return boolean for dispatch

src/runtime/internal/dom.ts

@@ -630,14 +630,10 @@ export function toggle_class(element, name, toggle) {
 	element.classList[toggle ? 'add' : 'remove'](name);
 }
 
-export interface CustomEventOpts {
-	bubbles?: boolean;
-	cancelable?: boolean;
-}
 
-export function custom_event<T=any>(type: string, detail?: T, opts: CustomEventOpts = {}) {
+export function custom_event<T=any>(type: string, detail?: T, { bubbles = false, cancelable = false } = {}): CustomEvent<T> {
 	const e: CustomEvent<T> = document.createEvent('CustomEvent');
-	e.initCustomEvent(type, opts.bubbles, opts.cancelable, detail);
+	e.initCustomEvent(type, bubbles, cancelable, detail);
 	return e;
 }
 

src/runtime/internal/lifecycle.ts

@@ -1,4 +1,4 @@
-import { custom_event, CustomEventOpts } from './dom';
+import { custom_event } from './dom';
 
 export let current_component;
 
@@ -32,17 +32,20 @@ export function createEventDispatcher<
 >(): <EventKey extends Extract<keyof EventMap, string>>(type: EventKey, detail?: EventMap[EventKey]) => void {
 	const component = get_current_component();
 
-	return (type: string, detail?: any, opts?: CustomEventOpts) => {
+	return (type: string, detail?: any, { cancelable = false } = {}): boolean => {
 		const callbacks = component.$$.callbacks[type];
 
 		if (callbacks) {
 			// TODO are there situations where events could be dispatched
 			// in a server (non-DOM) environment?
-			const event = custom_event(type, detail, opts);
+			const event = custom_event(type, detail, { cancelable });
 			callbacks.slice().forEach(fn => {
 				fn.call(component, event);
 			});
+			return !event.defaultPrevented;
 		}
+
+		return true;
 	};
 }