From 5b43717c5c94b2821f7eab9e69be51c854daefd3 Mon Sep 17 00:00:00 2001
From: "Maarten A. Breddels" <maartenbreddels@gmail.com>
Date: Tue, 4 Aug 2020 13:41:19 +0200
Subject: [PATCH] feat: when a widget fails, show a fallback error widget

---
 js/src/manager.js | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/js/src/manager.js b/js/src/manager.js
index 3f927d4dc..5845aa726 100644
--- a/js/src/manager.js
+++ b/js/src/manager.js
@@ -31,6 +31,7 @@ import { MessageLoop } from '@phosphor/messaging';
 
 import { requireLoader } from './loader';
 import { batchRateMap } from './utils';
+import * as errorwidget from './errorwidget';
 
 if (typeof window !== "undefined" && typeof window.define !== "undefined") {
     window.define("@jupyter-widgets/base", base);
@@ -49,6 +50,8 @@ if (typeof window !== "undefined" && typeof window.define !== "undefined") {
     window.define("@phosphor/algorithm", PhosphorAlgorithm);
     window.define("@phosphor/commands", PhosphorCommands);
     window.define("@phosphor/domutils", PhosphorDomutils);
+
+    window.define("voila-errorwidget", errorwidget);
 }
 
 const WIDGET_MIMETYPE = 'application/vnd.jupyter.widget-view+json';
@@ -106,10 +109,19 @@ export class WidgetManager extends JupyterLabManager {
     }
 
     async loadClass(className, moduleName, moduleVersion) {
+        try {
+            return await this._loadClass(className, moduleName, moduleVersion)
+        } catch(error) {
+            return errorwidget.createErrorWidget(error);
+        };
+    }
+
+    async _loadClass(className, moduleName, moduleVersion) {
         if (
             moduleName === '@jupyter-widgets/base' ||
             moduleName === '@jupyter-widgets/controls' ||
-            moduleName === '@jupyter-widgets/output'
+            moduleName === '@jupyter-widgets/output' ||
+            moduleName === 'voila-errorwidget'
         ) {
             return super.loadClass(className, moduleName, moduleVersion);
         }
@@ -120,7 +132,7 @@ export class WidgetManager extends JupyterLabManager {
                     return module[className];
                 }
                 else {
-                    return Promise.reject("Class " + className + " not found in module " + moduleName + "@" + moduleVersion);
+                    throw Error("Class " + className + " not found in module " + moduleName + "@" + moduleVersion);
                 }
             })
         }
@@ -145,6 +157,11 @@ export class WidgetManager extends JupyterLabManager {
             version: output.OUTPUT_WIDGET_VERSION,
             exports: output
         });
+        this.register({
+            name: 'voila-errorwidget',
+            version: '1.0.0',
+            exports: errorwidget
+        });
     }
 
     async _build_models() {