Not sure if the post topic really explains what I'm trying to achieve - just exploring the possibilities of Razor, and trying to build out a simple site navigation. 

I want it to work as below - if a page is selected at level 1, show its children. If a page is selected of those children, show that nodes' children.

Level 1 page
Level 1 page
Level 1 page (the first page selected - show its children)
--- Level 2 page
--- Level 2 page 
--- Level 2 page (the second page selected -show its children)
------ Level 3 page
------ Level 3 page
--- Level 2 page 
Level 1 page
Level 1 page
Level 1 page
Level 1 page
Level 1 page

My current attempt is below - it works to build out the lists, but shows the entire nav tree - it should only show child nodes when the parent is selected. I guess what I need to do is build the nested lists, and as I add each item, check if it is an ancestor of the current page. 

This would be ideal (item being the nav item being created, page the current page being served):

item.IsAncestorOrSelf(page)

My code (it's very much a rough run, surely someone can show me a much more robust way of achieving the same):

@inherits RenderViewPage
@using Umbraco.Cms.Web;
@{
    var channel = DynamicModel;
    var page = DynamicModel;

    while (channel.Parent.ContentType.Alias != "homePage")
    {
        channel = channel.Parent;
    }
}

<ul>
    @foreach (var item in channel.Children) {
        var childName = item.Name ?? "(No name yet)";
        <li class="@(@page.Url == @item.Url ? "selected" : "")"><a href="@item.Url">@childName</a>
        @if (item.Children != null) {
            <ul>
            @foreach (var itemChild in item.Children) {
            var itemChildName = itemChild.Name;
                <li class="@(@page.Url == @itemChild.Url ? "selected" : "")"><a href="@itemChild.Url">@itemChildName</a>
                @if (itemChild.Children != null) {
                    <ul>
                    @foreach (var itemChildChild in itemChild.Children) {
                        var itemChildChildName = itemChildChild.Name;
                        <li class="@(@page.Url == @itemChildChild.Url ? "selected" : "")"><a href="@itemChildChild.Url">@itemChildChildName</a></li>
                    }
                    </ul>
                }
                </li>
            }
            </ul>
        }
        </li>
    }
</ul>