Added a shiny app example in streaming-async.Rmd using chat_async

vignettes/streaming-async.Rmd

@@ -61,7 +61,76 @@ chat$chat_async("How's your day going?") %...>% print()
 #> I'm just a computer program, so I don't have feelings, but I'm here to help you with any questions you have.
 ```
 
-TODO: Shiny example
+#### Shiny example
+
+The following shiny app uses `chat_async` to query the OpenAI API. The server function shows a loading element (spinner) while it is waiting for the API to return the answer. Then the recent response is displayed.
+The `%...>%` operator from the `promises` package is used to push the asynchronous chat response and render it, once it is ready. It also hides the loading element.
+
+```
+library(shiny)
+library(bslib)
+library(elmer)
+library(promises)
+library(shinyjs)
+
+# Initialize chat
+chat <- chat_openai(
+  system_prompt = "You like chatting about star trek, mostly TNG and onwards (not TOS). Answers should be concise and star trek inspired."
+)
+
+# The user interface is simple
+ui <- page_sidebar(
+  title = "Interactive chat with async",
+  sidebar = sidebar(
+    title = "Controls",
+    useShinyjs(),
+    textInput("user_query", "Enter query:"),
+    actionButton("ask_chat", label = "Ask the chat")
+  ),
+  card(
+    card_header("The chat's response"),
+    hidden(
+      div("I'm thinking...", 
+          div(class = "spinner-border text-primary", role = "status",
+              span(class = "sr-only")),
+          id = "loading")),
+    uiOutput("chat_response")
+  )
+)
+
+server <- function(input, output) {
+
+  # Once the user requests chat gpt output:
+  observeEvent(input$ask_chat, {
+
+    # Do something to show you're thinking
+    showElement("loading")
+
+    # Call the chat_async function
+    result_async <- chat$chat_async("Answer this question:",
+                                    input$user_query)
+
+    # Using the promises notation, this will run once the chat result comes through
+    result_async %...>% {
+      # Update the modal once the result is available
+      hideElement("loading") # results are in - hide the "loading".
+      output$chat_response <- renderUI({
+        markdown(.) # The `.` is the chat result, markdown is just used to turn content into html.
+      })
+    } %...!% {
+      # Error handling
+      output$chat_response <- renderUI({
+        p("Something went wrong... :(")
+      })
+    }
+  })
+}
+
+# Run the app
+shinyApp(ui = ui, server = server)
+```
+
+TODO: Extend example to stream_async, simplify example.
 
 ### Asynchronous streaming