Method BeginDragDropSource

lib/imgui/imgui.cpp:8948–9049

Call when current ID is active. When this returns true you need to: a) call SetDragDropPayload() exactly once, b) you may render the payload visual/description, c) call EndDragDropSource()

// Call when current ID is active.
// When this returns true you need to: a) call SetDragDropPayload() exactly once, b) you may render the payload visual/description, c) call EndDragDropSource()
bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
{
    ImGuiContext& g = *GImGui;
    ImGuiWindow* window = g.CurrentWindow;

    bool source_drag_active = false;
    ImGuiID source_id = 0;
    ImGuiID source_parent_id = 0;
    ImGuiMouseButton mouse_button = ImGuiMouseButton_Left;
    if (!(flags & ImGuiDragDropFlags_SourceExtern))
    {
        source_id = window->DC.LastItemId;
        if (source_id != 0 && g.ActiveId != source_id) // Early out for most common case
            return false;
        if (g.IO.MouseDown[mouse_button] == false)
            return false;

        if (source_id == 0)
        {
            // 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, C) Swallow your programmer pride.
            if (!(flags & ImGuiDragDropFlags_SourceAllowNullID))
            {
                IM_ASSERT(0);
                return false;
            }

            // Early out
            if ((window->DC.LastItemStatusFlags & ImGuiItemStatusFlags_HoveredRect) == 0 && (g.ActiveId == 0 || g.ActiveIdWindow != window))
                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.
            source_id = window->DC.LastItemId = window->GetIDFromRectangle(window->DC.LastItemRect);
            bool is_hovered = ItemHoverable(window->DC.LastItemRect, 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;
        }
        else
        {
            g.ActiveIdAllowOverlap = false;
        }
        if (g.ActiveId != source_id)
            return false;
        source_parent_id = window->IDStack.back();
        source_drag_active = IsMouseDragging(mouse_button);

        // Disable navigation and key inputs while dragging
        g.ActiveIdUsingNavDirMask = ~(ImU32)0;
        g.ActiveIdUsingNavInputMask = ~(ImU32)0;
        g.ActiveIdUsingKeyInputMask = ~(ImU64)0;