used on a smartphone, shiny interactive plot doesn't understand finger movements

While I can't solve the question completely, maybe a dirty workaround could also have some value to you. Or someone else can build on that answer.

I could reproduce the error that the clicks on the plot are not captured on mobile. But I noticed that I can add additional clickevents with javascript/shinyjs.

One way would be:

  onevent(event = "click", id = "plot", function(e){
    global$clickx = c(global$clickx, e$pageX - 88)
    global$clicky = c(global$clicky, 540 - e$pageY)
  })

It has a few drawbacks:

• shapes are drawn with lines only, it does not capture all hovering on the plot
• the position is quite imprecise since you have to account for borders and margins (very dirty but potential here)

I ran a bit out of time after a few hours, one can for sure improve it, but maybe its of interest for you anyway.

Test here: (link might change within next weeks)

http://ec2-3-121-215-255.eu-central-1.compute.amazonaws.com/shiny/rstudio/sample-apps/mobile/

Reproducible code: (tested on smartphone: Mi A2)

library(shiny)
library(shinyjs)
ui <- fluidPage(
  useShinyjs(),
  h4("Click on plot to start drawing, click again to pause"),

  plotOutput(outputId = "plot", width = "500px", height = "500px")
)


server <- function(input, output, session) {

  onevent(event = "click", id = "plot", function(e){
    global$clickx = c(global$clickx, e$pageX - 88)
    global$clicky = c(global$clicky, 540 - e$pageY)
  })

  global <- reactiveValues(clickx = NULL, clicky = NULL)

  output$plot= renderPlot({
    plot(x = NULL, y = NULL, xlim=c(0, 440), ylim=c(0, 440), ylab="y", xlab="x", type="l")
    len <- length(global$clickx)
    lines(x = global$clickx, y = global$clicky, type = "p")      
    if(len > 1){
      for(nr in 2:len){
        lines(x = global$clickx[(nr - 1):nr], y = global$clicky[(nr - 1):nr], type = "l")
      }
    }
  })
}
shinyApp(ui, server)

You can disable panning/zoom gestures on the plot using the touch-action CSS property:

#plot {
  touch-action: none;
}

Turning touch events into mouse events is a little trickier, but you could listen to touch events like touchstart, touchmove, touchend and simulate equivalent mouse events in JavaScript. See https://developer.mozilla.org/en-US/docs/Web/API/Touch_events/Using_Touch_Events and https://javascript.info/dispatch-events for more info.

It's not perfect, but I gave it a shot. I disabled touch gestures on the plot and added a script that converts touchmove to mousemove, and tells the server when to start drawing (on touchstart) and stop drawing (on touchend).

library(shiny)

ui <- fluidPage(
  h4("Click on plot to start drawing, click again to pause"),
  sliderInput("mywidth", "width of the pencil", min=1, max=30, step=1, value=10),
  actionButton("reset", "reset"),
  plotOutput("plot", width = "400px", height = "400px",
             hover=hoverOpts(id = "hover", delay = 100, delayType = "throttle", clip = TRUE, nullOutside = TRUE),
             click="click"),
  tags$head(
    tags$script("
      $(document).ready(function() {
        var plot = document.getElementById('plot')

        plot.addEventListener('touchmove', function (e) {
          var touch = e.changedTouches[0];
          var mouseEvent = new MouseEvent('mousemove', {
            view: window,
            bubbles: true,
            cancelable: true,
            screenX: touch.screenX,
            screenY: touch.screenY,
            clientX: touch.clientX,
            clientY: touch.clientY
          })
          touch.target.dispatchEvent(mouseEvent);
          e.preventDefault()
        }, { passive: false });

        plot.addEventListener('touchstart', function(e) {
          Shiny.onInputChange('draw', true)
          e.preventDefault()
        }, { passive: false });

        plot.addEventListener('touchend', function(e) {
          Shiny.onInputChange('draw', false)
          e.preventDefault()
        }, { passive: false });
      })
    "),
    tags$style("#plot { touch-action: none; }")
    )
)

server <- function(input, output, session) {
  vals = reactiveValues(x=NULL, y=NULL)
  draw = reactiveVal(FALSE)

  observeEvent(input$click, {
    draw(!draw())
    vals$x <- append(vals$x, NA)
    vals$y <- append(vals$y, NA)
  })

  observeEvent(input$draw, {
    draw(input$draw)
    vals$x <- append(vals$x, NA)
    vals$y <- append(vals$y, NA)
  })

  observeEvent(input$reset, handlerExpr = {
    vals$x <- NULL; vals$y <- NULL
  })

  observeEvent(input$hover, {
    if (draw()) {
      vals$x <- c(vals$x, input$hover$x)
      vals$y <- c(vals$y, input$hover$y)
    }
  })

  output$plot= renderPlot({
    plot(x=vals$x, y=vals$y, xlim=c(0, 28), ylim=c(0, 28), ylab="y", xlab="x", type="l", lwd=input$mywidth)
  })
}

shinyApp(ui, server)