test: 💍 add createStartServicesGetter() tests

src/plugins/kibana_utils/public/core/create_start_service_getter.test.ts

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+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import { StartServicesAccessor } from '../../../../core/public';
+import { createStartServicesGetter } from './create_start_service_getter';
+import { Defer } from '../../common/defer';
+
+describe('createStartServicesGetter', () => {
+  test('throws if services are accessed before accessor resolves', async () => {
+    const future = new Defer<any>();
+    const accessor: StartServicesAccessor = async () => await future.promise;
+    const start = createStartServicesGetter(accessor);
+
+    await new Promise(r => setTimeout(r, 1));
+
+    expect(() => start()).toThrowErrorMatchingInlineSnapshot(
+      `"Trying to access start services before start."`
+    );
+  });
+
+  test('returns services after accessor resolves even if first time called before it resolved', async () => {
+    const future = new Defer<any>();
+    const core = {};
+    const plugins = {};
+    const self = {};
+    const accessor: StartServicesAccessor = async () => await future.promise;
+    const start = createStartServicesGetter(accessor);
+
+    await new Promise(r => setTimeout(r, 1));
+
+    expect(() => start()).toThrow();
+
+    await new Promise(r => setTimeout(r, 1));
+    future.resolve([core, plugins, self]);
+    await future.promise;
+
+    expect(start()).toEqual({
+      core,
+      plugins,
+      self,
+    });
+  });
+
+  test('returns services if called after accessor resolves', async () => {
+    const future = new Defer<any>();
+    const core = {};
+    const plugins = {};
+    const self = {};
+    const accessor: StartServicesAccessor = async () => await future.promise;
+    const start = createStartServicesGetter(accessor);
+
+    await new Promise(r => setTimeout(r, 1));
+    future.resolve([core, plugins, self]);
+    await future.promise;
+
+    expect(start()).toEqual({
+      core,
+      plugins,
+      self,
+    });
+  });
+});