gistfile1.js

YUI().use('app-base', function (Y) {
    Y.BasicView = Y.Base.create('basicView', Y.View, [], {
        render: function () {
            this.get('container').set('text', this.get('text'));
            return this;
        }
    });

    Y.CompositeView = Y.Base.create('compositeView', Y.View, [], {
        initializer: function () {
            this.after(['sidebarChange', 'mainChange'], this.render, this);
        },

        render: function () {
            var container = this.get('container');

            container.get('children').remove();
            container.append(this.get('sidebar').get('container'));
            container.append(this.get('main').get('container'));

            return this;
        }
    });

    var app = new Y.App({
        serverRouting: false,
        linkSelector : '#nav a',

        views: {
            sidebar: {type: 'BasicView'},
            main   : {type: 'BasicView'},

            page: {
                type    : 'CompositeView',
                preserve: true
            }
        }
    });

    app.route('*', function (req, res, next) {
        var sidebar = this.get('sidebar');

        if (!sidebar) {
            sidebar = this.createView('sidebar', {text: 'Sidebar'});
            this.set('sidebar', sidebar.render());
        }

        next();
    });

    app.route('/', function (req, res) {
        this.showPage(this.createView('main', {text: 'Home'}));
    });

    app.route('/foo', function (req, res) {
        this.showPage(this.createView('main', {text: 'Foo'}));
    });

    app.showPage = function (mainView) {
        this.showView('page', {
            sidebar: this.get('sidebar'),
            main   : mainView.render()
        }, {
            update: true
        });
    };

    app.render().dispatch();
});