Method BeginDockableDragDropTarget

imgui_tiny_app/imgui/imgui.cpp:21317–21398

}

void ImGui::BeginDockableDragDropTarget(ImGuiWindow* window)
{
    ImGuiContext& g = *GImGui;

    //IM_ASSERT(window->RootWindowDockTree == window); // May also be a DockSpace
    IM_ASSERT((window->Flags & ImGuiWindowFlags_NoDocking) == 0);
    if (!g.DragDropActive)
        return;
    //GetForegroundDrawList(window)->AddRect(window->Pos, window->Pos + window->Size, IM_COL32(255, 255, 0, 255));
    if (!BeginDragDropTargetCustom(window->Rect(), window->ID))
        return;

    // Peek into the payload before calling AcceptDragDropPayload() so we can handle overlapping dock nodes with filtering
    // (this is a little unusual pattern, normally most code would call AcceptDragDropPayload directly)
    const ImGuiPayload* payload = &g.DragDropPayload;
    if (!payload->IsDataType(IMGUI_PAYLOAD_TYPE_WINDOW) || !DockNodeIsDropAllowed(window, *(ImGuiWindow**)payload->Data))
    {
        EndDragDropTarget();
        return;
    }

    ImGuiWindow* payload_window = *(ImGuiWindow**)payload->Data;
    if (AcceptDragDropPayload(IMGUI_PAYLOAD_TYPE_WINDOW, ImGuiDragDropFlags_AcceptBeforeDelivery | ImGuiDragDropFlags_AcceptNoDrawDefaultRect))
    {
        // Select target node
        // (Important: we cannot use g.HoveredDockNode here! Because each of our target node have filters based on payload, each candidate drop target will do its own evaluation)
        bool dock_into_floating_window = false;
        ImGuiDockNode* node = NULL;
        if (window->DockNodeAsHost)
        {
            // Cannot assume that node will != NULL even though we passed the rectangle test: it depends on padding/spacing handled by DockNodeTreeFindVisibleNodeByPos().
            node = DockNodeTreeFindVisibleNodeByPos(window->DockNodeAsHost, g.IO.MousePos);

            // There is an edge case when docking into a dockspace which only has _inactive_ nodes (because none of the windows are active)
            // In this case we need to fallback into any leaf mode, possibly the central node.
            // FIXME-20181220: We should not have to test for IsLeafNode() here but we have another bug to fix first.
            if (node && node->IsDockSpace() && node->IsRootNode())
                node = (node->CentralNode && node->IsLeafNode()) ? node->CentralNode : DockNodeTreeFindFallbackLeafNode(node);
        }
        else
        {
            if (window->DockNode)
                node = window->DockNode;
            else
                dock_into_floating_window = true; // Dock into a regular window
        }

        const ImRect explicit_target_rect = (node && node->TabBar && !node->IsHiddenTabBar() && !node->IsNoTabBar()) ? node->TabBar->BarRect : ImRect(window->Pos, window->Pos + ImVec2(window->Size.x, GetFrameHeight()));
        const bool is_explicit_target = g.IO.ConfigDockingWithShift || IsMouseHoveringRect(explicit_target_rect.Min, explicit_target_rect.Max);

        // Preview docking request and find out split direction/ratio
        //const bool do_preview = true;     // Ignore testing for payload->IsPreview() which removes one frame of delay, but breaks overlapping drop targets within the same window.
        const bool do_preview = payload->IsPreview() || payload->IsDelivery();
        if (do_preview && (node != NULL || dock_into_floating_window))
        {
            // If we have a non-leaf node it means we are hovering the border of a parent node, in which case only outer markers will appear.
            ImGuiDockPreviewData split_inner;
            ImGuiDockPreviewData split_outer;