feat: introduce DocsPageState for managing document navigation and state

Author: Shewart

src/ShellDocs.Components/DocsPageState.cs

@@ -0,0 +1,56 @@
+using Microsoft.AspNetCore.Components;
+using ShellDocs.Core;
+using ShellDocs.Markdown;
+
+namespace ShellDocs.Components;
+
+// Populated by MarkdownContent on each page render, read by DocsLayout's chrome
+// (TOC, PrevNext, Breadcrumb) so those components don't need per-page wiring.
+public sealed class DocsPageState : IDisposable
+{
+    private readonly NavigationGraph _graph;
+    private readonly NavigationManager _nav;
+
+    public DocsPageState(NavigationGraph graph, NavigationManager nav)
+    {
+        _graph = graph;
+        _nav = nav;
+        _nav.LocationChanged += OnLocationChanged;
+    }
+
+    public RenderedDocument? Document { get; private set; }
+    public NavigationNode? CurrentNode { get; private set; }
+    public NavigationNode? Prev { get; private set; }
+    public NavigationNode? Next { get; private set; }
+    public IReadOnlyList<NavigationNode> Breadcrumbs { get; private set; } = Array.Empty<NavigationNode>();
+
+    public event Action? OnChange;
+
+    public void SetDocument(RenderedDocument? document)
+    {
+        Document = document;
+        Recompute();
+    }
+
+    private void OnLocationChanged(object? sender, Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs e)
+        => Recompute();
+
+    private void Recompute()
+    {
+        var path = new Uri(_nav.Uri).AbsolutePath;
+        CurrentNode = _graph.ResolveByUrl(path);
+        if (CurrentNode is null)
+        {
+            Prev = Next = null;
+            Breadcrumbs = Array.Empty<NavigationNode>();
+        }
+        else
+        {
+            (Prev, Next) = _graph.GetPrevNext(CurrentNode);
+            Breadcrumbs = _graph.GetBreadcrumb(CurrentNode);
+        }
+        OnChange?.Invoke();
+    }
+
+    public void Dispose() => _nav.LocationChanged -= OnLocationChanged;
+}

src/ShellDocs.Components/ServiceCollectionExtensions.cs

@@ -19,6 +19,7 @@ public static IServiceCollection AddShellDocs(this IServiceCollection services,
         services.AddScoped<SearchState>();
         services.AddScoped<SidebarCollapseState>();
         services.AddScoped<CodeGroupSyncState>();
+        services.AddScoped<DocsPageState>();
 
         /* Auto-register the shipped content primitives so `razor:preview` blocks
            in markdown can reference <Callout>, <Card>, <Steps>, <FileTree> etc.