Drag and Drop: added BeginDragDropTargetViewport(), still in imgui_internal.h for now. (#5204)

imgui.cpp

@@ -4160,6 +4160,7 @@ ImGuiContext::ImGuiContext(ImFontAtlas* shared_font_atlas)
     DragDropSourceFrameCount = -1;
     DragDropMouseButton = -1;
     DragDropTargetId = 0;
+    DragDropTargetFullViewport = 0;
     DragDropAcceptFlags = ImGuiDragDropFlags_None;
     DragDropAcceptIdCurrRectSurface = 0.0f;
     DragDropAcceptIdPrev = DragDropAcceptIdCurr = 0;
@@ -14722,6 +14723,31 @@ bool ImGui::BeginDragDropTargetCustom(const ImRect& bb, ImGuiID id)
     g.DragDropTargetRect = bb;
     g.DragDropTargetClipRect = window->ClipRect; // May want to be overridden by user depending on use case?
     g.DragDropTargetId = id;
+    g.DragDropTargetFullViewport = 0;
+    g.DragDropWithinTarget = true;
+    return true;
+}
+
+// Typical usage would be:
+//   if (!ImGui::IsWindowHovered(ImGuiHoveredFlags_AnyWindow | ImGuiHoveredFlags_AllowWhenBlockedByActiveItem))
+//       if (ImGui::BeginDragDropTargetViewport(ImGui::GetMainViewport(), NULL))
+// But we are leaving the hover test to the caller for maximum flexibility.
+bool ImGui::BeginDragDropTargetViewport(ImGuiViewport* viewport, const ImRect* p_bb)
+{
+    ImGuiContext& g = *GImGui;
+    if (!g.DragDropActive)
+        return false;
+
+    ImRect bb = p_bb ? *p_bb : ((ImGuiViewportP*)viewport)->GetWorkRect();
+    ImGuiID id = viewport->ID;
+    if (!IsMouseHoveringRect(bb.Min, bb.Max, false) || (id == g.DragDropPayload.SourceId))
+        return false;
+
+    IM_ASSERT(g.DragDropWithinTarget == false && g.DragDropWithinSource == false); // Can't nest BeginDragDropSource() and BeginDragDropTarget()
+    g.DragDropTargetRect = bb;
+    g.DragDropTargetClipRect = bb;
+    g.DragDropTargetId = id;
+    g.DragDropTargetFullViewport = id;
     g.DragDropWithinTarget = true;
     return true;
 }
@@ -14792,8 +14818,17 @@ const ImGuiPayload* ImGui::AcceptDragDropPayload(const char* type, ImGuiDragDrop
     // Render default drop visuals
     payload.Preview = was_accepted_previously;
     flags |= (g.DragDropSourceFlags & ImGuiDragDropFlags_AcceptNoDrawDefaultRect); // Source can also inhibit the preview (useful for external sources that live for 1 frame)
-    if (!(flags & ImGuiDragDropFlags_AcceptNoDrawDefaultRect) && payload.Preview)
+    const bool draw_target_rect = payload.Preview && !(flags & ImGuiDragDropFlags_AcceptNoDrawDefaultRect);
+    if (draw_target_rect && g.DragDropTargetFullViewport != 0)
+    {
+        ImRect bb = g.DragDropTargetRect;
+        bb.Expand(-3.5f);
+        RenderDragDropTargetRectEx(GetForegroundDrawList(), bb);
+    }
+    else if (draw_target_rect)
+    {
         RenderDragDropTargetRectForItem(r);
+    }
 
     g.DragDropAcceptFrameCount = g.FrameCount;
     if ((g.DragDropSourceFlags & ImGuiDragDropFlags_SourceExtern) && g.DragDropMouseButton == -1)

imgui_internal.h

@@ -2372,6 +2372,7 @@ struct ImGuiContext
     ImRect                  DragDropTargetRect;                 // Store rectangle of current target candidate (we favor small targets when overlapping)
     ImRect                  DragDropTargetClipRect;             // Store ClipRect at the time of item's drawing
     ImGuiID                 DragDropTargetId;
+    ImGuiID                 DragDropTargetFullViewport;
     ImGuiDragDropFlags      DragDropAcceptFlags;
     float                   DragDropAcceptIdCurrRectSurface;    // Target item surface (we resolve overlapping targets by prioritizing the smaller surface)
     ImGuiID                 DragDropAcceptIdCurr;               // Target item id (set at the time of accepting the payload)
@@ -3421,6 +3422,7 @@ namespace ImGui
     // Drag and Drop
     IMGUI_API bool          IsDragDropActive();
     IMGUI_API bool          BeginDragDropTargetCustom(const ImRect& bb, ImGuiID id);
+    IMGUI_API bool          BeginDragDropTargetViewport(ImGuiViewport* viewport, const ImRect* p_bb = NULL);
     IMGUI_API void          ClearDragDrop();
     IMGUI_API bool          IsDragDropPayloadBeingAccepted();
     IMGUI_API void          RenderDragDropTargetRectForItem(const ImRect& bb);