Add a drop zone which reports dropped paths

app/MindWork AI Studio/Components/PathDropZone.razor

@@ -0,0 +1,7 @@
+@inherits MSGComponentBase
+
+<div @onmouseenter="@this.OnMouseEnter" @onmouseleave="@this.OnMouseLeave">
+    <MudPaper Outlined="@true" Class="@this.dragClass">
+        @this.ChildContent
+    </MudPaper>
+</div>

app/MindWork AI Studio/Components/PathDropZone.razor.cs

@@ -0,0 +1,168 @@
+using AIStudio.Tools.Rust;
+
+using Microsoft.AspNetCore.Components;
+
+namespace AIStudio.Components;
+
+/// <summary>
+/// A drop zone which reports the paths of whatever was dropped on it, and nothing else.
+/// </summary>
+/// <remarks>
+/// Dropping is a native matter in AI Studio: the Tauri runtime reports real paths, which is why
+/// this zone can hand out folders just as well as files. What those paths mean is the consumer's
+/// business — this component reads no content and does not care whether a path leads to a file or
+/// to a folder.
+/// </remarks>
+public partial class PathDropZone : MSGComponentBase
+{
+    /// <summary>
+    /// The content shown inside the zone.
+    /// </summary>
+    [Parameter]
+    public RenderFragment? ChildContent { get; set; }
+
+    /// <summary>
+    /// Reports the dropped paths, in the order the runtime delivered them.
+    /// </summary>
+    [Parameter]
+    public EventCallback<List<string>> OnPathsDropped { get; set; }
+
+    /// <summary>
+    /// On which layer to register the drop area. Higher layers have priority over lower layers.
+    /// </summary>
+    [Parameter]
+    public int Layer { get; set; } = DropLayers.ROOT;
+
+    /// <summary>
+    /// Catch all documents that are hovered over the AI Studio window and not only over the drop zone.
+    /// </summary>
+    [Parameter]
+    public bool CatchAllDocuments { get; set; }
+
+    /// <summary>
+    /// When true, the zone ignores drops and is not highlighted.
+    /// </summary>
+    /// <remarks>
+    /// The drop area stays registered nevertheless. Releasing it during the lifetime of the
+    /// component would lower the count of every zone below this one, and those zones would then
+    /// catch files while this one is still on screen.
+    /// </remarks>
+    [Parameter]
+    public bool Disabled { get; set; }
+
+    [Inject]
+    private ILogger<PathDropZone> Logger { get; init; } = null!;
+
+    private const string DEFAULT_DRAG_CLASS = "relative rounded-lg border-2 border-dashed pa-3 mb-3 mud-width-full";
+
+    private string dragClass = DEFAULT_DRAG_CLASS;
+    private uint numDropAreasAboveThis;
+    private bool isComponentHovered;
+
+    #region Overrides of MSGComponentBase
+
+    protected override async Task OnInitializedAsync()
+    {
+        this.ApplyFilters([], [ Event.TAURI_EVENT_RECEIVED, Event.REGISTER_FILE_DROP_AREA, Event.UNREGISTER_FILE_DROP_AREA ]);
+        await this.MessageBus.SendMessage(this, Event.REGISTER_FILE_DROP_AREA, this.Layer);
+
+        await base.OnInitializedAsync();
+    }
+
+    /// <summary>
+    /// Releases the drop area.
+    /// </summary>
+    protected override void DisposeResources()
+    {
+        // Without this, drop areas below this one would count this component forever and would
+        // stop catching dropped files:
+        this.MessageBus.SendMessage(this, Event.UNREGISTER_FILE_DROP_AREA, this.Layer).Observe($"{nameof(PathDropZone)}: releasing the drop area");
+
+        base.DisposeResources();
+    }
+
+    protected override async Task ProcessIncomingMessage<T>(ComponentBase? sendingComponent, Event triggeredEvent, T? data) where T : default
+    {
+        // A disabled zone takes no files. It keeps track of the zones above it, though, because
+        // those come and go while this one is disabled:
+        if (this.Disabled && triggeredEvent == Event.TAURI_EVENT_RECEIVED)
+            return;
+
+        switch (triggeredEvent)
+        {
+            case Event.REGISTER_FILE_DROP_AREA when sendingComponent != this:
+            {
+                if(data is int layer && layer > this.Layer)
+                {
+                    this.numDropAreasAboveThis++;
+                    this.ClearDragClass();
+                }
+
+                break;
+            }
+
+            case Event.UNREGISTER_FILE_DROP_AREA when sendingComponent != this:
+            {
+                if(data is int layer && layer > this.Layer && this.numDropAreasAboveThis > 0)
+                    this.numDropAreasAboveThis--;
+
+                break;
+            }
+
+            case Event.TAURI_EVENT_RECEIVED when data is TauriEvent { EventType: TauriEventType.FILE_DROP_HOVERED }:
+                if(!this.CanCatchDroppedPath())
+                    return;
+
+                this.SetDragClass();
+                this.StateHasChanged();
+                break;
+
+            case Event.TAURI_EVENT_RECEIVED when data is TauriEvent { EventType: TauriEventType.FILE_DROP_CANCELED }:
+            case Event.TAURI_EVENT_RECEIVED when data is TauriEvent { EventType: TauriEventType.WINDOW_NOT_FOCUSED }:
+                this.isComponentHovered = false;
+                this.ClearDragClass();
+                this.StateHasChanged();
+                break;
+
+            case Event.TAURI_EVENT_RECEIVED when data is TauriEvent { EventType: TauriEventType.FILE_DROP_DROPPED, Payload: var paths }:
+                if(!this.CanCatchDroppedPath())
+                    return;
+
+                this.Logger.LogDebug("The path drop zone on layer {Layer} caught {Count} path(s).", this.Layer, paths.Count);
+                await this.OnPathsDropped.InvokeAsync(paths);
+                this.ClearDragClass();
+                this.StateHasChanged();
+                break;
+        }
+    }
+
+    #endregion
+
+    private bool CanCatchDroppedPath() => this.numDropAreasAboveThis is 0 && (this.isComponentHovered || this.CatchAllDocuments);
+
+    private void SetDragClass() => this.dragClass = $"{DEFAULT_DRAG_CLASS} mud-border-primary border-2";
+
+    private void ClearDragClass() => this.dragClass = DEFAULT_DRAG_CLASS;
+
+    private void OnMouseEnter(EventArgs _)
+    {
+        if(this.Disabled || this.numDropAreasAboveThis > 0)
+            return;
+
+        // The webview gets no DOM drag events while a native drag is in progress, so the mouse is
+        // what tells us whether the user is aiming at this zone:
+        this.isComponentHovered = true;
+        this.SetDragClass();
+        this.StateHasChanged();
+    }
+
+    private void OnMouseLeave(EventArgs _)
+    {
+        if(this.Disabled)
+            return;
+
+        this.isComponentHovered = false;
+        this.ClearDragClass();
+        this.StateHasChanged();
+    }
+}