Add loading callback to widget

doc/user/events/widget.rst

@@ -206,6 +206,21 @@ If you want, you can suppress us loading the widget and/or modify the user data
 
 If you then later want to trigger loading the widgets, just call ``window.PretixWidget.buildWidgets()``.
 
+Waiting for the widget to load
+------------------------------
+
+If you want to run custom JavaScript once the widget is fully loaded, you can register a callback function. Note that
+this function might be run multiple times, for example if you have multiple widgets on a page or if the user switches
+e.g. from an event list to an event detail view::
+
+    <script type="text/javascript">
+    window.pretixWidgetCallback = function () {
+        window.PretixWidget.addLoadListener(function () {
+            console.log("Widget has loaded!");
+        });
+    }
+    </script>
+
 
 Passing user data to the widget
 -------------------------------

src/pretix/static/pretixpresale/js/widget/widget.js

@@ -784,6 +784,7 @@ Vue.component('pretix-widget-event-form', {
             this.$root.target_url = this.$root.parent_stack.pop();
             this.$root.error = null;
             this.$root.subevent = null;
+            this.$root.trigger_load_callback();
             if (this.$root.events !== undefined) {
                 this.$root.view = "events";
             } else {
@@ -1086,6 +1087,13 @@ var shared_root_methods = {
             return;
         }
     },
+    trigger_load_callback: function () {
+        this.$nextTick(function () {
+            for (var i = 0; i < window.PretixWidget._loaded.length; i++) {
+                window.PretixWidget._loaded[i]()
+            }
+        });
+    },
     reload: function () {
         var url;
         if (this.$root.subevent) {
@@ -1148,6 +1156,7 @@ var shared_root_methods = {
             }
             if (root.loading > 0) {
                 root.loading--;
+                root.trigger_load_callback();
             }
         }, function (error) {
             root.categories = [];
@@ -1155,6 +1164,7 @@ var shared_root_methods = {
             root.error = strings['loading_error'];
             if (root.loading > 0) {
                 root.loading--;
+                root.trigger_load_callback();
             }
         });
     },
@@ -1385,6 +1395,10 @@ var create_button = function (element) {
 /* Find all widgets on the page and render them */
 widgetlist = [];
 buttonlist = [];
+window.PretixWidget._loaded = [];
+window.PretixWidget.addLoadListener = function (f) {
+    window.PretixWidget._loaded.push(f);
+}
 window.PretixWidget.buildWidgets = function () {
     document.createElement("pretix-widget");
     document.createElement("pretix-button");