Allow extensions to use route resolvers (#2275)

- mapRoutes: don't wrap components in resolvers if they are already resolvers
- Extract defaultResolver into its own class
- Allow either route resolver instances, or components with an optional resolverClass which should accept the component and route name in its constructor.
- Introduce a resolver for DiscussionPage, so that routing from one post to another on the same discussion triggers a scroll instead of rerendering

js/src/forum/resolver/DiscussionPageResolver.ts

@@ -0,0 +1,47 @@
+import DefaultResolver from '../../common/resolvers/DefaultResolver';
+
+/**
+ * This isn't exported as it is a temporary measure.
+ * A more robust system will be implemented alongside UTF-8 support in beta 15.
+ */
+function getDiscussionIdFromSlug(slug: string | undefined) {
+  if (!slug) return;
+  return slug.split('-')[0];
+}
+
+/**
+ * A custom route resolver for DiscussionPage that generates the same key to all posts
+ * on the same discussion. It triggers a scroll when going from one post to another
+ * in the same discussion.
+ */
+export default class DiscussionPageResolver extends DefaultResolver {
+  static scrollToPostNumber: number | null = null;
+
+  makeKey() {
+    const params = { ...m.route.param() };
+    if ('near' in params) {
+      delete params.near;
+    }
+    params.id = getDiscussionIdFromSlug(params.id);
+    return this.routeName.replace('.near', '') + JSON.stringify(params);
+  }
+
+  onmatch(args, requestedPath, route) {
+    if (route.includes('/d/:id') && getDiscussionIdFromSlug(args.id) === getDiscussionIdFromSlug(m.route.param('id'))) {
+      DiscussionPageResolver.scrollToPostNumber = parseInt(args.near);
+    }
+
+    return super.onmatch(args, requestedPath, route);
+  }
+
+  render(vnode) {
+    if (DiscussionPageResolver.scrollToPostNumber !== null) {
+      const number = DiscussionPageResolver.scrollToPostNumber;
+      // Scroll after a timeout to avoid clashes with the render.
+      setTimeout(() => app.current.get('stream').goToNumber(number));
+      DiscussionPageResolver.scrollToPostNumber = null;
+    }
+
+    return super.render(vnode);
+  }
+}