feat: 🎸 expose .fork() method in plugin contract

src/plugins/expressions/common/service/expressions_services.ts

@@ -130,6 +130,15 @@ export class ExpressionsService {
     context?: ExtraContext
   ): Promise<Output> => this.executor.run<Input, Output, ExtraContext>(ast, input, context);
 
+  /**
+   * Create a new instance of `ExpressionsService`. The new instance inherits
+   * all state of the original `ExpressionsService`, including all expression
+   * types, expression functions and context. Also, all new types and functions
+   * registered in the original services AFTER the forking event will be
+   * available in the forked instance. However, all new types and functions
+   * registered in the forked instances will NOT be available to the original
+   * service.
+   */
   public readonly fork = (): ExpressionsService => {
     const executor = this.executor.fork();
     const renderers = this.renderers;
@@ -139,7 +148,7 @@ export class ExpressionsService {
   };
 
   public setup() {
-    const { executor, renderers, registerFunction, run } = this;
+    const { executor, renderers, registerFunction, run, fork } = this;
 
     const getFunction = executor.getFunction.bind(executor);
     const getFunctions = executor.getFunctions.bind(executor);
@@ -151,6 +160,7 @@ export class ExpressionsService {
     const registerType = executor.registerType.bind(executor);
 
     return {
+      fork,
       getFunction,
       getFunctions,
       getRenderer,

src/plugins/expressions/public/mocks.tsx

@@ -31,6 +31,7 @@ export type Start = jest.Mocked<ExpressionsStart>;
 
 const createSetupContract = (): Setup => {
   const setupContract: Setup = {
+    fork: jest.fn(),
     getFunction: jest.fn(),
     getFunctions: jest.fn(),
     getRenderer: jest.fn(),

src/plugins/expressions/public/plugin.test.ts

@@ -19,6 +19,7 @@
 
 import { expressionsPluginMock } from './mocks';
 import { add } from '../common/test_helpers/expression_functions/add';
+import { ExpressionsService } from '../common';
 
 describe('ExpressionsPublicPlugin', () => {
   test('can instantiate from mocks', async () => {
@@ -27,6 +28,13 @@ describe('ExpressionsPublicPlugin', () => {
   });
 
   describe('setup contract', () => {
+    test('.fork() method returns ExpressionsService', async () => {
+      const { setup } = await expressionsPluginMock.createPlugin();
+      const fork = setup.fork();
+
+      expect(fork).toBeInstanceOf(ExpressionsService);
+    });
+
     describe('.registerFunction()', () => {
       test('can register a function', async () => {
         const { setup } = await expressionsPluginMock.createPlugin();