Added alt param

[jooyoungseo]

This ready-for-review PR attempts to follow up with the PR #2494 to address the alt text issue #612.

We have to make sure alt attribute to be dynamic (beyond static).

Adding alt param to render*() instead of *Output() makes more sense to me (the actual screen reader user) given that we may want to have dynamic alt text depending on graph changes.

Reprex

• Confirmed the following dynamic alt text works very well for screen readers

library(shiny)

shinyApp(
  ui = fluidPage(
    actionButton("btn1", "Update plots"),
    plotOutput("plot1", width = "400px")
  ),
  server = function(input, output, session) {
    dt <- reactive({
      invalidateLater(3 * 1000)
      rnorm(30)
    })
    output$plot1 <- renderPlot(
      {
        hist(dt())
      },
      alt = reactive({
        paste("Dynamic alt text. Mean(x):", round(mean(dt()), 3))
      })
    )
  }
)

[schloerke]

Let's allow for alt to be reactive. (Sorry for my misleading comments earlier!)

[schloerke]

reprex goal:

shinyApp(
  ui = fluidPage(
    actionButton("btn1", "Update plots"),
    plotOutput("plot1", width = "400px")
  ),
  server = function(input, output, session) {
    dt <- reactive({
      invalidateLater(3 * 1000)
      rnorm(30)
    })
    output$plot1 <- renderPlot(
      {
        hist(dt())
      },
      alt = reactive({
        paste("Dynamic alt text. Mean(x):", round(mean(dt()), 3))
      })
    )
  }
)

[jooyoungseo]

@schloerke, we've made it!

I have confirmed the reactive expr is dynamically passed to alt param. When not provided, defaulting to "plot object" announced to screen readers :)

[cpsievert]

LGTM after default alt value is addressed

[schloerke]

LGTM pending alt retrieval in renderCachedPlot.