Added alt param

NEWS.md

@@ -2,6 +2,8 @@
 shiny 1.5.0.9000
 ================
 
+## Full changelog
+
 ### Accessibility
 
 * Added [bootstrap accessibility plugin](https://github.com/paypal/bootstrap-accessibility-plugin) under the hood to improve accessibility of shiny apps for screen-reader and keyboard users: the enhancements include better navigations for alert, tooltip, popover, modal dialog, dropdown, tab Panel, collapse, and carousel elements. (#2911)
@@ -14,6 +16,8 @@ shiny 1.5.0.9000
 
 * Closed #2847: `selectInput()` is reasonably accessible for screen readers even when `selectize` option is set to TRUE. To improve `selectize.js` accessibility, We have added [selectize-plugin-a11y](https://github.com/SLMNBJ/selectize-plugin-a11y) by default. (#2993)
 
+* Closed #612: Added `alt` argument to `renderPlot()` and `renderCachedPlot()` to specify descriptive texts for `plotOutput()` objects, which is essential for screen readers. By default, alt text is set to the static text, "Plot object," but even dynamic text can be made with reactive function. (#3006, thanks @trafficonese and @leonawicz for the original PR and discussion via #2494)
+
 ### Minor new features and improvements
 
 * When UI is specified as a function (e.g. `ui <- function(req) { ... }`), the response can now be an HTTP response as returned from the (newly exported) `httpResponse()` function. (#2970)

R/render-cached-plot.R

@@ -293,6 +293,7 @@ renderCachedPlot <- function(expr,
   cacheKeyExpr,
   sizePolicy = sizeGrowthRatio(width = 400, height = 400, growthRate = 1.2),
   res = 72,
+  alt = "Plot object",
   cache = "app",
   ...,
   outputArgs = list()
@@ -341,6 +342,14 @@ renderCachedPlot <- function(expr,
   # values get filled by an observer below.
   fitDims <- reactiveValues(width = NULL, height = NULL)
 
+  # Make sure alt param to be reactive function
+  if (is.reactive(alt))
+    altWrapper <- alt
+  else if (is.function(alt))
+    altWrapper <- reactive({ alt() })
+  else
+    altWrapper <- function() { alt }
+
   resizeObserver <- NULL
   ensureResizeObserver <- function() {
     if (!is.null(resizeObserver))
@@ -387,6 +396,8 @@ renderCachedPlot <- function(expr,
         isolate({
           width  <- fitDims$width
           height <- fitDims$height
+          # Make sure alt text to be reactive function
+          alt <- altWrapper() 
         })
 
         pixelratio <- session$clientData$pixelratio %OR% 1
@@ -398,6 +409,7 @@ renderCachedPlot <- function(expr,
             func = isolatedFunc,
             width = width,
             height = height,
+            alt = alt,
             pixelratio = pixelratio,
             res = res
           ),
@@ -471,6 +483,7 @@ renderCachedPlot <- function(expr,
               plotObj = drawReactiveResult,
               width = width,
               height = height,
+              alt = alt,
               pixelratio = pixelratio
             )
           }
@@ -480,6 +493,7 @@ renderCachedPlot <- function(expr,
         hybrid_chain(possiblyAsyncResult, function(result) {
           width      <- result$width
           height     <- result$height
+          alt     <- result$alt
           pixelratio <- result$pixelratio
 
           # Three possibilities when we get here:
@@ -500,6 +514,7 @@ renderCachedPlot <- function(expr,
                 result$plotObj,
                 width,
                 height,
+                alt,
                 pixelratio,
                 res
               ),

R/render-plot.R

@@ -36,6 +36,12 @@
 #' @param res Resolution of resulting plot, in pixels per inch. This value is
 #'   passed to [grDevices::png()]. Note that this affects the resolution of PNG
 #'   rendering in R; it won't change the actual ppi of the browser.
+#' @param alt Alternate text for the HTML `<img>` tag
+#'   if it cannot be displayed or viewed (i.e., the user uses a screen reader).
+#'   In addition to a character string, the value may be a reactive expression 
+#'   (or a function referencing reactive values) that returns a character string.
+#'   NULL or "" is not recommended because those should be limited to decorative images
+#'   (the default is "Plot object").
 #' @param ... Arguments to be passed through to [grDevices::png()].
 #'   These can be used to set the width, height, background color, etc.
 #' @param env The environment in which to evaluate `expr`.
@@ -51,7 +57,7 @@
 #'   call to [plotOutput()] when `renderPlot` is used in an
 #'   interactive R Markdown document.
 #' @export
-renderPlot <- function(expr, width='auto', height='auto', res=72, ...,
+renderPlot <- function(expr, width='auto', height='auto', res=72, alt="Plot object", ...,
                        env=parent.frame(), quoted=FALSE,
                        execOnResize=FALSE, outputArgs=list()
 ) {
@@ -75,6 +81,13 @@ renderPlot <- function(expr, width='auto', height='auto', res=72, ...,
   else
     heightWrapper <- function() { height }
 
+  if (is.reactive(alt))
+    altWrapper <- alt
+  else if (is.function(alt))
+    altWrapper <- reactive({ alt() })
+  else
+    altWrapper <- function() { alt }
+
   getDims <- function() {
     width <- widthWrapper()
     height <- heightWrapper()
@@ -112,6 +125,7 @@ renderPlot <- function(expr, width='auto', height='auto', res=72, ...,
             func = func,
             width = dims$width,
             height = dims$height,
+            alt = altWrapper(),
             pixelratio = pixelratio,
             res = res
           ), args))
@@ -140,7 +154,7 @@ renderPlot <- function(expr, width='auto', height='auto', res=72, ...,
         dims <- getDims()
         pixelratio <- session$clientData$pixelratio %OR% 1
         result <- do.call("resizeSavedPlot", c(
-          list(name, shinysession, result, dims$width, dims$height, pixelratio, res),
+          list(name, shinysession, result, dims$width, dims$height, altWrapper(), pixelratio, res),
           args
         ))
 
@@ -159,7 +173,7 @@ renderPlot <- function(expr, width='auto', height='auto', res=72, ...,
   markRenderFunction(outputFunc, renderFunc, outputArgs = outputArgs)
 }
 
-resizeSavedPlot <- function(name, session, result, width, height, pixelratio, res, ...) {
+resizeSavedPlot <- function(name, session, result, width, height, alt, pixelratio, res, ...) {
   if (result$img$width == width && result$img$height == height &&
       result$pixelratio == pixelratio && result$res == res) {
     return(result)
@@ -181,14 +195,15 @@ resizeSavedPlot <- function(name, session, result, width, height, pixelratio, re
     src = session$fileUrl(name, outfile, contentType = "image/png"),
     width = width,
     height = height,
+    alt = alt,
     coordmap = coordmap,
     error = attr(coordmap, "error", exact = TRUE)
   )
 
   result
 }
 
-drawPlot <- function(name, session, func, width, height, pixelratio, res, ...) {
+drawPlot <- function(name, session, func, width, height, alt, pixelratio, res, ...) {
   #  1. Start PNG
   #  2. Enable displaylist recording
   #  3. Call user-defined func
@@ -272,6 +287,7 @@ drawPlot <- function(name, session, func, width, height, pixelratio, res, ...) {
         src = session$fileUrl(name, outfile, contentType='image/png'),
         width = width,
         height = height,
+        alt = alt,
         coordmap = result$coordmap,
         # Get coordmap error message if present
         error = attr(result$coordmap, "error", exact = TRUE)