Typed channel api proposal

lib/api/src/modules/channel.ts

@@ -3,6 +3,37 @@ import { STORY_CHANGED } from '@storybook/core-events';
 import { Channel, Listener } from '@storybook/channels';
 
 import { Module } from '../index';
+import { Story, StoriesList } from './stories';
+
+interface Event<T> {
+  type: string;
+}
+
+const SELECT_STORY: Event<Story> = {
+  type: 'selectStory',
+};
+
+const SET_STORIES: Event<StoriesList> = {
+  type: 'setStories',
+};
+
+// we wouldn't actually use a map..
+const callbacks = new Map<string, any>();
+
+const on = <T>(event: Event<T>, callback: (param: T) => void) => {
+  callbacks.set(event.type, callback);
+};
+
+const emit = <T>(event: Event<T>, payload: T) => {
+  const callback = callbacks.get(event.type) as ((param: T) => void);
+  callback(payload);
+};
+
+on(SELECT_STORY, story => {
+  // story type inferred as `Story`
+});
+
+emit(SET_STORIES, 'foo'); // Err, Argument of type '"foo"' is not assignable to parameter of type '(Group | Story)
 
 export interface SubAPI {
   getChannel: () => Channel;

lib/api/src/modules/stories.ts

@@ -60,7 +60,7 @@ interface StoryInput {
   isLeaf: boolean;
 }
 
-type Story = StoryInput & Group;
+export type Story = StoryInput & Group;
 
 export interface StoriesHash {
   [id: string]: Group | Story;