Add STORY_STATE_CHANGED event

lib/client-api/src/story_store.test.ts

@@ -93,6 +93,21 @@ describe('preview.story_store', () => {
       store.setStoryState('a--1', { baz: 'bing' });
       expect(store.getRawStory('a', '1').state).toEqual({ foo: 'bar', baz: 'bing' });
     });
+
+    it('synchronously emits STORY_STATE_CHANGED if different', () => {
+      const onStateChanged = jest.fn();
+      const testChannel = createChannel({ page: 'preview' });
+      testChannel.on(Events.STORY_STATE_CHANGED, onStateChanged);
+
+      const store = new StoryStore({ channel: testChannel });
+      addStoryToStore(store, 'a', '1', () => 0);
+
+      store.setStoryState('a--1', { foo: 'bar' });
+      expect(onStateChanged).toHaveBeenCalledWith('a--1', { foo: 'bar' });
+
+      store.setStoryState('a--1', { baz: 'bing' });
+      expect(onStateChanged).toHaveBeenCalledWith('a--1', { foo: 'bar', baz: 'bing' });
+    });
   });
 
   describe('storySort', () => {

lib/client-api/src/story_store.ts

@@ -349,8 +349,11 @@ export default class StoryStore extends EventEmitter {
   }
 
   setStoryState(id: string, newState: StoryState) {
+    if (!this._stories[id]) throw new Error(`No story for id ${id}`);
     const { state } = this._stories[id];
     this._stories[id].state = { ...state, ...newState };
+
+    this._channel.emit(Events.STORY_STATE_CHANGED, id, this._stories[id].state);
   }
 
   // This API is a reimplementation of Storybook's original getStorybook() API.

lib/core-events/src/index.ts

@@ -10,6 +10,7 @@ enum events {
   STORY_ADDED = 'storyAdded',
   STORY_CHANGED = 'storyChanged',
   STORY_UNCHANGED = 'storyUnchanged',
+  STORY_STATE_CHANGED = 'storyStateChanged',
   FORCE_RE_RENDER = 'forceReRender',
   REGISTER_SUBSCRIPTION = 'registerSubscription',
   STORY_INIT = 'storyInit',
@@ -44,6 +45,7 @@ export const {
   REGISTER_SUBSCRIPTION,
   STORY_INIT,
   STORY_ADDED,
+  STORY_STATE_CHANGED,
   STORY_RENDER,
   STORY_RENDERED,
   STORY_MISSING,