Drag and Drop: Added ImGuiDragDropFlags_SourceExtern to facilitate interfacing with WM_DROPFILES (#143)

imgui.cpp

@@ -11220,50 +11220,67 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags, int mouse_button)
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
-    if (g.IO.MouseDown[mouse_button] == false)
-        return false;
 
-    ImGuiID id = window->DC.LastItemId;
+    bool source_drag_active = false;
-    if (id == 0)
+    ImGuiID source_id = 0;
+    ImGuiID source_parent_id = 0;
+    if (!(flags & ImGuiDragDropFlags_SourceExtern))
     {
-        // If you want to use BeginDragDropSource() on an item with no unique identifier for interaction, such as Text() or Image(), you need to:
+        source_id = window->DC.LastItemId;
-        // A) Read the explanation below, B) Use the ImGuiDragDropFlags_SourceAllowNullID flag, C) Swallow your programmer pride.
+        if (source_id != 0 && g.ActiveId != source_id) // Early out for most common case
-        if (!(flags & ImGuiDragDropFlags_SourceAllowNullID))
+            return false;
-        {
+        if (g.IO.MouseDown[mouse_button] == false)
-            IM_ASSERT(0);
             return false;
-        }
 
-        // Magic fallback (=somehow reprehensible) to handle items with no assigned ID, e.g. Text(), Image()
+        if (source_id == 0)
-        // We build a throwaway ID based on current ID stack + relative AABB of items in window. 
-        // THE IDENTIFIER WON'T SURVIVE ANY REPOSITIONING OF THE WIDGET, so if your widget moves your dragging operation will be canceled. 
-        // We don't need to maintain/call ClearActiveID() as releasing the button will early out this function and trigger !ActiveIdIsAlive.
-        bool is_hovered = window->DC.LastItemRectHoveredRect;
-        if (!is_hovered && (g.ActiveId == 0 || g.ActiveIdWindow != window))
-            return false;
-        id = window->DC.LastItemId = window->GetIDFromRectangle(window->DC.LastItemRect);
-        if (is_hovered)
-            SetHoveredID(id);
-        if (is_hovered && g.IO.MouseClicked[mouse_button])
         {
-            SetActiveID(id, window);
+            // If you want to use BeginDragDropSource() on an item with no unique identifier for interaction, such as Text() or Image(), you need to:
-            FocusWindow(window);
+            // A) Read the explanation below, B) Use the ImGuiDragDropFlags_SourceAllowNullID flag, C) Swallow your programmer pride.
+            if (!(flags & ImGuiDragDropFlags_SourceAllowNullID))
+            {
+                IM_ASSERT(0);
+                return false;
+            }
+
+            // Magic fallback (=somehow reprehensible) to handle items with no assigned ID, e.g. Text(), Image()
+            // We build a throwaway ID based on current ID stack + relative AABB of items in window. 
+            // THE IDENTIFIER WON'T SURVIVE ANY REPOSITIONING OF THE WIDGET, so if your widget moves your dragging operation will be canceled. 
+            // We don't need to maintain/call ClearActiveID() as releasing the button will early out this function and trigger !ActiveIdIsAlive.
+            bool is_hovered = window->DC.LastItemRectHoveredRect;
+            if (!is_hovered && (g.ActiveId == 0 || g.ActiveIdWindow != window))
+                return false;
+            source_id = window->DC.LastItemId = window->GetIDFromRectangle(window->DC.LastItemRect);
+            if (is_hovered)
+                SetHoveredID(source_id);
+            if (is_hovered && g.IO.MouseClicked[mouse_button])
+            {
+                SetActiveID(source_id, window);
+                FocusWindow(window);
+            }
+            if (g.ActiveId == source_id) // Allow the underlying widget to display/return hovered during the mouse release frame, else we would get a flicker.
+                g.ActiveIdAllowOverlap = is_hovered;
         }
-        if (g.ActiveId == id) // Allow the underlying widget to display/return hovered during the mouse release frame, else we would get a flicker.
+        if (g.ActiveId != source_id)
-            g.ActiveIdAllowOverlap = is_hovered;
+            return false;
+        source_parent_id = window->IDStack.back();
+        source_drag_active = IsMouseDragging(mouse_button);
+    }
+    else
+    {
+        window = NULL;
+        source_id = ImHash("#SourceExtern", 0);
+        source_drag_active = true;
     }
-    if (g.ActiveId != id)
-        return false;
 
-    if (IsMouseDragging(mouse_button))
+    if (source_drag_active)
     {
         if (!g.DragDropActive)
         {
-            IM_ASSERT(id != 0);
+            IM_ASSERT(source_id != 0);
             ClearDragDrop();
             ImGuiPayload& payload = g.DragDropPayload;
-            payload.SourceId = id;
+            payload.SourceId = source_id;
-            payload.SourceParentId = window->IDStack.back();
+            payload.SourceParentId = source_parent_id;
             g.DragDropActive = true;
             g.DragDropSourceFlags = flags;
             g.DragDropMouseButton = mouse_button;
@@ -11279,7 +11296,7 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags, int mouse_button)
             BeginTooltipEx(ImGuiWindowFlags_NoInputs);
         }
 
-        if (!(flags & ImGuiDragDropFlags_SourceNoDisableHover))
+        if (!(flags & ImGuiDragDropFlags_SourceNoDisableHover) && !(flags & ImGuiDragDropFlags_SourceExtern))
             window->DC.LastItemRectHoveredRect = false;
 
         return true;
@@ -11434,7 +11451,7 @@ const ImGuiPayload* ImGui::AcceptDragDropPayload(const char* type, ImGuiDragDrop
     }
 
     g.DragDropAcceptFrameCount = g.FrameCount;
-    payload.Delivery = was_accepted_previously && IsMouseReleased(g.DragDropMouseButton);
+    payload.Delivery = was_accepted_previously && !IsMouseDown(g.DragDropMouseButton); // For extern drag sources affecting os window focus, it's easier to just test !IsMouseDown() instead of IsMouseReleased()
     if (!payload.Delivery && !(flags & ImGuiDragDropFlags_AcceptBeforeDelivery))
         return NULL;
 

imgui.h

@@ -643,6 +643,7 @@ enum ImGuiDragDropFlags_
     ImGuiDragDropFlags_SourceNoDisableHover         = 1 << 1,       // By default, when dragging we clear data so that IsItemHovered() will return true, to avoid subsequent user code submitting tooltips. This flag disable this behavior so you can still call IsItemHovered() on the source item.
     ImGuiDragDropFlags_SourceNoHoldToOpenOthers     = 1 << 2,       // Disable the behavior that allows to open tree nodes and collapsing header by holding over them while dragging a source item.
     ImGuiDragDropFlags_SourceAllowNullID            = 1 << 3,       // Allow items such as Text(), Image() that have no unique identifier to be used as drag source, by manufacturing a temporary identifier based on their window-relative position. This is extremely unusual within the dear imgui ecosystem and so we made it explicit.
+    ImGuiDragDropFlags_SourceExtern                 = 1 << 4,       // External source (from outside of imgui), won't attempt to read current item/window info. Will always return true. Only one Extern source can be active simultaneously.
     // AcceptDragDropPayload() flags
     ImGuiDragDropFlags_AcceptBeforeDelivery         = 1 << 10,      // AcceptDragDropPayload() will returns true even before the mouse button is released. You can then call IsDelivery() to test if the payload needs to be delivered.
     ImGuiDragDropFlags_AcceptNoDrawDefaultRect      = 1 << 11,      // Do not draw the default highlight rectangle when hovering over target.