chartJSRadar downloadhandler creating empty png - r

Question

I would like to create a download button within my shiny app to download a reactive plot created with chartJSRadar. I am not able to solve this problem!
As I have been through the documented problem on the internet, I was not able to solve it, receiving all the time an empty png. As suggested (Save plots made in a shiny app), https://groups.google.com/forum/#!msg/shiny-discuss/u7gwXc8_vyY/IZK_o7b7I8gJ
I built a function ...
So my code is an example code:
ui.R:
library(radarchart)
shinyUI(pageWithSidebar(
headerPanel('Radarchart Shiny Example'),
sidebarPanel(
checkboxGroupInput('selectedPeople', 'Who to include',
names(radarchart::skills)[-1], selected="Rich")
),
mainPanel(
chartJSRadarOutput("plot1", width = "450", height = "300"), width = 7,
radioButtons(inputId = "var3", label = "Select the file type", choices = list("png", "pdf")),
downloadButton('downloadPlot', 'Download Plot')
)
))
server.R
library(radarchart)
shinyServer(function(input, output) {
output$plot1 <- renderChartJSRadar({
chartJSRadar(skills[, c("Label", input$selectedPeople)],
maxScale = 10, showToolTipLabel=TRUE)
})
plot2 <- function(){
chartJSRadar(skills[, c("Label", input$selectedPeople)],
maxScale = 10, showToolTipLabel=TRUE)
}
output$downloadPlot <- downloadHandler(
filename = "Shinyplot.png",
content = function(file) {
png(file)
plot2()
print(plot2())
dev.off()
})
})

Answer 1

Here is a JavaScript way which should be faster than webshot, I think.
library(shiny)
library(radarchart)
library(htmlwidgets) # to use the 'onRender' function
js <- c(
"function(el, x){",
" $('#downloadPlot').on('click', function(){",
" // Clone the chart to add a background color.",
" var cloneCanvas = document.createElement('canvas');",
" cloneCanvas.width = el.width;",
" cloneCanvas.height = el.height;",
" var ctx = cloneCanvas.getContext('2d');",
" ctx.fillStyle = '#FFFFFF';",
" ctx.fillRect(0, 0, el.width, el.height);",
" ctx.drawImage(el, 0, 0);",
" // Download.",
" const a = document.createElement('a');",
" document.body.append(a);",
" a.download = 'radarchart.png';",
" a.href = cloneCanvas.toDataURL('image/png');",
" a.click();",
" a.remove();",
" });",
"}"
)
ui <- pageWithSidebar(
headerPanel('Radarchart Shiny Example'),
sidebarPanel(
checkboxGroupInput('selectedPeople', 'Who to include',
names(radarchart::skills)[-1], selected="Rich"),
actionButton('downloadPlot', 'Download Plot')
),
mainPanel(
chartJSRadarOutput("plot1", width = "450", height = "300"), width = 7
)
)
server <- function(input, output) {
output$plot1 <- renderChartJSRadar({
chartJSRadar(skills[, c("Label", input$selectedPeople)],
maxScale = 10, showToolTipLabel=TRUE) %>%
onRender(js)
})
}
shinyApp(ui, server)
This exports to png only. Use webshot to export to pdf.
EDIT
library(shiny)
library(radarchart)
js <- paste0(c(
"$(document).ready(function(){",
" $('#downloadPlot').on('click', function(){",
" var el = document.getElementById('plot1');",
" // Clone the chart to add a background color.",
" var cloneCanvas = document.createElement('canvas');",
" cloneCanvas.width = el.width;",
" cloneCanvas.height = el.height;",
" var ctx = cloneCanvas.getContext('2d');",
" ctx.fillStyle = '#FFFFFF';",
" ctx.fillRect(0, 0, el.width, el.height);",
" ctx.drawImage(el, 0, 0);",
" // Download.",
" const a = document.createElement('a');",
" document.body.append(a);",
" a.download = 'radarchart.png';",
" a.href = cloneCanvas.toDataURL('image/png');",
" a.click();",
" a.remove();",
" cloneCanvas.remove();",
" });",
"});"
), collapse = "\n")
ui <- pageWithSidebar(
headerPanel('Radarchart Shiny Example'),
sidebarPanel(
checkboxGroupInput('selectedPeople', 'Who to include',
names(radarchart::skills)[-1], selected="Rich"),
actionButton('downloadPlot', 'Download Plot')
),
mainPanel(
tags$head(tags$script(HTML(js))),
chartJSRadarOutput("plot1", width = "450", height = "300"), width = 7
)
)
server <- function(input, output) {
output$plot1 <- renderChartJSRadar({
chartJSRadar(skills[, c("Label", input$selectedPeople)],
maxScale = 10, showToolTipLabel=TRUE)
})
}
shinyApp(ui, server)

Answer 2

chartJSRadar returns an htmlWidget. To save, try using saveWidget and then webshot of the temporary html file. Add webshot library:
library(webshot)
and try substituting this for downloadHandler in your server function:
output$downloadPlot <- downloadHandler(
filename = "Shinyplot.png",
content = function(file) {
saveWidget(plot2(), "temp.html", selfcontained = TRUE)
webshot("temp.html", file = file)
}
)