ModelMerge comments

Author: deansheather

App/Utils/ModelMerge.cs

@@ -0,0 +1,97 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Coder.Desktop.App.Utils;
+
+public interface IModelUpdateable<in T>
+{
+    /// <summary>
+    ///     Applies changes from obj to `this` if they represent the same
+    ///     object based on some identifier like an ID or fixed name.
+    /// </summary>
+    /// <returns>
+    ///     True if the two objects represent the same item and the changes
+    ///     were applied.
+    /// </returns>
+    public bool TryApplyChanges(T obj);
+}
+
+/// <summary>
+///     A static utility class providing methods for applying model updates
+///     with as little UI updates as possible.
+///     The main goal of the utilities in this class is to prevent redraws in
+///     ItemsRepeater items when nothing has changed.
+/// </summary>
+public static class ModelUpdate
+{
+    /// <summary>
+    ///     Takes all items in `update` and either applies them to existing
+    ///     items in `target`, or adds them to `target` if there are no
+    ///     matching items.
+    ///     Any items in `target` that don't have a corresponding item in
+    ///     `update` will be removed from `target`.
+    ///     Items are inserted in their correct sort position according to
+    ///     `sorter`. It's assumed that the target list is already sorted by
+    ///     `sorter`.
+    /// </summary>
+    /// <param name="target">Target list to be updated</param>
+    /// <param name="update">Incoming list to apply to `target`</param>
+    /// <param name="sorter">
+    ///     Comparison to use for sorting. Note that the sort order does not
+    ///     need to be the ID/name field used in the <c>IModelUpdateable</c>
+    ///     implementation, and can be by any order.
+    ///     New items will be sorted after existing items.
+    /// </param>
+    public static void ApplyLists<T>(IList<T> target, IEnumerable<T> update, Comparison<T> sorter)
+        where T : IModelUpdateable<T>
+    {
+        var newItems = update.ToList();
+
+        // Update and remove existing items. We use index-based for loops here
+        // because we remove items, and removing items while using the list as
+        // an IEnumerable will throw an exception.
+        for (var i = 0; i < target.Count; i++)
+        {
+            // Even though we're removing items before a "break", we still use
+            // index-based for loops here to avoid exceptions.
+            for (var j = 0; j < newItems.Count; j++)
+            {
+                if (!target[i].TryApplyChanges(newItems[j])) continue;
+
+                // Prevent it from being added below, or checked again. We
+                // don't need to decrement `j` here because we're breaking
+                // out of this inner loop.
+                newItems.RemoveAt(j);
+                goto OuterLoopEnd; // continue outer loop
+            }
+
+            // A merge couldn't occur, so we need to remove the old item and
+            // decrement `i` for the next iteration.
+            target.RemoveAt(i);
+            i--;
+
+            OuterLoopEnd: ;
+        }
+
+        // Add any items that were missing into their correct sorted place.
+        // It's assumed the list is already sorted.
+        foreach (var newItem in newItems)
+        {
+            for (var i = 0; i < target.Count; i++)
+                // If the new item sorts before the current item, insert it
+                // after.
+                if (sorter(newItem, target[i]) < 0)
+                {
+                    target.Insert(i, newItem);
+                    goto OuterLoopEnd;
+                }
+
+            // Handle the case where target is empty or the new item is
+            // equal to or after every other item.
+            target.Add(newItem);
+
+            OuterLoopEnd: ;
+        }
+    }
+}

App/Utils/ModelUpdate.cs

@@ -38,7 +38,7 @@ public AgentAppViewModel Create(Uuid id, string name, Uri appUri, Uri? iconUrl)
     }
 }
 
-public partial class AgentAppViewModel : ObservableObject, IModelMergeable<AgentAppViewModel>
+public partial class AgentAppViewModel : ObservableObject, IModelUpdateable<AgentAppViewModel>
 {
     private readonly ILogger<AgentAppViewModel> _logger;
 
@@ -97,7 +97,7 @@ public AgentAppViewModel(ILogger<AgentAppViewModel> logger)
         _logger = logger;
     }
 
-    public bool ApplyMerge(AgentAppViewModel obj)
+    public bool TryApplyChanges(AgentAppViewModel obj)
     {
         if (Id != obj.Id) return false;
 

App/ViewModels/AgentAppViewModel.cs

@@ -65,7 +65,7 @@ public enum AgentConnectionStatus
     Gray,
 }
 
-public partial class AgentViewModel : ObservableObject, IModelMergeable<AgentViewModel>
+public partial class AgentViewModel : ObservableObject, IModelUpdateable<AgentViewModel>
 {
     private const string DefaultDashboardUrl = "https://coder.com";
     private const int MaxAppsPerRow = 6;
@@ -182,7 +182,7 @@ public AgentViewModel(ILogger<AgentViewModel> logger, ICoderApiClientFactory cod
         };
     }
 
-    public bool ApplyMerge(AgentViewModel model)
+    public bool TryApplyChanges(AgentViewModel model)
     {
         if (Id != model.Id) return false;
 
@@ -285,7 +285,7 @@ private void ContinueFetchApps(Task<WorkspaceAgent> task)
         }
 
         // Sort by name.
-        ModelMerge.MergeLists(Apps, apps, (a, b) => string.Compare(a.Name, b.Name, StringComparison.Ordinal));
+        ModelUpdate.ApplyLists(Apps, apps, (a, b) => string.Compare(a.Name, b.Name, StringComparison.Ordinal));
     }
 
     [RelayCommand]

App/ViewModels/AgentViewModel.cs

@@ -191,7 +191,7 @@ private void UpdateFromRpcModel(RpcModel rpcModel)
                 workspace.Name));
 
         // Sort by status green, red, gray, then by hostname.
-        ModelMerge.MergeLists(Agents, agents, (a, b) =>
+        ModelUpdate.ApplyLists(Agents, agents, (a, b) =>
         {
             if (a.ConnectionStatus != b.ConnectionStatus)
                 return a.ConnectionStatus.CompareTo(b.ConnectionStatus);