From 6b534bb66ff3047e0d5a3a5bdbf8392408c548f8 Mon Sep 17 00:00:00 2001
From: Riad Benguella <benguella@gmail.com>
Date: Tue, 24 Jul 2018 10:47:50 +0100
Subject: [PATCH] Don't repersist if the persisted changes are kept identical

---
 packages/data/src/persist.js      |  8 +++++
 packages/data/src/registry.js     | 24 +++++++++++----
 packages/data/src/test/persist.js | 49 ++++++++++++++++++++++++++++++-
 3 files changed, 74 insertions(+), 7 deletions(-)

diff --git a/packages/data/src/persist.js b/packages/data/src/persist.js
index 56134daabde3c..05a2c42b3f389 100644
--- a/packages/data/src/persist.js
+++ b/packages/data/src/persist.js
@@ -47,6 +47,14 @@ export function restrictPersistence( reducer, keyToPersist ) {
 		const nextState = reducer( state, action );
 
 		if ( action.type === 'SERIALIZE' ) {
+			// Returning the same instance if the state is kept identical avoids reserializing again
+			if (
+				action.previousState &&
+				action.previousState[ keyToPersist ] === nextState[ keyToPersist ]
+			) {
+				return action.previousState;
+			}
+
 			return { [ keyToPersist ]: nextState[ keyToPersist ] };
 		}
 
diff --git a/packages/data/src/registry.js b/packages/data/src/registry.js
index 42d938e8cef26..d0e3df3ff7fb3 100644
--- a/packages/data/src/registry.js
+++ b/packages/data/src/registry.js
@@ -4,6 +4,11 @@
 import { createStore } from 'redux';
 import { flowRight, without, mapValues, overEvery, get } from 'lodash';
 
+/**
+ * WordPress dependencies
+ */
+import isShallowEqual from '@wordpress/is-shallow-equal';
+
 /**
  * Internal dependencies
  */
@@ -334,20 +339,27 @@ export function createRegistry( storeConfigs = {} ) {
 			previousValue = persistedData;
 		}
 
-		// Persist updated preferences
-		subscribe( () => {
+		const triggerPersist = () => {
 			const newValue = Object.entries( namespaces )
 				.filter( ( [ , { persist } ] ) => persist )
 				.reduce( ( memo, [ reducerKey, { reducer, store } ] ) => {
-					memo[ reducerKey ] = reducer( store.getState(), { type: 'SERIALIZE' } );
+					memo[ reducerKey ] = reducer( store.getState(), {
+						type: 'SERIALIZE',
+						previousState: get( previousValue, reducerKey ),
+					} );
 					return memo;
 				}, {} );
 
-			if ( newValue !== previousValue ) {
+			if ( ! isShallowEqual( newValue, previousValue ) ) {
 				persistenceStorage.setItem( storageKey, JSON.stringify( newValue ) );
-				previousValue = newValue;
 			}
-		} );
+
+			previousValue = newValue;
+		};
+
+		// Persist updated preferences
+		subscribe( triggerPersist );
+		triggerPersist();
 	}
 
 	Object.entries( {
diff --git a/packages/data/src/test/persist.js b/packages/data/src/test/persist.js
index b78e33b58ea4d..4a55ae18905e7 100644
--- a/packages/data/src/test/persist.js
+++ b/packages/data/src/test/persist.js
@@ -1,13 +1,14 @@
 /**
  * Internal dependencies
  */
-import { getPersistenceStorage, restrictPersistence } from '../persist';
+import { getPersistenceStorage, setPersistenceStorage, restrictPersistence } from '../persist';
 import { createRegistry } from '../registry';
 
 describe( 'persiss registry', () => {
 	let registry;
 	beforeEach( () => {
 		registry = createRegistry();
+		setPersistenceStorage( window.localStorage );
 	} );
 
 	it( 'should load the initial value from the local storage integrating it into reducer default value.', () => {
@@ -54,6 +55,52 @@ describe( 'persiss registry', () => {
 		expect( JSON.parse( getPersistenceStorage().getItem( storageKey ) ) )
 			.toEqual( { storeKey: { chicken: true } } );
 	} );
+
+	it( 'should not trigger persistence if the value doesn\'t change', () => {
+		const storageKey = 'dumbStorageKey2';
+		let countCalls = 0;
+		const storage = {
+			getItem() {
+				return this.item;
+			},
+			setItem( key, value ) {
+				countCalls++;
+				this.item = value;
+			},
+		};
+		setPersistenceStorage( storage );
+		const reducer = ( state = { ribs: true }, action ) => {
+			if ( action.type === 'UPDATE' ) {
+				return { chicken: true };
+			}
+
+			return state;
+		};
+		registry.registerStore( 'store1', {
+			reducer,
+			persist: true,
+			actions: {
+				update: () => ( { type: 'UPDATE' } ),
+			},
+		} );
+		registry.registerStore( 'store2', {
+			reducer,
+			actions: {
+				update: () => ( { type: 'UPDATE' } ),
+			},
+		} );
+		registry.setupPersistence( storageKey );
+
+		expect( countCalls ).toBe( 1 ); // Setup trigger initial persistence.
+
+		registry.dispatch( 'store1' ).update();
+
+		expect( countCalls ).toBe( 2 ); // Updating state trigger persistence.
+
+		registry.dispatch( 'store2' ).update();
+
+		expect( countCalls ).toBe( 2 ); // If the persisted state doesn't change, don't persist.
+	} );
 } );
 
 describe( 'restrictPersistence', () => {