add second argument to writable - fixes #2072

store.mjs

@@ -34,7 +34,8 @@ export function readable(start, value) {
 	};
 }
 
-export function writable(value) {
+export function writable(value, start = noop) {
+	let stop;
 	const subscribers = [];
 
 	function set(newValue) {
@@ -51,11 +52,13 @@ export function writable(value) {
 	function subscribe(run, invalidate = noop) {
 		const subscriber = [run, invalidate];
 		subscribers.push(subscriber);
+		if (subscribers.length === 1) stop = start() || noop;
 		run(value);
 
 		return () => {
 			const index = subscribers.indexOf(subscriber);
 			if (index !== -1) subscribers.splice(index, 1);
+			if (subscribers.length === 0) stop();
 		};
 	}
 

test/store/index.js

@@ -21,6 +21,27 @@ describe('store', () => {
 
 			assert.deepEqual(values, [0, 1, 2]);
 		});
+
+		it('calls provided subscribe handler', () => {
+			let called = 0;
+
+			const store = writable(0, () => {
+				called += 1;
+				return () => called -= 1;
+			});
+
+			const unsubscribe1 = store.subscribe(() => {});
+			assert.equal(called, 1);
+
+			const unsubscribe2 = store.subscribe(() => {});
+			assert.equal(called, 1);
+
+			unsubscribe1();
+			assert.equal(called, 1);
+
+			unsubscribe2();
+			assert.equal(called, 0);
+		});
 	});
 
 	describe('readable', () => {