSource)
mainTitle <- paste("Number of cases =" , nrow(file$dfSource))
return(mainTitle)
})
## Capture selected filter variables in global object
testfilter <- eventReactive(input$filterButton, {
appliedfilter <- paste0(input$filterVar, "(", input$filterGroup,")")
if (is.null(allfilters)) {
allfilters <<- paste("Selected Filters:", appliedfilter)
} else {
allfilters <<- paste(allfilters, "&", appliedfilter)
}
return(allfilters)
})
# Print out filter variables in global object
output$selectedfilters <- renderText({
filteroutput <- testfilter()
print(filteroutput)
return(filteroutput)
})
## Save out case data file
output$downloadData <- downloadHandler(
filename = function() {
paste("data-", Sys.Date(), ".csv", sep="")
},
content = function(file) {
write.csv(file$dfSource, file)
}
)
})
shinyApp(ui, server)

Answer 1

1) Storing it a global variable is probably not a good idea (scope in shiny is already complicated enough!). You already have a reactiveValues object, why not use that?
This alone, however, is not enough; the problem seems to be the eventReactive - I'm not quite sure why.
This works:
# this replaces the testfilter eventReactive
observeEvent(input$filterButton, {
appliedfilter <- paste0(input$filterVar, "(", input$filterGroup,")")
if (is.null(file$allfilters)) {
file$allfilters <- paste("Selected Filters:", appliedfilter)
} else {
file$allfilters <- paste(file$allfilters, "&", appliedfilter)
}
})
# Print out filter variables in global object
output$selectedfilters <- renderText({
filteroutput <- file$allfilters
print(filteroutput)
return(filteroutput)
})
2) The error is in the content function you pass to downloadHandler. The parameter is called file, which shadows the file reactiveValues. This works:
## Save out case data file
output$downloadData <- downloadHandler(
filename = function() {
paste("data-", Sys.Date(), ".csv", sep="")
},
content = function(filetarget) {
write.csv(file$dfSource, filetarget)
}
)
PS ad 1: It might be better to store the filters, instead of storing the filtered data frame and a string listing the filters. If your users change their mind, they have to start over from the beginning, but if you store the filters you can have a table or similar that allows deleting/editing individual filters. You could just store a list of two-element vectors, then iterate though the list to filter the data.