From 0aef2a57d7638ac2f04dd79c269e259b8a3fb2da Mon Sep 17 00:00:00 2001
From: Warren R Bank <warren.r.bank@gmail.com>
Date: Sun, 28 Jan 2018 13:34:52 -0800
Subject: [PATCH 1/3] enhancement: store.subscribe(listener, filter)

new (optional) input parameter: `filter`

* its purpose is to conditionally filter when `listener` is called
* when its value is:
  * undefined:
    * `listener` is called as normal
  * `function`:
    * input: (oldState, newState)
    * output: boolean
      * when truthy, `listener` is not informed of this state change
  * `boolean` && true:
    * `listener` is not informed when `newState`
      is equal (by reference) to `oldState`
---
 src/createStore.js | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/src/createStore.js b/src/createStore.js
index d4a6ad9fd3..d0c29a605f 100644
--- a/src/createStore.js
+++ b/src/createStore.js
@@ -98,11 +98,17 @@ export default function createStore(reducer, preloadedState, enhancer) {
    * @param {Function} listener A callback to be invoked on every dispatch.
    * @returns {Function} A function to remove this change listener.
    */
-  function subscribe(listener) {
+  function subscribe(listener, filter) {
     if (typeof listener !== 'function') {
       throw new Error('Expected the listener to be a function.')
     }
 
+    if (typeof filter !== 'function') {
+      filter = (filter === true) ? pure_filter : false
+    }
+
+    const nextListener = {listener, filter}
+
     if (isDispatching) {
       throw new Error(
         'You may not call store.subscribe() while the reducer is executing. ' +
@@ -115,7 +121,7 @@ export default function createStore(reducer, preloadedState, enhancer) {
     let isSubscribed = true
 
     ensureCanMutateNextListeners()
-    nextListeners.push(listener)
+    nextListeners.push(nextListener)
 
     return function unsubscribe() {
       if (!isSubscribed) {
@@ -132,11 +138,15 @@ export default function createStore(reducer, preloadedState, enhancer) {
       isSubscribed = false
 
       ensureCanMutateNextListeners()
-      const index = nextListeners.indexOf(listener)
+      const index = nextListeners.indexOf(nextListener)
       nextListeners.splice(index, 1)
     }
   }
 
+  function pure_filter(oldState, newState) {
+    return oldState === newState
+  }
+
   /**
    * Dispatches an action. It is the only way to trigger a state change.
    *
@@ -181,6 +191,8 @@ export default function createStore(reducer, preloadedState, enhancer) {
       throw new Error('Reducers may not dispatch actions.')
     }
 
+    const oldState = currentState
+
     try {
       isDispatching = true
       currentState = currentReducer(currentState, action)
@@ -190,8 +202,11 @@ export default function createStore(reducer, preloadedState, enhancer) {
 
     const listeners = (currentListeners = nextListeners)
     for (let i = 0; i < listeners.length; i++) {
-      const listener = listeners[i]
-      listener()
+      const {listener, filter} = listeners[i]
+      const apply_filter = filter && filter(oldState, currentState)
+      if (! apply_filter) {
+        listener()
+      }
     }
 
     return action

From 5d27ee079aaaa7321d579702c3fb525a42f53d79 Mon Sep 17 00:00:00 2001
From: Warren R Bank <warren.r.bank@gmail.com>
Date: Sun, 28 Jan 2018 13:59:47 -0800
Subject: [PATCH 2/3] lint

---
 src/createStore.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/createStore.js b/src/createStore.js
index d0c29a605f..3a2e27be73 100644
--- a/src/createStore.js
+++ b/src/createStore.js
@@ -104,7 +104,7 @@ export default function createStore(reducer, preloadedState, enhancer) {
     }
 
     if (typeof filter !== 'function') {
-      filter = (filter === true) ? pure_filter : false
+      filter = filter === true ? pure_filter : false
     }
 
     const nextListener = {listener, filter}

From a2a7470ce09a028d184f6023739e02de97519a47 Mon Sep 17 00:00:00 2001
From: Warren R Bank <warren.r.bank@gmail.com>
Date: Sun, 28 Jan 2018 14:51:00 -0800
Subject: [PATCH 3/3] lint

npm install
node_modules/.bin/prettier "src/createStore.js" >"src/createStore.js"
---
 src/createStore.js | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/createStore.js b/src/createStore.js
index 3a2e27be73..cf5738063d 100644
--- a/src/createStore.js
+++ b/src/createStore.js
@@ -107,7 +107,7 @@ export default function createStore(reducer, preloadedState, enhancer) {
       filter = filter === true ? pure_filter : false
     }
 
-    const nextListener = {listener, filter}
+    const nextListener = { listener, filter }
 
     if (isDispatching) {
       throw new Error(
@@ -202,9 +202,9 @@ export default function createStore(reducer, preloadedState, enhancer) {
 
     const listeners = (currentListeners = nextListeners)
     for (let i = 0; i < listeners.length; i++) {
-      const {listener, filter} = listeners[i]
+      const { listener, filter } = listeners[i]
       const apply_filter = filter && filter(oldState, currentState)
-      if (! apply_filter) {
+      if (!apply_filter) {
         listener()
       }
     }