Desktop: swap the host lists in whole instead of announcing every row to listeners that redo all their work per row

src/DodoSSH.Client.Shell/ViewModels/VaultViewModel.cs

@@ -1663,7 +1663,7 @@ internal sealed partial class VaultViewModel(
     /// host key list checks itself against, so a filter applied here would change what the application can
     /// do rather than what it shows. <see cref="VisibleHosts"/> is the filtered view.
     /// </remarks>
-    internal ObservableCollection<HostRowViewModel> Hosts { get; } = [];
+    internal ReplaceableCollection<HostRowViewModel> Hosts { get; } = [];
 
     /// <summary>The hosts the grid is showing: one level of the group tree, narrowed by the box.</summary>
     /// <remarks>
@@ -1677,7 +1677,7 @@ internal sealed partial class VaultViewModel(
     /// alone: the phone draws <see cref="SidebarRows"/>, which is the same hosts flattened under headings.
     /// </para>
     /// </remarks>
-    internal ObservableCollection<HostRowViewModel> VisibleHosts { get; } = [];
+    internal ReplaceableCollection<HostRowViewModel> VisibleHosts { get; } = [];
 
     /// <summary>Whether the grid has anything to draw.</summary>
     /// <remarks>
@@ -1891,7 +1891,7 @@ internal sealed partial class VaultViewModel(
     /// groupless keychain is one flat grid and nothing above it.
     /// </para>
     /// </remarks>
-    internal ObservableCollection<HostSectionViewModel> HostSections { get; } = [];
+    internal ReplaceableCollection<HostSectionViewModel> HostSections { get; } = [];
 
     /// <summary>Every host on the board, in the order its cards are drawn — every section, one after another.</summary>
     /// <remarks>
@@ -2003,7 +2003,7 @@ internal sealed partial class VaultViewModel(
     internal ObservableCollection<CredentialRowViewModel> Credentials { get; } = [];
 
     /// <summary>The buckets to show, unpushed local state included.</summary>
-    internal ObservableCollection<ObjectStoreRowViewModel> ObjectStores { get; } = [];
+    internal ReplaceableCollection<ObjectStoreRowViewModel> ObjectStores { get; } = [];
 
     /// <summary>The tags to show, with the number of hosts wearing each.</summary>
     /// <remarks>
@@ -4714,18 +4714,14 @@ internal sealed partial class VaultViewModel(
 
         var (rows, unreadable) = await FetchHostRowsAsync(cancellationToken).ConfigureAwait(true);
 
-        Hosts.Clear();
-
         // Grouped by vault, with the one new items go into first, then by name inside each. Two vaults can
         // hold a host with the same label and both are shown: which vault it is in is what tells them
-        // apart, which is why the row carries the name rather than the list deduplicating.
-        foreach (var host in rows
+        // apart, which is why the row carries the name rather than the list deduplicating. Swapped in whole,
+        // and announced once: see ReplaceableCollection for what a row at a time cost.
+        Hosts.ReplaceAll(rows
             .OrderByDescending(row => row.VaultId == session.ActiveVaultId)
             .ThenBy(row => row.VaultName, StringComparer.CurrentCulture)
-            .ThenBy(row => row.Label, StringComparer.CurrentCulture))
-        {
-            Hosts.Add(host);
-        }
+            .ThenBy(row => row.Label, StringComparer.CurrentCulture));
 
         SelectedHost = SelectionAfterReload(selectedId);
         ApplyTheChosenHosts();
@@ -4816,13 +4812,9 @@ internal sealed partial class VaultViewModel(
 
         var selectedId = SelectedObjectStore?.EntityId;
 
-        ObjectStores.Clear();
-
-        foreach (var store in listing.Items
-            .OrderBy(store => store.Secret.Label, StringComparer.CurrentCulture))
-        {
-            ObjectStores.Add(new ObjectStoreRowViewModel(store));
-        }
+        ObjectStores.ReplaceAll(listing.Items
+            .OrderBy(store => store.Secret.Label, StringComparer.CurrentCulture)
+            .Select(store => new ObjectStoreRowViewModel(store)));
 
         SelectedObjectStore = ObjectStores.FirstOrDefault(row => row.EntityId == selectedId);
 
@@ -5388,12 +5380,7 @@ internal sealed partial class VaultViewModel(
         // a walk up the group tree, and this is the map that walk is made against.
         var parents = EffectiveParents();
 
-        VisibleHosts.Clear();
-
-        foreach (var host in Hosts.Where(host => Matches(host, parents)))
-        {
-            VisibleHosts.Add(host);
-        }
+        VisibleHosts.ReplaceAll(Hosts.Where(host => Matches(host, parents)));
 
         RebuildSidebarRows();
 
@@ -5403,7 +5390,7 @@ internal sealed partial class VaultViewModel(
         // property directly, and a host that is not on screen is not one any of them should act on.
         SelectedHost = selected is not null && VisibleHosts.Contains(selected) ? selected : null;
 
-        // After the host selection, not before: this mirrors it, and the ListBox's own answer to the Clear()
+        // After the host selection, not before: this mirrors it, and the ListBox's own answer to the reset
         // above is a null that has to be overwritten rather than read.
         SelectedSidebarRow = SelectedHost;
 
@@ -5473,12 +5460,7 @@ internal sealed partial class VaultViewModel(
     /// </remarks>
     private void RebuildHostSections()
     {
-        HostSections.Clear();
-
-        foreach (var built in BuildHostSections())
-        {
-            HostSections.Add(built);
-        }
+        HostSections.ReplaceAll(BuildHostSections());
 
         // Computed rather than stored, and none of the four has a change notification of its own — the same
         // arrangement RebuildVisibleHosts holds for HasVisibleHosts and NoVisibleHostsMessage, and for the