dashboard: lazy-load BrowsePage via DashboardBrowseService

src/ZB.MOM.WW.MxGateway.Server/Dashboard/Components/Shared/BrowseTreeNodeView.razor

@@ -2,15 +2,21 @@
 
 @*
     Recursive Browse hierarchy node. Renders one Galaxy object, its child
-    objects (recursively), and its attributes as right-clickable tag rows.
-    Expansion state is local; children render only while expanded.
+    objects (recursively, lazy-loaded on first expand), and its attributes as
+    right-clickable tag rows. Expansion state is local; children render only
+    while expanded.
+
+    The expand triangle is shown whenever the server's child_has_children
+    projector hint is set (HasChildrenHint), even before children have been
+    loaded — clicking it triggers OnLoadChildren so the parent page can fill
+    in Node.Children, then the view re-renders.
 *@
 
 <div class="tree-node">
     <div class="tree-row @(Node.IsArea ? "tree-row-area" : "tree-row-object")">
-        @if (Node.HasChildren)
+        @if (ShowToggle())
         {
-            <button type="button" class="tree-toggle" @onclick="Toggle" aria-label="Toggle">
+            <button type="button" class="tree-toggle" @onclick="ToggleAsync" aria-label="Toggle">
                 @(_expanded ? "▾" : "▸")
             </button>
         }
@@ -18,7 +24,7 @@
         {
             <span class="tree-toggle tree-toggle-empty"></span>
         }
-        <span class="tree-label" @onclick="Toggle">
+        <span class="tree-label" @onclick="ToggleAsync">
             <span class="tree-icon">@(Node.IsArea ? "▣" : "◇")</span>
             <span class="tree-name">@Node.DisplayName</span>
             @if (!string.IsNullOrWhiteSpace(Node.Object.TagName)
@@ -31,9 +37,27 @@
     @if (_expanded)
     {
         <div class="tree-children">
+            @if (Node.LoadState == BrowseLoadState.Loading)
+            {
+                <div class="tree-load-status text-secondary">
+                    <span class="tree-toggle tree-toggle-empty"></span>
+                    <span>⌛ Loading…</span>
+                </div>
+            }
+            else if (Node.LoadState == BrowseLoadState.Error)
+            {
+                <div class="tree-load-status text-danger">
+                    <span class="tree-toggle tree-toggle-empty"></span>
+                    <span>Failed to load: @Node.LoadError</span>
+                </div>
+            }
+
             @foreach (DashboardBrowseNode child in Node.Children)
             {
-                <BrowseTreeNodeView Node="child" OnAddTag="OnAddTag" OnTagContextMenu="OnTagContextMenu" />
+                <BrowseTreeNodeView Node="child"
+                                    OnAddTag="OnAddTag"
+                                    OnTagContextMenu="OnTagContextMenu"
+                                    OnLoadChildren="OnLoadChildren" />
             }
             @foreach (GalaxyAttribute attr in Node.Attributes)
             {
@@ -75,13 +99,52 @@
     [Parameter]
     public EventCallback<(MouseEventArgs Event, GalaxyAttribute Attribute)> OnTagContextMenu { get; set; }
 
+    /// <summary>
+    ///     Invoked on first expand when the projector hint says this node has children
+    ///     but they have not been fetched yet. The callback is expected to populate
+    ///     <see cref="DashboardBrowseNode.Children"/> on the node it receives and then
+    ///     trigger a re-render.
+    /// </summary>
+    [Parameter]
+    public Func<DashboardBrowseNode, Task>? OnLoadChildren { get; set; }
+
     private bool _expanded;
 
-    private void Toggle()
+    // The triangle is shown whenever the projector says children exist (even
+    // pre-load), or attributes are already present, or already-loaded children
+    // are sitting on the node.
+    private bool ShowToggle()
     {
-        if (Node.HasChildren)
+        return Node.HasChildrenHint
+            || Node.Attributes.Count > 0
+            || Node.Children.Count > 0;
+    }
+
+    private async Task ToggleAsync()
+    {
+        if (!ShowToggle())
         {
-            _expanded = !_expanded;
+            return;
+        }
+
+        _expanded = !_expanded;
+
+        if (_expanded
+            && Node.HasChildrenHint
+            && Node.LoadState == BrowseLoadState.NotLoaded
+            && OnLoadChildren is not null)
+        {
+            Node.LoadState = BrowseLoadState.Loading;
+            try
+            {
+                await OnLoadChildren(Node);
+                Node.LoadState = BrowseLoadState.Loaded;
+            }
+            catch (Exception ex)
+            {
+                Node.LoadState = BrowseLoadState.Error;
+                Node.LoadError = ex.Message;
+            }
         }
     }