Implemented onNavigate

runtime/src/app/app.ts

@@ -72,6 +72,7 @@ export function set_cid(n) {
 }
 
 const _history = typeof history !== 'undefined' ? history : {
+	go: (delta: number) => {},
 	pushState: (state: any, title: string, href: string) => {},
 	replaceState: (state: any, title: string, href: string) => {},
 	scrollRestoration: ''

runtime/src/app/goto/index.ts

@@ -1,9 +1,14 @@
 import { history, select_target, navigate, cid } from '../app';
+import { canNavigate } from '../onNavigate/index';
 
-export default function goto(href: string, opts = { replaceState: false }) {
+export default async function goto(href: string, opts = { replaceState: false }) {
 	const target = select_target(new URL(href, document.baseURI));
 
 	if (target) {
+		if (!(await canNavigate(target.page))) {
+			return;
+		}
+
 		history[opts.replaceState ? 'replaceState' : 'pushState']({ id: cid }, '', href);
 		return navigate(target, null).then(() => {});
 	}

runtime/src/app/index.ts

@@ -6,4 +6,5 @@ export const stores = () => getContext(CONTEXT_KEY);
 export { default as start } from './start/index';
 export { default as goto } from './goto/index';
 export { default as prefetch } from './prefetch/index';
-export { default as prefetchRoutes } from './prefetchRoutes/index';
+export { default as prefetchRoutes } from './prefetchRoutes/index';
+export { default as onNavigate } from './onNavigate/index';

runtime/src/app/onNavigate/index.ts

@@ -0,0 +1,21 @@
+import { OnNavigateCallback, Page } from '../types';
+
+const callbacks: OnNavigateCallback[] = [];
+
+export default function onNavigate(callback: OnNavigateCallback): () => void {
+	if (!callbacks.includes(callback)) {
+		callbacks.push(callback);
+	}
+
+	return () => {
+		const index = callbacks.indexOf(callback);
+		if (index !== -1) {
+			callbacks.splice(index, 1);
+		}
+	};
+}
+
+export async function canNavigate(page: Page): Promise<boolean> {
+	const results = await Promise.all(callbacks.map(callback => callback(page)));
+	return !results.some(result => result === false);
+}

runtime/src/app/start/index.ts

@@ -13,6 +13,7 @@ import {
 	set_cid
 } from '../app';
 import prefetch from '../prefetch/index';
+import { canNavigate } from '../onNavigate/index';
 
 export default function start(opts: {
 	target: Node
@@ -60,7 +61,7 @@ function trigger_prefetch(event: MouseEvent | TouchEvent) {
 	prefetch(a.href);
 }
 
-function handle_click(event: MouseEvent) {
+async function handle_click(event: MouseEvent) {
 	// Adapted from https://github.com/visionmedia/page.js
 	// MIT license https://github.com/visionmedia/page.js#license
 	if (which(event) !== 1) return;
@@ -97,9 +98,13 @@ function handle_click(event: MouseEvent) {
 
 	const target = select_target(url);
 	if (target) {
+		event.preventDefault();
+		if (!(await canNavigate(target.page))) {
+			return;
+		}
+
 		const noscroll = a.hasAttribute('sapper-noscroll');
 		navigate(target, null, noscroll, url.hash);
-		event.preventDefault();
 		history.pushState({ id: cid }, '', url.href);
 	}
 }
@@ -113,13 +118,18 @@ function find_anchor(node: Node) {
 	return node;
 }
 
-function handle_popstate(event: PopStateEvent) {
+async function handle_popstate(event: PopStateEvent) {
 	scroll_history[cid] = scroll_state();
 
 	if (event.state) {
 		const url = new URL(location.href);
 		const target = select_target(url);
 		if (target) {
+			if (!(await canNavigate(target.page))) {
+				history.go(cid - event.state.id);
+				return;
+			}
+
 			navigate(target, event.state.id);
 		} else {
 			location.href = location.href;

runtime/src/app/types.ts

@@ -60,4 +60,6 @@ export type Page = {
 	path: string;
 	params: Record<string, string>;
 	query: Record<string, string | string[]>;
-};
+};
+
+export type OnNavigateCallback = (page: Page) => any;