Added some tests for event emitter

Author: Brian Vaughn

packages/react-devtools-shared/src/__tests__/events-test.js

@@ -0,0 +1,96 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+describe('events', () => {
+  let dispatcher;
+
+  beforeEach(() => {
+    const EventEmitter = require('../events').default;
+
+    dispatcher = new EventEmitter();
+  });
+
+  it('can dispatch an event with no listeners', () => {
+    dispatcher.emit('event', 123);
+  });
+
+  it('handles a listener being attached multiple times', () => {
+    const callback = jest.fn();
+
+    dispatcher.addListener('event', callback);
+    dispatcher.addListener('event', callback);
+  });
+
+  it('notifies all attached listeners of events', () => {
+    const callback1 = jest.fn();
+    const callback2 = jest.fn();
+    const callback3 = jest.fn();
+
+    dispatcher.addListener('event', callback1);
+    dispatcher.addListener('event', callback2);
+    dispatcher.addListener('other-event', callback3);
+    dispatcher.emit('event', 123);
+
+    expect(callback1).toHaveBeenCalledTimes(1);
+    expect(callback1).toHaveBeenCalledWith(123);
+    expect(callback2).toHaveBeenCalledTimes(1);
+    expect(callback2).toHaveBeenCalledWith(123);
+    expect(callback3).not.toHaveBeenCalled();
+  });
+
+  it('calls later listeners before re-throwing if an earlier one throws', () => {
+    const callbackThatThrows = jest.fn(() => {
+      throw Error('expected');
+    });
+    const callback = jest.fn();
+
+    dispatcher.addListener('event', callbackThatThrows);
+    dispatcher.addListener('event', callback);
+
+    expect(() => {
+      dispatcher.emit('event', 123);
+    }).toThrow('expected');
+
+    expect(callbackThatThrows).toHaveBeenCalledTimes(1);
+    expect(callbackThatThrows).toHaveBeenCalledWith(123);
+    expect(callback).toHaveBeenCalledTimes(1);
+    expect(callback).toHaveBeenCalledWith(123);
+  });
+
+  it('removes attached listeners', () => {
+    const callback1 = jest.fn();
+    const callback2 = jest.fn();
+
+    dispatcher.addListener('event', callback1);
+    dispatcher.addListener('other-event', callback2);
+    dispatcher.removeListener('event', callback1);
+    dispatcher.emit('event', 123);
+    expect(callback1).not.toHaveBeenCalled();
+    dispatcher.emit('other-event', 123);
+    expect(callback2).toHaveBeenCalledTimes(1);
+    expect(callback2).toHaveBeenCalledWith(123);
+  });
+
+  it('removes all listeners', () => {
+    const callback1 = jest.fn();
+    const callback2 = jest.fn();
+    const callback3 = jest.fn();
+
+    dispatcher.addListener('event', callback1);
+    dispatcher.addListener('event', callback2);
+    dispatcher.addListener('other-event', callback3);
+    dispatcher.removeAllListeners();
+    dispatcher.emit('event', 123);
+    dispatcher.emit('other-event', 123);
+
+    expect(callback1).not.toHaveBeenCalled();
+    expect(callback2).not.toHaveBeenCalled();
+    expect(callback3).not.toHaveBeenCalled();
+  });
+});