(core) Exposing custom widgets on the UI

Summary:
Exposing custom widgets as a dropdown menu in custom section configuration panel.

Adding new environmental variable GRIST_WIDGET_LIST_URL that points to a
json file with an array of available widgets. When not present, custom widget menu is
hidden, exposing only Custom URL option.

Available widget list can be fetched from:
https://github.com/gristlabs/grist-widget/releases/download/latest/manifest.json

Test Plan: New tests, and updated old ones.

Reviewers: paulfitz, dsagal

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3127

app/client/components/CursorMonitor.ts

@@ -52,7 +52,9 @@ export class CursorMonitor extends Disposable {
     this.autoDispose(doc.cursorPosition.addListener(pos => {
       // if current position is not restored yet, don't change it
       if (!this._restored) { return; }
-      if (pos) { this._storePosition(pos); }
+      // store position only when we have valid rowId
+      // for some views (like CustomView) cursor position might not reflect actual row
+      if (pos && pos.rowId !== undefined) { this._storePosition(pos); }
     }));
   }
 

app/client/components/ViewConfigTab.js

@@ -771,20 +771,7 @@ ViewConfigTab.prototype._buildCustomTypeItems = function() {
   }, {
 
     // 2)
-    showObs: () => activeSection().customDef.mode() === "url",
-    buildDom: () => kd.scope(activeSection, ({customDef}) => dom('div',
-      kf.row(18, kf.text(customDef.url, {placeholder: "Full URL of webpage to show"}, dom.testId('ViewConfigTab_url'))),
-      kf.row(5, "Access", 13, dom(kf.select(customDef.access, ['none', 'read table', 'full']), dom.testId('ViewConfigTab_customView_access'))),
-      kf.helpRow('none: widget has no access to document.',
-                 kd.style('text-align', 'left'),
-                 kd.style('margin-top', '1.5rem')),
-      kf.helpRow('read table: widget can read the selected table.',
-                 kd.style('text-align', 'left'),
-                 kd.style('margin-top', '1.5rem')),
-      kf.helpRow('full: widget can read, modify, and copy the document.',
-                 kd.style('text-align', 'left'),
-                 kd.style('margin-top', '1.5rem'))
-    )),
+    // TODO: refactor this part, Custom Widget moved to separate file.
   }, {
 
     // 3)

app/client/models/entities/ViewSectionRec.ts

@@ -1,6 +1,7 @@
 import * as BaseView from 'app/client/components/BaseView';
 import { ColumnRec, FilterRec, TableRec, ViewFieldRec, ViewRec } from 'app/client/models/DocModel';
 import * as modelUtil from 'app/client/models/modelUtil';
+import {ICustomWidget} from 'app/common/CustomWidget';
 import * as ko from 'knockout';
 import { CursorPos, } from 'app/client/components/Cursor';
 import { KoArray, } from 'app/client/lib/koArray';
@@ -131,29 +132,36 @@ export interface ViewSectionRec extends IRowModel<"_grist_Views_section"> {
 
   // Apply `filter` to the field or column identified by `colRef`.
   setFilter(colRef: number, filter: string): void;
+
+  // Saves custom definition (bundles change)
+  saveCustomDef(): Promise<void>;
 }
 
 export interface CustomViewSectionDef {
   /**
    * The mode.
    */
-  mode: ko.Observable<"url"|"plugin">;
+  mode: modelUtil.KoSaveableObservable<"url"|"plugin">;
   /**
    * The url.
    */
-  url: ko.Observable<string>;
+  url: modelUtil.KoSaveableObservable<string|null>;
+   /**
+   * Custom widget information.
+   */
+  widgetDef: modelUtil.KoSaveableObservable<ICustomWidget|null>;
   /**
    * Access granted to url.
    */
-  access: ko.Observable<string>;
+  access: modelUtil.KoSaveableObservable<string>;
   /**
    * The plugin id.
    */
-  pluginId: ko.Observable<string>;
+  pluginId: modelUtil.KoSaveableObservable<string>;
   /**
    * The section id.
    */
-  sectionId: ko.Observable<string>;
+  sectionId: modelUtil.KoSaveableObservable<string>;
 }
 
 // Information about filters for a field or hidden column.
@@ -185,7 +193,8 @@ export function createViewSectionRec(this: ViewSectionRec, docModel: DocModel):
 
   const customViewDefaults = {
     mode: 'url',
-    url: '',
+    url: null,
+    widgetDef: null,
     access: '',
     pluginId: '',
     sectionId: ''
@@ -196,11 +205,16 @@ export function createViewSectionRec(this: ViewSectionRec, docModel: DocModel):
   this.customDef = {
     mode: customDefObj.prop('mode'),
     url: customDefObj.prop('url'),
+    widgetDef: customDefObj.prop('widgetDef'),
     access: customDefObj.prop('access'),
     pluginId: customDefObj.prop('pluginId'),
     sectionId: customDefObj.prop('sectionId')
   };
 
+  this.saveCustomDef = () => {
+    return customDefObj.save();
+  };
+
   this.themeDef = modelUtil.fieldWithDefault(this.theme, 'form');
   this.chartTypeDef = modelUtil.fieldWithDefault(this.chartType, 'bar');
   this.view = refRecord(docModel.views, this.parentId);

app/client/models/errors.ts

@@ -123,9 +123,14 @@ export function reportError(err: Error|string): void {
     } else {
       // If we don't recognize it, consider it an application error (bug) that the user should be
       // able to report.
+      if (details?.userError) {
+        // If we have user friendly error, show it instead.
+        _notifier.createAppError(Error(details.userError));
+      } else {
         _notifier.createAppError(err);
       }
     }
+  }
 }
 
 /**

app/client/ui/CustomSectionConfig.ts

@@ -0,0 +1,280 @@
+import * as kf from 'app/client/lib/koForm';
+import {ViewSectionRec} from 'app/client/models/DocModel';
+import {reportError} from 'app/client/models/errors';
+import {cssLabel, cssRow, cssTextInput} from 'app/client/ui/RightPanel';
+import {basicButton, primaryButton} from 'app/client/ui2018/buttons';
+import {colors} from 'app/client/ui2018/cssVars';
+import {cssLink} from 'app/client/ui2018/links';
+import {IOptionFull, select} from 'app/client/ui2018/menus';
+import {AccessLevel, ICustomWidget} from 'app/common/CustomWidget';
+import {GristLoadConfig} from 'app/common/gristUrls';
+import {nativeCompare} from 'app/common/gutil';
+import {UserAPI} from 'app/common/UserAPI';
+import {bundleChanges, Computed, Disposable, dom,
+        makeTestId, MultiHolder, Observable, styled} from 'grainjs';
+import {icon} from 'app/client/ui2018/icons';
+
+// Custom URL widget id - used as mock id for selectbox.
+const CUSTOM_ID = "custom";
+const testId = makeTestId('test-config-widget-');
+
+
+/**
+ * Custom Widget section.
+ * Allows to select custom widget from the list of available widgets
+ * (taken from /widgets endpoint), or enter a Custom URL.
+ * When Custom Widget has a desired access level (in accessLevel field),
+ * will prompt user to approve it. "None" access level is auto approved,
+ * so prompt won't be shown.
+ *
+ * When gristConfig.enableWidgetRepository is set to false, it will only
+ * allow to specify Custom URL.
+ */
+
+export class CustomSectionConfig extends Disposable {
+  // Holds all available widget definitions.
+  private _widgets: Observable<ICustomWidget[]>;
+  // Holds selected option (either custom or a widgetId).
+  private _selected: Computed<string|null>;
+  // Holds custom widget URL.
+  private _url: Computed<string>;
+  // Enable or disable widget repository.
+  private _canSelect = true;
+  // Selected access level.
+  private _selectedAccess: Computed<AccessLevel>;
+  // When widget is changed, it sets its desired access level. We will prompt
+  // user to approve or reject it.
+  private _desiredAccess: Observable<AccessLevel>;
+  // Current access level (stored inside a section).
+  private _currentAccess: Computed<AccessLevel>;
+
+  constructor(section: ViewSectionRec, api: UserAPI) {
+    super();
+
+    // Test if we can offer widget list.
+    const gristConfig: GristLoadConfig = (window as any).gristConfig || {};
+    this._canSelect = gristConfig.enableWidgetRepository ?? true;
+
+    // Array of available widgets - will be updated asynchronously.
+    this._widgets = Observable.create(this, []);
+
+    if (this._canSelect) {
+      // From the start we will provide single widget definition
+      // that was chosen previously.
+      if (section.customDef.widgetDef.peek()) {
+        this._widgets.set([section.customDef.widgetDef.peek()!]);
+      }
+      // Request for rest of the widgets.
+      api.getWidgets().then(widgets => {
+        if (this.isDisposed()) {
+          return;
+        }
+        const existing = section.customDef.widgetDef.peek();
+        // Make sure we have current widget in place.
+        if (existing && !widgets.some(w => w.widgetId === existing.widgetId)) {
+          widgets.push(existing);
+        }
+        this._widgets.set(widgets.sort((a, b) => nativeCompare(a.name.toLowerCase(), b.name.toLowerCase())));
+      }).catch(err => {
+        reportError(err);
+      });
+    }
+
+    // Create temporary variable that will hold blank Custom Url state. When url is blank and widgetDef is not stored
+    // we can either show "Select Custom Widget" or a Custom Url with a blank url.
+    // To distinguish those states, we will mark Custom Url state at start (by checking that url is not blank and
+    // widgetDef is not set). And then switch it during selectbox manipulation.
+    const wantsToBeCustom = Observable.create(this,
+      Boolean(section.customDef.url.peek() && !section.customDef.widgetDef.peek())
+    );
+
+    // Selected value from the dropdown (contains widgetId or "custom" string for Custom URL)
+    this._selected = Computed.create(this, use => {
+      if (use(section.customDef.widgetDef)) {
+        return section.customDef.widgetDef.peek()!.widgetId;
+      }
+      if (use(section.customDef.url) || use(wantsToBeCustom)) {
+        return CUSTOM_ID;
+      }
+      return null;
+    });
+    this._selected.onWrite(async (value) => {
+      if (value === CUSTOM_ID) {
+        // Select Custom URL
+        bundleChanges(() => {
+          // Clear url.
+          section.customDef.url(null);
+          // Clear widget definition.
+          section.customDef.widgetDef(null);
+          // Set intermediate state
+          wantsToBeCustom.set(true);
+          // Reset access level to none.
+          section.customDef.access(AccessLevel.none);
+          this._desiredAccess.set(AccessLevel.none);
+        });
+        await section.saveCustomDef();
+      } else {
+        // Select Widget
+        const selectedWidget = this._widgets.get().find(w => w.widgetId === value);
+        if (!selectedWidget) {
+          // should not happen
+          throw new Error("Error accessing widget from the list");
+        }
+        // If user selected the same one, do nothing.
+        if (section.customDef.widgetDef.peek()?.widgetId === value) {
+          return;
+        }
+        bundleChanges(() => {
+          // Clear access level
+          section.customDef.access(AccessLevel.none);
+          // When widget wants some access, set desired access level.
+          this._desiredAccess.set(selectedWidget.accessLevel || AccessLevel.none);
+          // Update widget definition.
+          section.customDef.widgetDef(selectedWidget);
+          // Update widget URL.
+          section.customDef.url(selectedWidget.url);
+          // Clear intermediate state.
+          wantsToBeCustom.set(false);
+        });
+        await section.saveCustomDef();
+      }
+    });
+
+    // Url for the widget, taken either from widget definition, or provided by hand for Custom URL.
+    // For custom widget, we will store url also in section definition.
+    this._url = Computed.create(this, use => use(section.customDef.url) || "");
+    this._url.onWrite((newUrl) => section.customDef.url.setAndSave(newUrl));
+
+    // Compute current access level.
+    this._currentAccess = Computed.create(this,
+      use => use(section.customDef.access) as AccessLevel || AccessLevel.none);
+
+    // From the start desired access level is the same as current one.
+    this._desiredAccess = Observable.create(this, this._currentAccess.get());
+
+    // Selected access level will show desired one, but will updated both (desired and current).
+    this._selectedAccess = Computed.create(this, use => use(this._desiredAccess));
+    this._selectedAccess.onWrite(async newAccess => {
+      this._desiredAccess.set(newAccess);
+      await section.customDef.access.setAndSave(newAccess);
+    });
+
+    // Clear intermediate state when section changes.
+    this.autoDispose(section.id.subscribe(() => wantsToBeCustom.set(false)));
+    this.autoDispose(section.id.subscribe(() => this._reject()));
+  }
+
+  public buildDom() {
+    // UI observables holder.
+    const holder = new MultiHolder();
+
+    // Show prompt, when desired access level is different from actual one.
+    const prompt = Computed.create(holder, use => use(this._currentAccess) !== use(this._desiredAccess));
+    // If this is empty section or not.
+    const isSelected = Computed.create(holder, use => Boolean(use(this._selected)));
+    // If user is using custom url.
+    const isCustom = Computed.create(holder, use => use(this._selected) === CUSTOM_ID || !this._canSelect);
+    // Options for the selectbox (all widgets definitions and Custom URL)
+    const options = Computed.create(holder, use => [
+      {label: 'Custom URL', value: 'custom'},
+      ...use(this._widgets).map(w => ({label: w.name, value: w.widgetId})),
+    ]);
+    // Options for access level.
+    const levels: IOptionFull<string>[] = [
+      {label: 'No document access', value: AccessLevel.none},
+      {label: 'Read selected table', value: AccessLevel.read_table},
+      {label: 'Full document access', value: AccessLevel.full},
+    ];
+    return dom(
+      'div',
+      dom.autoDispose(holder),
+      this._canSelect ?
+      cssRow(
+        select(this._selected, options, {
+          defaultLabel: 'Select Custom Widget',
+          menuCssClass: cssMenu.className
+        }),
+        testId('select')
+      ) : null,
+      dom.maybe(isCustom, () => [
+        cssRow(
+          cssTextInput(
+            this._url,
+            async value => this._url.set(value),
+            dom.attr('placeholder', 'Enter Custom URL'),
+            testId('url')
+          )
+        ),
+      ]),
+      cssSection(
+        cssLink(
+          dom.attr('href', 'https://support.getgrist.com/widget-custom'),
+          dom.attr('target', '_blank'),
+          'Learn more about custom widgets'
+        )
+      ),
+      dom.maybe((use) => use(isSelected) || !this._canSelect, () => [
+        cssLabel('ACCESS LEVEL'),
+        cssRow(select(this._selectedAccess, levels), testId('access')),
+        dom.maybe(prompt, () =>
+          kf.prompt(
+            {tabindex: '-1'},
+            cssColumns(
+              cssWarningWrapper(
+                icon('Lock'),
+              ),
+              dom('div',
+                cssConfirmRow(
+                  "Approve requested access level?"
+                ),
+                cssConfirmRow(
+                  primaryButton("Accept",
+                    testId('access-accept'),
+                    dom.on('click', () => this._accept())),
+                  basicButton("Reject",
+                    testId('access-reject'),
+                    dom.on('click', () => this._reject()))
+                )
+              )
+            )
+          )
+        )
+      ])
+    );
+  }
+
+  private _accept() {
+    this._selectedAccess.set(this._desiredAccess.get());
+    this._reject();
+  }
+
+  private _reject() {
+    this._desiredAccess.set(this._currentAccess.get());
+  }
+}
+
+const cssWarningWrapper = styled('div', `
+  padding-left: 8px;
+  padding-top: 6px;
+  --icon-color: ${colors.lightGreen}
+`);
+
+const cssColumns = styled('div', `
+  display: flex;
+`);
+
+const cssConfirmRow = styled('div', `
+  display: flex;
+  padding: 8px;
+  gap: 8px;
+`);
+
+const cssSection = styled('div', `
+  margin: 16px 16px 12px 16px;
+`);
+
+const cssMenu = styled('div', `
+  & > li:first-child {
+    border-bottom: 1px solid ${colors.mediumGrey};
+  }
+`);

app/client/ui/RightPanel.ts

@@ -40,6 +40,7 @@ import {bundleChanges, Computed, Disposable, dom, domComputed, DomContents,
         DomElementArg, DomElementMethod, IDomComponent} from 'grainjs';
 import {MultiHolder, Observable, styled, subscribe} from 'grainjs';
 import * as ko from 'knockout';
+import {CustomSectionConfig} from 'app/client/ui/CustomSectionConfig';
 
 // Represents a top tab of the right side-pane.
 const TopTab = StringUnion("pageWidget", "field");
@@ -337,7 +338,7 @@ export class RightPanel extends Disposable {
             // In the default url mode, allow picking a url and granting/forbidding
             // access to data.
             dom.maybe(use => use(activeSection.customDef.mode) === 'url',
-                      () => dom('div', parts[1].buildDom())),
+                      () => dom.create(CustomSectionConfig, activeSection, this._gristDoc.app.topAppModel.api)),
           ];
         }
       ]),

app/common/CustomWidget.ts

@@ -0,0 +1,39 @@
+/**
+ * Custom widget manifest definition.
+ */
+export interface ICustomWidget {
+  /**
+   * Widget friendly name, used on the UI.
+   */
+  name: string;
+  /**
+   * Widget unique id, probably in npm package format @gristlabs/custom-widget-name.
+   */
+  widgetId: string;
+  /**
+   * Custom widget main page URL.
+   */
+  url: string;
+  /**
+   * Optional desired access level.
+   */
+  accessLevel?: AccessLevel;
+}
+
+/**
+ * Widget access level.
+ */
+export enum AccessLevel {
+  /**
+   * Default, no access to Grist.
+   */
+  none = "none",
+  /**
+   * Read only access to table the widget is based on.
+   */
+  read_table = "read table",
+  /**
+   * Full access to document on user's behalf.
+   */
+  full = "full",
+}

app/common/UserAPI.ts

@@ -6,6 +6,7 @@ import {BrowserSettings} from 'app/common/BrowserSettings';
 import {BulkColValues, TableColValues, UserAction} from 'app/common/DocActions';
 import {DocCreationInfo, OpenDocMode} from 'app/common/DocListAPI';
 import {Features} from 'app/common/Features';
+import {ICustomWidget} from 'app/common/CustomWidget';
 import {isClient} from 'app/common/gristUrls';
 import {FullUser} from 'app/common/LoginSessionAPI';
 import {OrgPrefs, UserOrgPrefs, UserPrefs} from 'app/common/Prefs';
@@ -321,6 +322,7 @@ export interface UserAPI {
   deleteUser(userId: number, name: string): Promise<void>;
   getBaseUrl(): string;  // Get the prefix for all the endpoints this object wraps.
   forRemoved(): UserAPI; // Get a version of the API that works on removed resources.
+  getWidgets(): Promise<ICustomWidget[]>;
 }
 
 /**
@@ -428,6 +430,10 @@ export class UserAPIImpl extends BaseAPI implements UserAPI {
     return this.requestJson(`${this._url}/api/templates?onlyFeatured=${onlyFeatured ? 1 : 0}`, { method: 'GET' });
   }
 
+  public async getWidgets(): Promise<ICustomWidget[]> {
+    return await this.requestJson(`${this._url}/api/widgets`, { method: 'GET' });
+  }
+
   public async getDoc(docId: string): Promise<Document> {
     return this.requestJson(`${this._url}/api/docs/${docId}`, { method: 'GET' });
   }

app/common/gristUrls.ts

@@ -469,6 +469,9 @@ export interface GristLoadConfig {
 
   // List of registered plugins (used by HomePluginManager and DocPluginManager)
   plugins?: LocalPlugin[];
+
+  // If custom widget list is available.
+  enableWidgetRepository?: boolean;
 }
 
 // Acceptable org subdomains are alphanumeric (hyphen also allowed) and of

app/gen-server/ApiServer.ts

@@ -12,6 +12,7 @@ import {RequestWithOrg} from 'app/server/lib/extractOrg';
 import * as log from 'app/server/lib/log';
 import {addPermit, getDocScope, getScope, integerParam, isParameterOn, sendOkReply,
         sendReply, stringParam} from 'app/server/lib/requestUtils';
+import {IWidgetRepository} from 'app/server/lib/WidgetRepository';
 import {Request} from 'express';
 
 import {User} from './entity/User';
@@ -98,7 +99,8 @@ export class ApiServer {
    */
   constructor(
     private _app: express.Application,
-    private _dbManager: HomeDBManager
+    private _dbManager: HomeDBManager,
+    private _widgetRepository: IWidgetRepository
   ) {
     this._addEndpoints();
   }
@@ -238,6 +240,13 @@ export class ApiServer {
       return sendReply(req, res, query);
     }));
 
+    // GET /api/widgets/
+    // Get all widget definitions from external source.
+    this._app.get('/api/widgets/', expressWrap(async (req, res) => {
+      const widgetList = await this._widgetRepository.getWidgets();
+      return sendOkReply(req, res, widgetList);
+    }));
+
     // PATCH /api/docs/:did
     // Update the specified doc.
     this._app.patch('/api/docs/:did', expressWrap(async (req, res) => {

app/server/lib/FlexServer.ts

@@ -54,6 +54,7 @@ import * as shutdown from 'app/server/lib/shutdown';
 import {TagChecker} from 'app/server/lib/TagChecker';
 import {startTestingHooks} from 'app/server/lib/TestingHooks';
 import {addUploadRoute} from 'app/server/lib/uploads';
+import {buildWidgetRepository, IWidgetRepository} from 'app/server/lib/WidgetRepository';
 import axios from 'axios';
 import * as bodyParser from 'body-parser';
 import * as express from 'express';
@@ -121,6 +122,7 @@ export class FlexServer implements GristServer {
   private _sessionStore: SessionStore;
   private _storageManager: IDocStorageManager;
   private _docWorkerMap: IDocWorkerMap;
+  private _widgetRepository: IWidgetRepository;
   private _internalPermitStore: IPermitStore;  // store for permits that stay within our servers
   private _externalPermitStore: IPermitStore;  // store for permits that pass through outside servers
   private _disabled: boolean = false;
@@ -271,6 +273,11 @@ export class FlexServer implements GristServer {
     return this._storageManager;
   }
 
+  public getWidgetRepository(): IWidgetRepository {
+    if (!this._widgetRepository) { throw new Error('no widget repository available'); }
+    return this._widgetRepository;
+  }
+
   public addLogging() {
     if (this._check('logging')) { return; }
     if (process.env.GRIST_LOG_SKIP_HTTP) { return; }
@@ -524,7 +531,7 @@ export class FlexServer implements GristServer {
 
     // ApiServer's constructor adds endpoints to the app.
     // tslint:disable-next-line:no-unused-expression
-    new ApiServer(this.app, this._dbManager);
+    new ApiServer(this.app, this._dbManager, this._widgetRepository = buildWidgetRepository());
   }
 
   public addBillingApi() {

app/server/lib/ITestingHooks-ti.ts

@@ -19,6 +19,7 @@ export const ITestingHooks = t.iface([], {
   "getDocClientCounts": t.func(t.array(t.tuple("string", "number"))),
   "setActiveDocTimeout": t.func("number", t.param("seconds", "number")),
   "setDiscourseConnectVar": t.func(t.union("string", "null"), t.param("varName", "string"), t.param("value", t.union("string", "null"))),
+  "setWidgetRepositoryUrl": t.func("void", t.param("url", "string")),
 });
 
 const exportedTypeSuite: t.ITypeSuite = {

app/server/lib/ITestingHooks.ts

@@ -15,4 +15,5 @@ export interface ITestingHooks {
   getDocClientCounts(): Promise<Array<[string, number]>>;
   setActiveDocTimeout(seconds: number): Promise<number>;
   setDiscourseConnectVar(varName: string, value: string|null): Promise<string|null>;
+  setWidgetRepositoryUrl(url: string): Promise<void>;
 }

app/server/lib/TestingHooks.ts

@@ -12,6 +12,7 @@ import {FlexServer} from './FlexServer';
 import {ITestingHooks} from './ITestingHooks';
 import ITestingHooksTI from './ITestingHooks-ti';
 import {connect, fromCallback} from './serverUtils';
+import {WidgetRepositoryImpl} from 'app/server/lib/WidgetRepository';
 
 const tiCheckers = t.createCheckers(ITestingHooksTI, {UserProfile: t.name("object")});
 
@@ -194,4 +195,12 @@ export class TestingHooks implements ITestingHooks {
     }
     return prev;
   }
+
+  public async setWidgetRepositoryUrl(url: string): Promise<void> {
+    const repo = this._server.getWidgetRepository() as WidgetRepositoryImpl;
+    if (!(repo instanceof WidgetRepositoryImpl)) {
+      throw new Error("Unsupported widget repository");
+    }
+    repo.testOverrideUrl(url);
+  }
 }

app/server/lib/WidgetRepository.ts

@@ -0,0 +1,92 @@
+import {ICustomWidget} from 'app/common/CustomWidget';
+import * as log from 'app/server/lib/log';
+import fetch from 'node-fetch';
+import {ApiError} from 'app/common/ApiError';
+import * as LRUCache from 'lru-cache';
+
+/**
+ * Widget Repository returns list of available Custom Widgets.
+ */
+export interface IWidgetRepository {
+  getWidgets(): Promise<ICustomWidget[]>;
+}
+
+// Static url for StaticWidgetRepository
+const STATIC_URL = process.env.GRIST_WIDGET_LIST_URL;
+
+/**
+ * Default repository that gets list of available widgets from a static URL.
+ */
+export class WidgetRepositoryImpl implements IWidgetRepository {
+  constructor(protected _staticUrl = STATIC_URL) {}
+
+  /**
+   * Method exposed for testing, overrides widget url.
+   */
+  public testOverrideUrl(url: string) {
+    this._staticUrl = url;
+  }
+
+  public async getWidgets(): Promise<ICustomWidget[]> {
+    if (!this._staticUrl) {
+      log.warn(
+        'WidgetRepository: Widget repository is not configured.' + !STATIC_URL
+          ? ' Missing GRIST_WIDGET_LIST_URL environmental variable.'
+          : ''
+      );
+      return [];
+    }
+    try {
+      const response = await fetch(this._staticUrl);
+      if (!response.ok) {
+        if (response.status === 404) {
+          throw new ApiError('WidgetRepository: Remote widget list not found', 404);
+        } else {
+          const body = await response.text().catch(() => '');
+          throw new ApiError(
+            `WidgetRepository: Remote server returned an error: ${body || response.statusText}`, response.status
+          );
+        }
+      }
+      const widgets = await response.json().catch(() => null);
+      if (!widgets || !Array.isArray(widgets)) {
+        throw new ApiError('WidgetRepository: Error reading widget list', 500);
+      }
+      return widgets;
+    } catch (err) {
+      if (!(err instanceof ApiError)) {
+        throw new ApiError(String(err), 500);
+      }
+      throw err;
+    }
+  }
+}
+
+/**
+ * Version of WidgetRepository that caches successful result for 2 minutes.
+ */
+class CachedWidgetRepository extends WidgetRepositoryImpl {
+  private _cache = new LRUCache<1, ICustomWidget[]>({maxAge : 1000 * 60 /* minute */ * 2});
+  public async getWidgets() {
+    if (this._cache.has(1)) {
+      log.debug("WidgetRepository: Widget list taken from the cache.");
+      return this._cache.get(1)!;
+    }
+    const list = await super.getWidgets();
+    // Cache only if there are some widgets.
+    if (list.length) { this._cache.set(1, list); }
+    return list;
+  }
+
+  public testOverrideUrl(url: string) {
+    super.testOverrideUrl(url);
+    this._cache.reset();
+  }
+}
+
+/**
+ * Returns widget repository implementation.
+ */
+export function buildWidgetRepository() {
+  return new CachedWidgetRepository();
+}

app/server/lib/sendAppPage.ts

@@ -45,6 +45,7 @@ export function makeGristConfig(homeUrl: string|null, extra: Partial<GristLoadCo
     maxUploadSizeImport: (Number(process.env.GRIST_MAX_UPLOAD_IMPORT_MB) * 1024 * 1024) || undefined,
     maxUploadSizeAttachment: (Number(process.env.GRIST_MAX_UPLOAD_ATTACHMENT_MB) * 1024 * 1024) || undefined,
     timestampMs: Date.now(),
+    enableWidgetRepository: Boolean(process.env.GRIST_WIDGET_LIST_URL),
     ...extra,
   };
 }