Trigger an event in shiny when a variable exceeds a threshold - r

Question

I have a continuous variable (the zoom on a leaflet map) and I want to activate some action (polygon drawing) only when this variable exceeds a given threshold (only at after a given zoom level).
Here is a similar - but simpler and easier to reproduce - toy problem:
ui <- bootstrapPage(
sliderInput("slider", label='a number', min=100, max=400, value = 150),
plotOutput("plot")
)
server <- function(input, output, session) {
observeEvent(input$slider > 200, {
output$plot <- renderPlot(plot(rnorm(10000), rnorm(10000)))
})
}
shinyApp(ui, server)
The problem is that, because of Shiny's reactivity system, the plot (in the toy problem) or the map (in the real problem) keep being updated, even though I would like them to be updated only when the threshold is passed, in either direction.
I tried constructions with observeEvent, eventReactive, reactiveValues, etc. mixed with if ... else declarations. But is seems like whenever an input is updated, it triggers the whole chain of events, regardless of whether the dependent variables have changed or not. In the toy problem, it does not matter that input$slider > 200 stays TRUE when input$slider goes from 100 to 101, it triggers the plotting anyways.
Please tell me I am wrong!

Answer 1

Instead of using observeEvent(), you could use a regular observe() and use the condition for an if statement and simply return() if the condition fails.
Something like
observe({
if (input$slider <= 200) return()
...
})
EDIT: In the comments you wanted to somehow track the last value. Here's how you can do this.
library(shiny)
ui <- fluidPage(
sliderInput("slider", "Slider", 1, 500, 100)
)
server <- function(input, output, session) {
values <- reactiveValues(last = 0)
observe({
if (input$slider <= 200 & values$last > 200) {
cat("check!")
}
values$last <- input$slider
})
}
shinyApp(ui = ui, server = server)

Answer 2

So, inspired by #daattali and our discussion, one possible solution is to use a reactiveValue to store the last value of the slider in order to test when the threshold is passed in either directions. The test itself is carried inside an observe function:
ui <- bootstrapPage(
sliderInput("slider", label='a number', min=100, max=400, value = 150),
plotOutput("plot")
)
server <- function(input, output, session) {
output$plot <- renderPlot(plot(rnorm(10000), rnorm(10000)))
last <- reactiveValues(value = 0)
observe({
if ((input$slider <= 200 & last$value > 200)|(input$slider > 200 & last$value <= 200)) {
output$plot <- renderPlot(plot(rnorm(10000), rnorm(10000)))
}
last$value <- input$slider
})
}
shinyApp(ui, server)
With this solution, I correctly have an update only when the threshold (200 on the slider) is passed in either directions.