fix subscribe implementation, add test for unsubscribing observables

Author: Rich-Harris

src/internal/utils.js

@@ -48,12 +48,11 @@ export function validate_store(store, name) {
 }
 
 export function subscribe(component, store, callback) {
-	let unsub = store.subscribe(callback);
-	// Prevent memory leaks for RxJS users.
-	if (unsub.unsubscribe) {
-		unsub = () => unsub.unsubscribe();
-	}
-	component.$$.on_destroy.push(unsub);
+	const unsub = store.subscribe(callback);
+
+	component.$$.on_destroy.push(unsub.unsubscribe
+		? () => unsub.unsubscribe()
+		: unsub);
 }
 
 export function create_slot(definition, ctx, fn) {

test/runtime/samples/observable-auto-subscribe/Nested.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let observable;
+</script>
+
+<p>value: {$observable}</p>

test/runtime/samples/observable-auto-subscribe/_config.js

@@ -0,0 +1,49 @@
+const subscribers = [];
+
+let value = 'initial';
+
+const observable = {
+	subscribe: fn => {
+		subscribers.push(fn);
+
+		fn(value);
+
+		return {
+			unsubscribe: () => {
+				const i = subscribers.indexOf(fn);
+				subscribers.splice(i, 1);
+			}
+		};
+	}
+};
+
+export default {
+	props: {
+		observable,
+		visible: false
+	},
+
+	html: ``,
+
+	async test({ assert, component, target }) {
+		assert.equal(subscribers.length, 0);
+
+		component.visible = true;
+
+		assert.equal(subscribers.length, 1);
+		assert.htmlEqual(target.innerHTML, `
+			<p>value: initial</p>
+		`);
+
+		value = 42;
+		await subscribers.forEach(fn => fn(value));
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>value: 42</p>
+		`);
+
+		component.visible = false;
+
+		assert.equal(subscribers.length, 0);
+	}
+};

test/runtime/samples/observable-auto-subscribe/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	import Nested from './Nested.svelte';
+
+	export let observable;
+	export let visible;
+</script>
+
+{#if visible}
+	<Nested {observable}/>
+{/if}