From a2ff93cb721b786f34e467b9bddfbf6eebcfde43 Mon Sep 17 00:00:00 2001
From: Rich Harris <richard.a.harris@gmail.com>
Date: Sun, 16 Dec 2018 19:49:42 -0500
Subject: [PATCH] glitch-free reactive stores

---
 store.js            | 46 +++++++++++++++++++++++++++++----------------
 test/store/index.js | 22 ++++++++++++++++++++++
 2 files changed, 52 insertions(+), 16 deletions(-)

diff --git a/store.js b/store.js
index ffef5c7ca8..d22afdedee 100644
--- a/store.js
+++ b/store.js
@@ -1,4 +1,4 @@
-import { run_all } from './internal.js';
+import { run_all, noop } from './internal.js';
 
 export function readable(start, value) {
 	const subscribers = [];
@@ -7,20 +7,22 @@ export function readable(start, value) {
 	function set(newValue) {
 		if (newValue === value) return;
 		value = newValue;
-		subscribers.forEach(fn => fn(value));
+		subscribers.forEach(s => s[1]());
+		subscribers.forEach(s => s[0](value));
 	}
 
 	return {
-		subscribe(fn) {
+		subscribe(run, invalidate = noop) {
 			if (subscribers.length === 0) {
 				stop = start(set);
 			}
 
-			subscribers.push(fn);
-			fn(value);
+			const subscriber = [run, invalidate];
+			subscribers.push(subscriber);
+			run(value);
 
 			return function() {
-				const index = subscribers.indexOf(fn);
+				const index = subscribers.indexOf(subscriber);
 				if (index !== -1) subscribers.splice(index, 1);
 
 				if (subscribers.length === 0) {
@@ -38,19 +40,21 @@ export function writable(value) {
 	function set(newValue) {
 		if (newValue === value) return;
 		value = newValue;
-		subscribers.forEach(fn => fn(value));
+		subscribers.forEach(s => s[1]());
+		subscribers.forEach(s => s[0](value));
 	}
 
 	function update(fn) {
 		set(fn(value));
 	}
 
-	function subscribe(fn) {
-		subscribers.push(fn);
-		fn(value);
+	function subscribe(run, invalidate = noop) {
+		const subscriber = [run, invalidate];
+		subscribers.push(subscriber);
+		run(value);
 
 		return () => {
-			const index = subscribers.indexOf(fn);
+			const index = subscribers.indexOf(subscriber);
 			if (index !== -1) subscribers.splice(index, 1);
 		};
 	}
@@ -63,20 +67,30 @@ export function derive(stores, fn) {
 	if (single) stores = [stores];
 
 	const auto = fn.length === 1;
+	let value = {};
 
 	return readable(set => {
 		let inited = false;
 		const values = [];
 
+		let pending = 0;
+
 		const sync = () => {
+			if (pending) return;
 			const result = fn(single ? values[0] : values, set);
-			if (auto) set(result);
+			if (auto && (value !== (value = result))) set(result);
 		}
 
-		const unsubscribers = stores.map((store, i) => store.subscribe(value => {
-			values[i] = value;
-			if (inited) sync();
-		}));
+		const unsubscribers = stores.map((store, i) => store.subscribe(
+			value => {
+				values[i] = value;
+				pending &= ~(1 << i);
+				if (inited) sync();
+			},
+			() => {
+				pending |= (1 << i);
+			})
+		);
 
 		inited = true;
 		sync();
diff --git a/test/store/index.js b/test/store/index.js
index 355cea4442..fb518cf24d 100644
--- a/test/store/index.js
+++ b/test/store/index.js
@@ -128,5 +128,27 @@ describe('store', () => {
 			number.set(7);
 			assert.deepEqual(values, [0, 2, 4]);
 		});
+
+		it('prevents glitches', () => {
+			const lastname = writable('Jekyll');
+			const firstname = derive(lastname, n => n === 'Jekyll' ? 'Henry' : 'Edward');
+
+			const fullname = derive([firstname, lastname], names => names.join(' '));
+
+			const values = [];
+
+			const unsubscribe = fullname.subscribe(value => {
+				values.push(value);
+			});
+
+			lastname.set('Hyde');
+
+			assert.deepEqual(values, [
+				'Henry Jekyll',
+				'Edward Hyde'
+			]);
+
+			unsubscribe();
+		});
 	});
 });