Method BeginDragDropSource

src/imgui/imgui.cpp:14513–14630

When this returns true you need to: a) call SetDragDropPayload() exactly once, b) you may render the payload visual/description, c) call EndDragDropSource() If the item has an identifier: - This assume/require the item to be activated (typically via ButtonBehavior). - Therefore if you want to use this with a mouse button other than left mouse button, it is up to the item itself to activate with an

// If the item has no identifier:
// - Currently always assume left mouse button.
bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
{
    ImGuiContext& g = *GImGui;
    ImGuiWindow* window = g.CurrentWindow;

    // FIXME-DRAGDROP: While in the common-most "drag from non-zero active id" case we can tell the mouse button,
    // in both SourceExtern and id==0 cases we may requires something else (explicit flags or some heuristic).
    ImGuiMouseButton mouse_button = ImGuiMouseButton_Left;

    bool source_drag_active = false;
    ImGuiID source_id = 0;
    ImGuiID source_parent_id = 0;
    if ((flags & ImGuiDragDropFlags_SourceExtern) == 0)
    {
        source_id = g.LastItemData.ID;
        if (source_id != 0)
        {
            // Common path: items with ID
            if (g.ActiveId != source_id)
                return false;
            if (g.ActiveIdMouseButton != -1)
                mouse_button = g.ActiveIdMouseButton;
            if (g.IO.MouseDown[mouse_button] == false || window->SkipItems)
                return false;
            g.ActiveIdAllowOverlap = false;
        }
        else
        {
            // Uncommon path: items without ID
            if (g.IO.MouseDown[mouse_button] == false || window->SkipItems)
                return false;
            if ((g.LastItemData.StatusFlags & ImGuiItemStatusFlags_HoveredRect) == 0 && (g.ActiveId == 0 || g.ActiveIdWindow != window))
                return false;

            // If you want to use BeginDragDropSource() on an item with no unique identifier for interaction, such as Text() or Image(), you need to:
            // A) Read the explanation below, B) Use the ImGuiDragDropFlags_SourceAllowNullID flag.
            if (!(flags & ImGuiDragDropFlags_SourceAllowNullID))
            {
                IM_ASSERT(0);
                return false;
            }

            // Magic fallback 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/RESIZINGG 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.
            // Rely on keeping other window->LastItemXXX fields intact.
            source_id = g.LastItemData.ID = window->GetIDFromRectangle(g.LastItemData.Rect);
            KeepAliveID(source_id);
            bool is_hovered = ItemHoverable(g.LastItemData.Rect, source_id, g.LastItemData.ItemFlags);
            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;
        }