make callback queue handling reusable and implement onUiTabChange()

script.js

@@ -2,24 +2,40 @@ function gradioApp(){
     return document.getElementsByTagName('gradio-app')[0].shadowRoot;
 }
 
+function get_uiCurrentTab() {
+    return gradioApp().querySelector('.tabs button:not(.border-transparent)')
+}
+
 uiUpdateCallbacks = []
+uiTabChangeCallbacks = []
+let uiCurrentTab = null
+
 function onUiUpdate(callback){
     uiUpdateCallbacks.push(callback)
 }
+function onUiTabChange(callback){
+    uiTabChangeCallbacks.push(callback)
+}
 
-function uiUpdate(root){
-	uiUpdateCallbacks.forEach(function(x){
+function runCallback(x){
     try {
         x()
     } catch (e) {
         (console.error || console.log).call(console, e.message, e);
     }
-	})
+}
+function executeCallbacks(queue) {
+    queue.forEach(runCallback)
 }
 
 document.addEventListener("DOMContentLoaded", function() {
     var mutationObserver = new MutationObserver(function(m){
-        uiUpdate(gradioApp());
+        executeCallbacks(uiUpdateCallbacks);
+        const newTab = get_uiCurrentTab();
+        if ( newTab && ( newTab !== uiCurrentTab ) ) {
+            uiCurrentTab = newTab;
+            executeCallbacks(uiTabChangeCallbacks);
+        }
     });
     mutationObserver.observe( gradioApp(), { childList:true, subtree:true })
 });