Display a subset of a data frame in a Shiny app - shiny

Question

New to Shiny, I am trying to create a very simple app respecting the following sequence of events:
(1) Upload a dataframe,
(2) Wait until the user set the filtering parameter (Category in the example below),
(3) Press a Go! button,
(4) Display the first rows of the subset data frame.
Let's say I have a file df.tab to upload and process.
df <- data.frame(Category=c("A","A","A","B","B","B"), X=c(1,2,3,1,2,3), Y=c(1,2,3,34,21,1))
df
Category X Y
1 A 1 1
2 A 2 2
3 A 3 3
4 B 1 34
5 B 2 21
6 B 3 1
write.table(df, file="df.tab", row.names=F, quote=F, sep="\t")
My app.R looks like:
library(shiny)
# Define UI ----
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
fileInput("input_df",label=h4("Dataset")),
selectInput("category",h4("Category"), choices = list("A" = 1,"B" = 2),selected = 1),
actionButton("goButton",label = "Go!")
),
mainPanel(
tableOutput("view")
)
)
)
# Define server logic ----
server <- function(input, output) {
data <- eventReactive(
input$input_df,
{
File <- input$input_df
if(is.null(File)){
return(NULL)
}else{
df <- read.table(File$datapath, header = T, sep = "\t")
}
}
)
data_sub <- eventReactive(
input$category,
{
df_sub <- subset(data(), Category == input$category)
}
)
output$view <- renderTable(
{
head(data_sub())
}
)
}
# Run the app ----
shinyApp(ui = ui, server = server)
However, the app is either not responsive or does not display any rows.
Note that I created 2 distinct reactive events data and data_sub in order to avoid loading the file every time I select a different category (and potentially to avoid stack errors with a recursive function).
Any help would be greatly appreciated.

Answer 1

Here is a working server function. Use reactive, not eventReactive and it is quite straightforward.
NOTE that your example assumes you have a Category column, I modified below to make it work with anything.
# Define server logic ----
server <- function(input, output) {
dataset <- reactive({
File <- input$input_df
req(File)
read.table(File$datapath, header = TRUE, sep = "\t")
})
data_sub <- reactive({
if("Category" %in% names(dataset())){
subset(dataset(), Category == input$category)
} else {
dataset()
}
})
output$view <- renderTable({
head(data_sub())
})
}