R Shiny - observeEvent do not pick up clear selectInput event - r

Question

I am not sure if what I am missing anything here, it seemed that event is not caught by observeEvent when an selectInput (multi-select on) is cleared. However, it is caught using reactive().
See example below, the goal is that with any changes in the selectInput, the program will pick up the change and display on screen. I used 2 examples:
Reactive to display on html_component2
ReactiveValues using observeEvent to display on html_component
For reactive function, it works perfectly. For the later, it works for all combinations, except when if user unselect everything. I am really confused on why, has anyone seen this issue before and if there are any workarounds for it? I'd prefer to use reactive values in this case for my application.
library(shiny)
# Define UI for application that draws a histogram
ui <- fluidPage(
# Application title
titlePanel("Old Faithful Geyser Data"),
# Sidebar with a slider input for number of bins
sidebarLayout(
sidebarPanel(
selectInput("sinput", "select here", c(1,2,3,4,5), multiple = T),
),
# Show a plot of the generated distribution
mainPanel(
htmlOutput("html_component"),
htmlOutput("html_component2"),
)
)
)
# Define server logic required to draw a histogram
server <- function(input, output) {
rv <- reactiveValues()
sel <- reactive({
input$sinput
})
observeEvent(input$sinput, {
rv$selected = input$sinput
})
output$html_component <- renderUI({
HTML(paste(c("1:", rv$selected)))
})
output$html_component2 <- renderUI({
HTML(paste(c("2:", sel())))
})
}
# Run the application
shinyApp(ui = ui, server = server)

Answer 1

By default observeEvent will ignore NULL in it's eventExpr, you need to set ignoreNULL = FALSE:
library(shiny)
# Define UI for application that draws a histogram
ui <- fluidPage(
# Application title
titlePanel("Old Faithful Geyser Data"),
# Sidebar with a slider input for number of bins
sidebarLayout(
sidebarPanel(
selectInput("sinput", "select here", c(1,2,3,4,5), multiple = T),
),
# Show a plot of the generated distribution
mainPanel(
htmlOutput("html_component"),
htmlOutput("html_component2"),
)
)
)
# Define server logic required to draw a histogram
server <- function(input, output) {
rv <- reactiveValues()
sel <- reactive({
input$sinput
})
observeEvent(input$sinput, {
rv$selected = input$sinput
}, ignoreNULL = FALSE)
output$html_component <- renderUI({
HTML(paste(c("1:", rv$selected)))
})
output$html_component2 <- renderUI({
HTML(paste(c("2:", sel())))
})
}
# Run the application
shinyApp(ui = ui, server = server)