Failing test for onError handler.

Rather than _always_ adding a try/catch this adds a try/catch only when on_error is present. Doesn't currently work because apparently updates and initial renders go through different paths. Also because I need maintainer support to figure this out...

src/runtime/index.ts

@@ -3,6 +3,7 @@ import './ambient';
 export {
 	onMount,
 	onDestroy,
+	onError,
 	beforeUpdate,
 	afterUpdate,
 	setContext,

src/runtime/internal/Component.ts

@@ -19,6 +19,7 @@ interface T$$ {
 	context: Map<any, any>;
 	on_mount: any[];
 	on_destroy: any[];
+	on_error: any[];
 }
 
 export function bind(component, name, callback) {
@@ -88,6 +89,7 @@ export function init(component, options, instance, create_fragment, not_equal, p
 
 		// lifecycle
 		on_mount: [],
+		on_error: [],
 		on_destroy: [],
 		before_update: [],
 		after_update: [],

src/runtime/internal/lifecycle.ts

@@ -27,6 +27,10 @@ export function onDestroy(fn) {
 	get_current_component().$$.on_destroy.push(fn);
 }
 
+export function onError(fn) {
+	get_current_component().$$.on_error.push(fn);
+}
+
 export function createEventDispatcher() {
 	const component = current_component;
 

src/runtime/internal/scheduler.ts

@@ -71,6 +71,15 @@ export function flush() {
 
 function update($$) {
 	if ($$.fragment) {
+		if ($$.on_error.length == 0) {
+			exec_update($$);
+		} else {
+			try_exec_update($$);
+		}
+	}
+}
+
+function exec_update($$) {
 	$$.update($$.dirty);
 	run_all($$.before_update);
 	$$.fragment.p($$.dirty, $$.ctx);
@@ -78,4 +87,19 @@ function update($$) {
 
 	$$.after_update.forEach(add_render_callback);
 }
+
+function try_exec_update($$) {
+	try {
+		exec_update($$);
+	} catch (e) {
+		let handled = false;
+		for (let i = 0; i < $$.on_error.length; i += 1) {
+			const callback = $$.on_error[i];
+			const result = callback(e);
+			if (result !== false) {
+				handled = true;
+			}
+		}
+		if (!handled) throw e;
+	}
 }

src/runtime/internal/ssr.ts

@@ -77,6 +77,7 @@ export function create_ssr_component(fn) {
 
 			// these will be immediately discarded
 			on_mount: [],
+			on_error: [],
 			before_update: [],
 			after_update: [],
 			callbacks: blank_object()

test/runtime/samples/onerror-fires-when-error/_config.js

@@ -0,0 +1,7 @@
+export default {
+	test({ assert, target }) {
+		const div = target.querySelector('div');
+
+		assert.equal('error', div.className);
+	}
+};

test/runtime/samples/onerror-fires-when-error/container.js

@@ -0,0 +1 @@
+export default {};

test/runtime/samples/onerror-fires-when-error/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	import { onError } from 'svelte';
+	let error;
+	onError(err => {
+		error = err;
+	});
+	function getWidget() {
+		throw new Error('Oops');
+	}
+</script>
+
+{#if error}
+<div class='error'>{error.message}</div>
+{:else}
+<div class='success'>{getWidget()}</div>
+{/if}