feat(ui): centralize retail selection state

src/AcDream.Core/Selection/SelectionState.cs

@@ -0,0 +1,106 @@
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Core.Selection;
+
+public enum SelectionChangeSource
+{
+    System,
+    World,
+    Radar,
+    Inventory,
+    Paperdoll,
+    Toolbar,
+    Keyboard,
+    Plugin,
+}
+
+public enum SelectionChangeReason
+{
+    Selected,
+    Cleared,
+    SelectedObjectRemoved,
+    PreviousSelection,
+}
+
+public readonly record struct SelectionTransition(
+    uint? PreviousObjectId,
+    uint? SelectedObjectId,
+    SelectionChangeSource Source,
+    SelectionChangeReason Reason);
+
+/// <summary>
+/// Single owner for the client's selected object. This ports the state behavior
+/// of retail <c>ACCWeenieObject::SetSelectedObject @ 0x0058C2E0</c>: changes are
+/// deduplicated, the old selection becomes the previous selection, and plugin/UI
+/// listeners receive one transition after state is committed.
+/// </summary>
+public sealed class SelectionState : ISelectionService
+{
+    private event Action<SelectionChangedEvent>? PluginChanged;
+
+    public uint? SelectedObjectId { get; private set; }
+    public uint? PreviousObjectId { get; private set; }
+    public uint? PreviousValidObjectId { get; private set; }
+
+    public event Action<SelectionTransition>? Changed;
+
+    event Action<SelectionChangedEvent> ISelectionService.Changed
+    {
+        add => PluginChanged += value;
+        remove => PluginChanged -= value;
+    }
+
+    public bool Select(
+        uint objectId,
+        SelectionChangeSource source,
+        SelectionChangeReason reason = SelectionChangeReason.Selected)
+        => objectId == 0
+            ? Clear(source)
+            : Set(objectId, source, reason);
+
+    public bool Clear(
+        SelectionChangeSource source,
+        SelectionChangeReason reason = SelectionChangeReason.Cleared)
+        => Set(null, source, reason);
+
+    public bool SelectPrevious(SelectionChangeSource source = SelectionChangeSource.Keyboard)
+        => PreviousObjectId is uint previous
+            && Set(previous, source, SelectionChangeReason.PreviousSelection);
+
+    bool ISelectionService.Select(uint objectId)
+        => Select(objectId, SelectionChangeSource.Plugin);
+
+    bool ISelectionService.Clear()
+        => Clear(SelectionChangeSource.Plugin);
+
+    private bool Set(
+        uint? selectedObjectId,
+        SelectionChangeSource source,
+        SelectionChangeReason reason)
+    {
+        if (SelectedObjectId == selectedObjectId)
+            return false;
+
+        uint? old = SelectedObjectId;
+        SelectedObjectId = selectedObjectId;
+        PreviousObjectId = old;
+        if (old is not null)
+            PreviousValidObjectId = old;
+
+        var transition = new SelectionTransition(old, selectedObjectId, source, reason);
+        Changed?.Invoke(transition);
+        DispatchPluginChanged(new SelectionChangedEvent(old, selectedObjectId));
+        return true;
+    }
+
+    private void DispatchPluginChanged(SelectionChangedEvent change)
+    {
+        Action<SelectionChangedEvent>? listeners = PluginChanged;
+        if (listeners is null) return;
+        foreach (Action<SelectionChangedEvent> listener in listeners.GetInvocationList())
+        {
+            try { listener(change); }
+            catch { /* plugin exceptions never escape into client state changes */ }
+        }
+    }
+}