From 81706ce05f7ac4eff73cd15da71b2bbfe3133abc Mon Sep 17 00:00:00 2001
From: "Maarten A. Breddels" <maartenbreddels@gmail.com>
Date: Thu, 4 Feb 2021 17:33:20 +0100
Subject: [PATCH] feat: when a widget fails, show a fallback error widget

---
 packages/voila/src/errorwidget.ts | 30 ++++++++++++++++++++++++++++++
 packages/voila/src/manager.ts     | 27 ++++++++++++++++++++++++---
 2 files changed, 54 insertions(+), 3 deletions(-)
 create mode 100644 packages/voila/src/errorwidget.ts

diff --git a/packages/voila/src/errorwidget.ts b/packages/voila/src/errorwidget.ts
new file mode 100644
index 000000000..cf0996ea3
--- /dev/null
+++ b/packages/voila/src/errorwidget.ts
@@ -0,0 +1,30 @@
+import * as widgets from '@jupyter-widgets/base';
+
+// Create a Widget Model that captures an error object
+export function createErrorWidget(error: string): any {
+  class ErrorWidget extends widgets.DOMWidgetModel {
+    constructor(attributes?: any, options?: any) {
+      attributes = {
+        ...attributes,
+        _view_name: 'ErrorWidgetView',
+        _view_module: 'voila-errorwidget',
+        _model_module_version: '1.0.0',
+        _view_module_version: '1.0.0',
+        failed_module: attributes._view_module,
+        failed_model_name: attributes._model_name,
+        error: error
+      };
+      super(attributes, options);
+    }
+  }
+  return ErrorWidget as any;
+}
+
+export class ErrorWidgetView extends widgets.DOMWidgetView {
+  render() {
+    const module = this.model.get('failed_module');
+    const name = this.model.get('failed_model_name');
+    const error = String(this.model.get('error').stack);
+    this.el.innerHTML = `Failed to load widget '${name}' from module '${module}', error:<pre>${error}</pre>`;
+  }
+}
diff --git a/packages/voila/src/manager.ts b/packages/voila/src/manager.ts
index 72898fc63..3251817dc 100644
--- a/packages/voila/src/manager.ts
+++ b/packages/voila/src/manager.ts
@@ -31,10 +31,11 @@ import * as PhosphorCommands from '@phosphor/commands';
 import * as PhosphorDomutils from '@phosphor/domutils';
 
 import { MessageLoop } from '@phosphor/messaging';
+import { Widget } from '@phosphor/widgets';
 
 import { requireLoader } from './loader';
 import { batchRateMap } from './utils';
-import { Widget } from '@phosphor/widgets';
+import * as errorwidget from './errorwidget';
 
 if (typeof window !== 'undefined' && typeof window.define !== 'undefined') {
   window.define('@jupyter-widgets/base', base);
@@ -53,6 +54,8 @@ 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';
@@ -129,11 +132,24 @@ export class WidgetManager extends JupyterLabManager {
     className: string,
     moduleName: string,
     moduleVersion: string
+  ): Promise<any> {
+    try {
+      return await this._loadClass(className, moduleName, moduleVersion);
+    } catch (error) {
+      return errorwidget.createErrorWidget(error);
+    }
+  }
+
+  async _loadClass(
+    className: string,
+    moduleName: string,
+    moduleVersion: string
   ): Promise<any> {
     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);
     } else {
@@ -142,7 +158,7 @@ export class WidgetManager extends JupyterLabManager {
         if (module[className]) {
           return module[className];
         } else {
-          return Promise.reject(
+          throw Error(
             'Class ' +
               className +
               ' not found in module ' +
@@ -175,6 +191,11 @@ export class WidgetManager extends JupyterLabManager {
       version: output.OUTPUT_WIDGET_VERSION,
       exports: output as any
     });
+    this.register({
+      name: 'voila-errorwidget',
+      version: '1.0.0',
+      exports: errorwidget as any
+    });
   }
 
   async _build_models(): Promise<{ [key: string]: base.WidgetModel }> {