From 9187a37f929f10c99e31b7ccfff0ab89e99645e1 Mon Sep 17 00:00:00 2001
From: "Maarten A. Breddels" <maartenbreddels@gmail.com>
Date: Mon, 3 Aug 2020 15:38:06 +0200
Subject: [PATCH] feat: when a widget fails, show a fallback error widget

---
 js/src/errorwidget.js | 32 ++++++++++++++++++++++++++++++++
 js/src/manager.js     | 43 +++++++++++++++++++++++++++++++++----------
 2 files changed, 65 insertions(+), 10 deletions(-)
 create mode 100644 js/src/errorwidget.js

diff --git a/js/src/errorwidget.js b/js/src/errorwidget.js
new file mode 100644
index 000000000..7e7bf2141
--- /dev/null
+++ b/js/src/errorwidget.js
@@ -0,0 +1,32 @@
+import * as widgets from "@jupyter-widgets/base";
+
+// create a Widget Model that captures an error object
+export
+function createErrorWidget(error) {
+    class ErrorWidget extends widgets.DOMWidgetModel {
+        constructor(attributes, options) {
+            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);
+            console.log(attributes);
+        }
+    }
+    return ErrorWidget;
+}
+
+export
+class ErrorWidgetView extends widgets.DOMWidgetView {
+    render() {
+        console.log('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>`;
+    }
+}
\ No newline at end of file
diff --git a/js/src/manager.js b/js/src/manager.js
index 3f927d4dc..d5b66b532 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';
@@ -109,20 +112,35 @@ export class WidgetManager extends JupyterLabManager {
         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);
+            return new Promise((resolve) =>  {
+                super.loadClass(className, moduleName, moduleVersion).then((cls) => {
+                    resolve(cls)
+                }, (err) => {
+                    // If we failed, fallback to the buildin error widget
+                    resolve(errorwidget.createErrorWidget(err));
+                });
+            });
         }
         else {
             // TODO: code duplicate from HTMLWidgetManager, consider a refactor
-            return this.loader(moduleName, moduleVersion).then((module) => {
-                if (module[className]) {
-                    return module[className];
-                }
-                else {
-                    return Promise.reject("Class " + className + " not found in module " + moduleName + "@" + moduleVersion);
-                }
-            })
+            return new Promise((resolve) =>  {
+                this.loader(moduleName, moduleVersion).then((module) => {
+                    if (module[className]) {
+                        resolve(module[className]);
+                    }
+                    else {
+                        // If we failed to find the widget, fallback to the buildin error widget
+                        const error = new Error("Class " + className + " not found in module " + moduleName + "@" + moduleVersion);
+                        resolve(errorwidget.createErrorWidget(error));
+                    }
+                }, (error) => {
+                    // If we failed to load the module, fallback to the buildin error widget
+                    resolve(errorwidget.createErrorWidget(error));
+                })
+            });
         }
     }
 
@@ -145,6 +163,11 @@ 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() {