Always disable UI cache (unless there's an existing Cache-Control header)

NEWS.md

@@ -6,7 +6,7 @@
 
 ### New features and improvements
 
-* Closed #789: `<script>` loaded from dynamic UI are no longer loaded using synchronous `XMLHttpRequest` (via jQuery). (#3666)
+* Closed #789: Dynamic UI is now rendered asynchronously, thanks in part to the newly exported `Shiny.renderDependenciesAsync()`, `Shiny.renderHtmlAsync()`, and `Shiny.renderContentAsync()`. Importantly, this means `<script>` tags are now loaded asynchronously (the old way used `XMLHttpRequest`, which is synchronous). In addition, `Shiny` now manages a queue of async tasks (exposed via `Shiny.shinyapp.taskQueue`) so that order of execution is preserved. (#3666)
 
 * For `reactiveValues()` objects, whenever the `$names()` or `$values()` methods are called, the keys are now returned in the order that they were inserted. (#3774)
 

R/bootstrap.R

@@ -1200,19 +1200,25 @@ uiOutput <- htmlOutput
 #' @examples
 #' \dontrun{
 #' ui <- fluidPage(
+#'   p("Choose a dataset to download."),
+#'   selectInput("dataset", "Dataset", choices = c("mtcars", "airquality")),
 #'   downloadButton("downloadData", "Download")
 #' )
 #'
 #' server <- function(input, output) {
-#'   # Our dataset
-#'   data <- mtcars
+#'   # The requested dataset
+#'   data <- reactive({
+#'     get(input$dataset)
+#'   })
 #'
 #'   output$downloadData <- downloadHandler(
 #'     filename = function() {
-#'       paste("data-", Sys.Date(), ".csv", sep="")
+#'       # Use the selected dataset as the suggested file name
+#'       paste0(input$dataset, ".csv")
 #'     },
 #'     content = function(file) {
-#'       write.csv(data, file)
+#'       # Write the dataset to the `file` that will be downloaded
+#'       write.csv(data(), file)
 #'     }
 #'   )
 #' }

R/shinyui.R

@@ -231,11 +231,32 @@ uiHttpHandler <- function(ui, uiPattern = "^/$") {
     if (is.null(uiValue))
       return(NULL)
 
-    if (inherits(uiValue, "httpResponse")) {
-      return(uiValue)
-    } else {
+    # Avoid caching the UI response to ensure that UI is always re-evaluated.
+    # This is necessary for getCurrentTheme() to be known, required for BS4+.
+    no_cache_headers <- list(
+      "Cache-Control" = "no-cache, no-store, must-revalidate",
+      "Pragma" = "no-cache",
+      "Expires" = "0"
+    )
+
+    if (!inherits(uiValue, "httpResponse")) {
       html <- renderPage(uiValue, showcaseMode, testMode)
-      return(httpResponse(200, content=html))
+      uiValue <- httpResponse(200, content=html)
     }
+
+    # 2023-04-23 jcheng5: Example app in PR comment
+    # https://github.com/rstudio/shiny/pull/3810#issuecomment-1513828996
+    # > I think we should always add the cache-busting headers. Without it,
+    # Connect and ShinyApps.io configurations will have problems in that the
+    # workerId could be a stale value.
+    if (
+      # The user has not set a `Cache-Control` policy within their UI func
+      !"Cache-Control" %in% names(uiValue$headers)
+    ) {
+      # Disable caching for the UI's response
+      uiValue$headers <- utils::modifyList(uiValue$headers, no_cache_headers)
+    }
+
+    uiValue
   }
 }

inst/www/shared/shiny.js

@@ -15583,7 +15583,7 @@
           if (opts.priority === "event") {
             this._sendNow();
           } else if (!this.sendIsEnqueued) {
-            this.shinyapp.actionQueue.enqueue(function() {
+            this.shinyapp.taskQueue.enqueue(function() {
               _this.sendIsEnqueued = false;
               _this._sendNow();
             });
@@ -17666,7 +17666,7 @@
     function ShinyApp2() {
       _classCallCheck36(this, ShinyApp2);
       _defineProperty18(this, "$socket", null);
-      _defineProperty18(this, "actionQueue", new AsyncQueue());
+      _defineProperty18(this, "taskQueue", new AsyncQueue());
       _defineProperty18(this, "config", null);
       _defineProperty18(this, "$inputValues", {});
       _defineProperty18(this, "$initialInput", null);
@@ -17875,7 +17875,7 @@
           _this.startActionQueueLoop();
         };
         socket.onmessage = function(e) {
-          _this.actionQueue.enqueue(/* @__PURE__ */ _asyncToGenerator8(/* @__PURE__ */ _regeneratorRuntime8().mark(function _callee2() {
+          _this.taskQueue.enqueue(/* @__PURE__ */ _asyncToGenerator8(/* @__PURE__ */ _regeneratorRuntime8().mark(function _callee2() {
             return _regeneratorRuntime8().wrap(function _callee2$(_context2) {
               while (1)
                 switch (_context2.prev = _context2.next) {
@@ -17918,7 +17918,7 @@
                     break;
                   }
                   _context3.next = 3;
-                  return this.actionQueue.dequeue();
+                  return this.taskQueue.dequeue();
                 case 3:
                   action = _context3.sent;
                   _context3.prev = 4;

man/downloadButton.Rd

@@ -36,19 +36,25 @@ function.
 \examples{
 \dontrun{
 ui <- fluidPage(
+  p("Choose a dataset to download."),
+  selectInput("dataset", "Dataset", choices = c("mtcars", "airquality")),
   downloadButton("downloadData", "Download")
 )
 
 server <- function(input, output) {
-  # Our dataset
-  data <- mtcars
+  # The requested dataset
+  data <- reactive({
+    get(input$dataset)
+  })
 
   output$downloadData <- downloadHandler(
     filename = function() {
-      paste("data-", Sys.Date(), ".csv", sep="")
+      # Use the selected dataset as the suggested file name
+      paste0(input$dataset, ".csv")
     },
     content = function(file) {
-      write.csv(data, file)
+      # Write the dataset to the `file` that will be downloaded
+      write.csv(data(), file)
     }
   )
 }

srcts/src/inputPolicies/inputBatchSender.ts

@@ -22,7 +22,7 @@ class InputBatchSender implements InputPolicy {
       if (opts.priority === "event") {
         this._sendNow();
       } else if (!this.sendIsEnqueued) {
-        this.shinyapp.actionQueue.enqueue(() => {
+        this.shinyapp.taskQueue.enqueue(() => {
           this.sendIsEnqueued = false;
           this._sendNow();
         });

srcts/src/shiny/shinyapp.ts

@@ -114,7 +114,7 @@ class ShinyApp {
   // without overlapping. This is used for handling incoming messages from the
   // server and scheduling outgoing messages to the server, and can be used for
   // other things tasks as well.
-  actionQueue = new AsyncQueue<() => Promise<void> | void>();
+  taskQueue = new AsyncQueue<() => Promise<void> | void>();
 
   config: {
     workerId: string;
@@ -240,7 +240,7 @@ class ShinyApp {
       this.startActionQueueLoop();
     };
     socket.onmessage = (e) => {
-      this.actionQueue.enqueue(async () => await this.dispatchMessage(e.data));
+      this.taskQueue.enqueue(async () => await this.dispatchMessage(e.data));
     };
     // Called when a successfully-opened websocket is closed, or when an
     // attempt to open a connection fails.
@@ -266,7 +266,7 @@ class ShinyApp {
   async startActionQueueLoop(): Promise<void> {
     // eslint-disable-next-line no-constant-condition
     while (true) {
-      const action = await this.actionQueue.dequeue();
+      const action = await this.taskQueue.dequeue();
 
       try {
         await action();

srcts/types/src/shiny/shinyapp.d.ts

@@ -20,7 +20,7 @@ type MessageValue = Parameters<WebSocket["send"]>[0];
 declare function addCustomMessageHandler(type: string, handler: Handler): void;
 declare class ShinyApp {
     $socket: ShinyWebSocket | null;
-    actionQueue: AsyncQueue<() => Promise<void> | void>;
+    taskQueue: AsyncQueue<() => Promise<void> | void>;
     config: {
         workerId: string;
         sessionId: string;