reactive expression for raster in shiny - r

Question

I am working on a simple shiny app.
Here is my data.
library(data.table)
library(ggthemes)
library(ggplot2)
library(shiny)
tempList <- list()
for(i in 1989:1991){
temp <- as.data.frame(cbind(runif(10,-10.85, 20.02),runif(10, 49.82,59.47)))
temp$value <- rnorm(10)
temp$Year <-i
tempList[[i]] <- temp
}
my.df <- rbindlist(tempList)
names(my.df)[1:2] <- c('lon', 'lat')
I want to make a shiny app that displays the raster for each year based on which year the users select
ui <- fluidPage(
titlePanel('My dat'),
sliderInput('yearRef','Select Year',min=1989,max=1991,value=1),
plotOutput(outputId = 'test')
)
server <- function(input, output) {
tempI <- reactive({my.df %>% dplyr::filter(Year == input$yearRef)})
output$test <- renderPlot({
ggplot() + geom_raster(data = tempI, aes(x = lon, y = lat, fill = value)) +
theme_map() + coord_equal() + scale_fill_viridis_c(option = 'C')
})
}
shinyApp(ui, server)
It gives me an error that tempI is not a dataframe which I understand is causing since tempI is class reactiveExpr. How do I correct it?

Answer 1

when working with reactive expressions in shiny you have to use paranthesis.
In your case:
renderPlot({
ggplot() + geom_raster(data = tempI(), aes(x = lon, y = lat, fill = value)) +
theme_map() + coord_equal() + scale_fill_viridis_c(option = 'C')
})
You can think of tempI() as a function that knows when its return-value is outdated. As soon as this happens (i.e. as soon as the user changes the slider)
tempI has to be reevaluated. Hence it works like a function. This also justifies the name reactive.
You can learn more about reactive expressions here.