Add types for current Observable interpolation interface (reduxjs#3067)

* Add types about Observable interpolation

* Run through Prettier

* Also run through Prettier

index.d.ts

@@ -1,3 +1,5 @@
+/// <reference types="symbol-observable" />
+
 /**
  * An *action* is a plain object that represents an intention to change the
  * state. Actions are the only way to get data into the store. Any data,
@@ -129,6 +131,32 @@ export interface Unsubscribe {
   (): void
 }
 
+/**
+ * A minimal observable of state changes.
+ * For more information, see the observable proposal:
+ * https://github.com/tc39/proposal-observable
+ */
+export type Observable<T> = {
+  /**
+   * The minimal observable subscription method.
+   * @param {Object} observer Any object that can be used as an observer.
+   * The observer object should have a `next` method.
+   * @returns {subscription} An object with an `unsubscribe` method that can
+   * be used to unsubscribe the observable from the store, and prevent further
+   * emission of values from the observable.
+   */
+  subscribe: (observer: Observer<T>) => { unsubscribe: Unsubscribe }
+  [Symbol.observable](): Observable<T>
+}
+
+/**
+ * An Observer is used to receive data from an Observable, and is supplied as
+ * an argument to subscribe.
+ */
+export type Observer<T> = {
+  next?(value: T): void
+}
+
 /**
  * A store is an object that holds the application's state tree.
  * There should only be a single store in a Redux app, as the composition
@@ -209,6 +237,14 @@ export interface Store<S = any, A extends Action = AnyAction> {
    * @param nextReducer The reducer for the store to use instead.
    */
   replaceReducer(nextReducer: Reducer<S, A>): void
+
+  /**
+   * Interoperability point for observable/reactive libraries.
+   * @returns {observable} A minimal observable of state changes.
+   * For more information, see the observable proposal:
+   * https://github.com/tc39/proposal-observable
+   */
+  [Symbol.observable](): Observable<S>
 }
 
 export type DeepPartial<T> = {

test/typescript/store.ts

@@ -6,8 +6,10 @@ import {
   StoreEnhancer,
   StoreCreator,
   StoreEnhancerStoreCreator,
-  Unsubscribe
+  Unsubscribe,
+  Observer
 } from 'redux'
+import 'symbol-observable'
 
 type State = {
   a: 'a'
@@ -101,3 +103,15 @@ unsubscribe()
 const newReducer: Reducer<State> = reducer
 
 store.replaceReducer(newReducer)
+
+/* observable */
+
+let observable = store[Symbol.observable]()
+observable = observable[Symbol.observable]()
+const observer: Observer<State> = {
+  next(state: State) {
+    console.log('current state:', state)
+  }
+}
+const unsubscribeFromObservable = observable.subscribe(observer).unsubscribe
+unsubscribeFromObservable()