From 393757db2982faaf0306e36a3604f50c0cb443c5 Mon Sep 17 00:00:00 2001
From: Rich Harris <richard.a.harris@gmail.com>
Date: Wed, 21 Aug 2019 11:33:02 -0400
Subject: [PATCH] conservative updates for await blocks

---
 .../compile/render_dom/wrappers/AwaitBlock.ts | 44 +++++++++++--------
 .../await-conservative-update/_config.js      | 16 +++++++
 .../await-conservative-update/main.svelte     | 19 ++++++++
 .../await-conservative-update/sleep.js        | 11 +++++
 4 files changed, 71 insertions(+), 19 deletions(-)
 create mode 100644 test/runtime/samples/await-conservative-update/_config.js
 create mode 100644 test/runtime/samples/await-conservative-update/main.svelte
 create mode 100644 test/runtime/samples/await-conservative-update/sleep.js

diff --git a/src/compiler/compile/render_dom/wrappers/AwaitBlock.ts b/src/compiler/compile/render_dom/wrappers/AwaitBlock.ts
index ff835ddef6..354143feab 100644
--- a/src/compiler/compile/render_dom/wrappers/AwaitBlock.ts
+++ b/src/compiler/compile/render_dom/wrappers/AwaitBlock.ts
@@ -188,29 +188,35 @@ export default class AwaitBlockWrapper extends Wrapper {
 			conditions.push(
 				`(${dependencies.map(dep => `'${dep}' in changed`).join(' || ')})`
 			);
-		}
 
-		conditions.push(
-			`${promise} !== (${promise} = ${snippet})`,
-			`@handle_promise(${promise}, ${info})`
-		);
+			conditions.push(
+				`${promise} !== (${promise} = ${snippet})`,
+				`@handle_promise(${promise}, ${info})`
+			);
 
-		block.builders.update.add_line(
-			`${info}.ctx = ctx;`
-		);
+			block.builders.update.add_line(
+				`${info}.ctx = ctx;`
+			);
 
-		if (this.pending.block.has_update_method) {
-			block.builders.update.add_block(deindent`
-				if (${conditions.join(' && ')}) {
-					// nothing
-				} else {
-					${info}.block.p(changed, @assign(@assign({}, ctx), ${info}.resolved));
-				}
-			`);
+			if (this.pending.block.has_update_method) {
+				block.builders.update.add_block(deindent`
+					if (${conditions.join(' && ')}) {
+						// nothing
+					} else {
+						${info}.block.p(changed, @assign(@assign({}, ctx), ${info}.resolved));
+					}
+				`);
+			} else {
+				block.builders.update.add_block(deindent`
+					${conditions.join(' && ')}
+				`);
+			}
 		} else {
-			block.builders.update.add_block(deindent`
-				${conditions.join(' && ')}
-			`);
+			if (this.pending.block.has_update_method) {
+				block.builders.update.add_block(deindent`
+					${info}.block.p(changed, @assign(@assign({}, ctx), ${info}.resolved));
+				`);
+			}
 		}
 
 		if (this.pending.block.has_outro_method) {
diff --git a/test/runtime/samples/await-conservative-update/_config.js b/test/runtime/samples/await-conservative-update/_config.js
new file mode 100644
index 0000000000..4e81ff8e37
--- /dev/null
+++ b/test/runtime/samples/await-conservative-update/_config.js
@@ -0,0 +1,16 @@
+import { sleep } from './sleep.js';
+
+export default {
+	html: `
+		<p>loading...</p>
+	`,
+
+	test({ assert, component, target }) {
+		return sleep(50).then(() => {
+			assert.htmlEqual(target.innerHTML, `
+				<p>the answer is 42</p>
+				<p>count: 1</p>
+			`);
+		});
+	}
+};
\ No newline at end of file
diff --git a/test/runtime/samples/await-conservative-update/main.svelte b/test/runtime/samples/await-conservative-update/main.svelte
new file mode 100644
index 0000000000..91c9a6c4c2
--- /dev/null
+++ b/test/runtime/samples/await-conservative-update/main.svelte
@@ -0,0 +1,19 @@
+<script>
+	import { sleep } from './sleep.js';
+
+	let count = 0;
+
+	const get_promise = () => {
+		return sleep(10).then(() => {
+			count += 1;
+			return 42;
+		});
+	};
+</script>
+
+{#await get_promise()}
+	<p>loading...</p>
+{:then value}
+	<p>the answer is {value}</p>
+	<p>count: {count}</p>
+{/await}
\ No newline at end of file
diff --git a/test/runtime/samples/await-conservative-update/sleep.js b/test/runtime/samples/await-conservative-update/sleep.js
new file mode 100644
index 0000000000..994f85f38a
--- /dev/null
+++ b/test/runtime/samples/await-conservative-update/sleep.js
@@ -0,0 +1,11 @@
+export let stopped = false;
+
+export const stop = () => stopped = true;
+
+export const sleep = ms => new Promise(f => {
+	if (stopped) return;
+	setTimeout(() => {
+		if (stopped) return;
+		f();
+	}, ms);
+});
\ No newline at end of file