Hitting: IM_ASSERT(node->Windows.Size > 0 || node->IsSplitNode() || node->IsCentralNode());

[gmagno]

Hello,

This is a crosspost from a message in discord.

Version/Branch of Dear ImGui:

Version: 4f5aac3
Branch: docking

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_glfw.cpp + imgui_impl_opengl3.cpp
Operating System: Arch Linux 5.6.19.a-1-hardened #1 SMP PREEMPT Sat, 20 Jun 2020 15:16:50 +0000 x86_64 GNU/Linux

My Issue/Question:

I'm hitting an IM_ASSERT when doing the following:

Am I doing something wrong? Is this an known issue? The IM_ASSERT is preceded by a FIXME-DOCK comment:

    // FIXME-DOCK: When we are trying to dock an existing single-window node into a loose window, transfer Node ID as well
    // When processing an interactive split, usually LastFrameAlive will be < g.FrameCount. But DockBuilder operations can make it ==.
    if (node)
        IM_ASSERT(node->LastFrameAlive <= g.FrameCount);
    if (node && target_window && node == target_window->DockNodeAsHost)
        IM_ASSERT(node->Windows.Size > 0 || node->IsSplitNode() || node->IsCentralNode());  // <------

Standalone, minimal, complete and verifiable example:

// Some code to reproduce the issue
void DockSpace(bool *p_open)
{
    static int initialized = 1;

    static bool opt_fullscreen_persistant = true;
    bool opt_fullscreen = opt_fullscreen_persistant;
    static ImGuiDockNodeFlags dockspace_flags = ImGuiDockNodeFlags_None;
    dockspace_flags |= ImGuiDockNodeFlags_PassthruCentralNode;
    // dockspace_flags |= ImGuiDockNodeFlags_AutoHideTabBar;

    // We are using the ImGuiWindowFlags_NoDocking flag to make the parent window not dockable into,
    // because it would be confusing to have two docking targets within each others.
    ImGuiWindowFlags window_flags = ImGuiWindowFlags_NoDocking;
    if (opt_fullscreen)
    {
        ImGuiViewport *viewport = ImGui::GetMainViewport();
        ImGui::SetNextWindowPos(viewport->GetWorkPos());
        ImGui::SetNextWindowSize(viewport->GetWorkSize());
        ImGui::SetNextWindowViewport(viewport->ID);
        ImGui::PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0f);
        ImGui::PushStyleVar(ImGuiStyleVar_WindowBorderSize, 0.0f);
        window_flags |= ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoCollapse;
        window_flags |= ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove;
        window_flags |= ImGuiWindowFlags_NoBringToFrontOnFocus | ImGuiWindowFlags_NoNavFocus;
    }

    // When using ImGuiDockNodeFlags_PassthruCentralNode, DockSpace() will render our background
    // and handle the pass-thru hole, so we ask Begin() to not render a background.
    if (dockspace_flags & ImGuiDockNodeFlags_PassthruCentralNode)
        window_flags |= ImGuiWindowFlags_NoBackground;

    // Important: note that we proceed even if Begin() returns false (aka window is collapsed).
    // This is because we want to keep our DockSpace() active. If a DockSpace() is inactive,
    // all active windows docked into it will lose their parent and become undocked.
    // We cannot preserve the docking relationship between an active window and an inactive docking, otherwise
    // any change of dockspace/settings would lead to windows being stuck in limbo and never being visible.
    ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(0.0f, 0.0f));
    ImGui::Begin("DockSpace", p_open, window_flags);

    ImGui::PopStyleVar();

    if (opt_fullscreen)
        ImGui::PopStyleVar(2);

    // DockSpace
    ImGuiIO &io = ImGui::GetIO();
    ImGuiID dockspace_id = 0;
    if (io.ConfigFlags & ImGuiConfigFlags_DockingEnable)
    {
        dockspace_id = ImGui::GetID("MyDockSpace");

        if (initialized == 1)
        {
            initialized = 2;
            ImGuiContext *ctx = ImGui::GetCurrentContext();
            ImGui::DockBuilderRemoveNode(dockspace_id); // Clear out existing layout
            ImGui::DockBuilderAddNode(dockspace_id);    // Add empty node

            ImGuiID dock_main_id = dockspace_id; // This variable will track the document node, however we are not using it here as we aren't docking anything into it.
            ImGuiID dock_id_left = ImGui::DockBuilderSplitNode(dock_main_id, ImGuiDir_Left, 0.20f, NULL, &dock_main_id);
            ImGuiID dock_id_bottom = ImGui::DockBuilderSplitNode(dock_main_id, ImGuiDir_Down, 0.20f, NULL, &dock_main_id);

            ImGui::DockBuilderDockWindow("Center", dock_main_id);
            ImGui::DockBuilderDockWindow("Left", dock_id_left);
            ImGui::DockBuilderDockWindow("Bottom", dock_id_bottom);
            ImGui::DockBuilderFinish(dockspace_id);
        }

        ImGui::DockSpace(dockspace_id, ImVec2(0.0f, 0.0f), dockspace_flags);

        if (initialized == 2)
        {
            ImGui::Begin("Center");
            ImGui::End();

            ImGui::Begin("Left");
            ImGui::End();

            ImGui::Begin("Bottom");
            ImGui::End();
        }
    }
    else
    {
        ShowDockingDisabledMessage();
    }

    ImGui::End();

}

[ocornut]

Thank you @gmagno for your report, I started looking into it.

[ocornut]

Am I doing something wrong?

The issue if you are creating a floating node if you call DockBuilderAddNode() without the ImGuiDockNodeFlags_DockSpace flag.

// - To create a DockSpace() node, make sure to set the ImGuiDockNodeFlags_DockSpace flag when calling DockBuilderAddNode().
//   You can create dockspace nodes (attached to a window) _or_ floating nodes (carry its own window) with this API.

An empty DockSpace has a node tagged "Central node" which keeps existing when it has no window, so any empty dockspace is generally a single "central node". Whereas floating node (e.g. when merging two floating windows) will cease to exist when the windows are undocked from it.

In this case it's a contradiction to create the node without the ImGuiDockNodeFlags_DockSpace flag and then calling DockSpace, but for various subtle reasons it's not easy at the moment to report a clear error. Going to investigate ways the error handling could be improved here.

[gmagno]

That fixed it. Many thanks for the quick reply.
I am taking the liberty of closing this issue.

[ocornut]

I pushed cbade7b which I think should help against this.
I think down the line we need to redesign the concept of _CentralNode as it currently has too many holes.