feat(store): add observable proposal interop to store

- Adds dependency on `symbol-observable` to pull in `Symbol.observable`
- Adds `Symbol.observable` method to the store that returns a generic observable
- Adds comprehensive tests to ensure interoperability. (rxjs 5 was used for a simple integration test, and
  is a dev only dependency)

closes #1631

Author: benlesh

package.json

@@ -63,7 +63,8 @@
   "dependencies": {
     "lodash": "^4.2.1",
     "lodash-es": "^4.2.1",
-    "loose-envify": "^1.1.0"
+    "loose-envify": "^1.1.0",
+    "symbol-observable": "^0.2.1"
   },
   "devDependencies": {
     "babel-cli": "^6.3.15",
@@ -101,6 +102,7 @@
     "isparta": "^4.0.0",
     "mocha": "^2.2.5",
     "rimraf": "^2.3.4",
+    "rxjs": "^5.0.0-beta.6",
     "typescript": "^1.8.0",
     "typescript-definition-tester": "0.0.4",
     "webpack": "^1.9.6"

src/createStore.js

@@ -1,4 +1,5 @@
 import isPlainObject from 'lodash/isPlainObject'
+import $$observable from 'symbol-observable'
 
 /**
  * These are private action types reserved by Redux.
@@ -198,6 +199,49 @@ export default function createStore(reducer, initialState, enhancer) {
     dispatch({ type: ActionTypes.INIT })
   }
 
+  /**
+   * Interoperability point for observable/reactive libraries.
+   * @returns {observable} A minimal observable of state changes.
+   * For more information, see the observable proposal:
+   * https://github.com/zenparsing/es-observable
+   */
+  function observable() {
+    var outerSubscribe = subscribe
+    return {
+      /**
+       * 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) {
+        if (typeof observer !== 'object') {
+          throw new TypeError('Expected observer to be an object')
+        }
+
+        var observeState = () => {
+          if (observer.next) {
+            observer.next(getState())
+          }
+        }
+
+        // send initial state to observer
+        observeState()
+
+        // send subsequent states to observer
+        var unsubscribe = outerSubscribe(observeState)
+
+        // return an unsubscribable
+        return { unsubscribe }
+      },
+      [$$observable]() {
+        return this
+      }
+    }
+  }
+
   // When a store is created, an "INIT" action is dispatched so that every
   // reducer returns their initial state. This effectively populates
   // the initial state tree.
@@ -207,6 +251,7 @@ export default function createStore(reducer, initialState, enhancer) {
     dispatch,
     subscribe,
     getState,
-    replaceReducer
+    replaceReducer,
+    [$$observable]: observable
   }
 }

test/createStore.spec.js

@@ -2,6 +2,8 @@ import expect from 'expect'
 import { createStore, combineReducers } from '../src/index'
 import { addTodo, dispatchInMiddle, throwError, unknownAction } from './helpers/actionCreators'
 import * as reducers from './helpers/reducers'
+import * as Rx from 'rxjs'
+import $$observable from 'symbol-observable'
 
 describe('createStore', () => {
   it('exposes the public API', () => {
@@ -610,4 +612,118 @@ describe('createStore', () => {
       store.subscribe(undefined)
     ).toThrow()
   })
+
+  describe('Symbol.observable interop point', () => {
+    it('should exist', () => {
+      const store = createStore(() => {})
+      expect(typeof store[$$observable]).toBe('function')
+    })
+
+    describe('returned value', () => {
+      it('should be subscribable', () => {
+        const store = createStore(() => {})
+        const obs = store[$$observable]()
+        expect(typeof obs.subscribe).toBe('function')
+      })
+
+      it('should throw a TypeError if an observer object is not supplied to subscribe', () => {
+        const store = createStore(() => {})
+        const obs = store[$$observable]()
+
+        expect(function () {
+          obs.subscribe()
+        }).toThrow()
+
+        expect(function () {
+          obs.subscribe(() => {})
+        }).toThrow()
+
+        expect(function () {
+          obs.subscribe({})
+        }).toNotThrow()
+      })
+
+      it('should return a subscription object when subscribed', () => {
+        const store = createStore(() => {})
+        const obs = store[$$observable]()
+        const sub = obs.subscribe({})
+        expect(typeof sub.unsubscribe).toBe('function')
+      })
+    })
+
+    it('should pass an integration test with no unsubscribe', () => {
+      function foo(state = 0, action) {
+        return action.type === 'foo' ? 1 : state
+      }
+
+      function bar(state = 0, action) {
+        return action.type === 'bar' ? 2 : state
+      }
+
+      const store = createStore(combineReducers({ foo, bar }))
+      const observable = store[$$observable]()
+      const results = []
+
+      observable.subscribe({
+        next(state) {
+          results.push(state)
+        }
+      })
+
+      store.dispatch({ type: 'foo' })
+      store.dispatch({ type: 'bar' })
+
+      expect(results).toEqual([ { foo: 0, bar: 0 }, { foo: 1, bar: 0 }, { foo: 1, bar: 2 } ])
+    })
+
+    it('should pass an integration test with an unsubscribe', () => {
+      function foo(state = 0, action) {
+        return action.type === 'foo' ? 1 : state
+      }
+
+      function bar(state = 0, action) {
+        return action.type === 'bar' ? 2 : state
+      }
+
+      const store = createStore(combineReducers({ foo, bar }))
+      const observable = store[$$observable]()
+      const results = []
+
+      const sub = observable.subscribe({
+        next(state) {
+          results.push(state)
+        }
+      })
+
+      store.dispatch({ type: 'foo' })
+      sub.unsubscribe()
+      store.dispatch({ type: 'bar' })
+
+      expect(results).toEqual([ { foo: 0, bar: 0 }, { foo: 1, bar: 0 } ])
+    })
+
+    it('should pass an integration test with a common library (RxJS)', () => {
+      function foo(state = 0, action) {
+        return action.type === 'foo' ? 1 : state
+      }
+
+      function bar(state = 0, action) {
+        return action.type === 'bar' ? 2 : state
+      }
+
+      const store = createStore(combineReducers({ foo, bar }))
+      const observable = Rx.Observable.from(store)
+      const results = []
+
+      const sub = observable
+        .map(state => ({ fromRx: true, ...state }))
+        .subscribe(state => results.push(state))
+
+      store.dispatch({ type: 'foo' })
+      sub.unsubscribe()
+      store.dispatch({ type: 'bar' })
+
+      expect(results).toEqual([ { foo: 0, bar: 0, fromRx: true }, { foo: 1, bar: 0, fromRx: true } ])
+    })
+  })
 })