InputTextMultiline with line numbers code example

Example: code for a text box in imgui

#define MAX(a, b)    (((a) < (b)) ? (b) : (a))
#define INTERNAL     static

struct MultilineScrollState
{
    // Input.
    float scrollRegionX;
    float scrollX;
    ImGuiStorage *storage;

    // Output.
    bool newScrollPositionAvailable;
    float newScrollX;
};

INTERNAL int MultilineScrollCallback(ImGuiTextEditCallbackData *data)
{
    MultilineScrollState *scrollState = (MultilineScrollState *)data->UserData;
    ImGuiID cursorId = ImGui::GetID("cursor");
    int oldCursorIndex = scrollState->storage->GetInt(cursorId, 0);

    if (oldCursorIndex != data->CursorPos)
    {
        int begin = data->CursorPos;

        while ((begin > 0) && (data->Buf[begin - 1] != '\n'))
        {
            --begin;
        }

        float cursorOffset = ImGui::CalcTextSize(data->Buf + begin, data->Buf + data->CursorPos).x;
        float SCROLL_INCREMENT = scrollState->scrollRegionX * 0.25f;

        if (cursorOffset < scrollState->scrollX)
        {
            scrollState->newScrollPositionAvailable = true;
            scrollState->newScrollX = MAX(0.0f, cursorOffset - SCROLL_INCREMENT);
        }
        else if ((cursorOffset - scrollState->scrollRegionX) >= scrollState->scrollX)
        {
            scrollState->newScrollPositionAvailable = true;
            scrollState->newScrollX = cursorOffset - scrollState->scrollRegionX + SCROLL_INCREMENT;
        }
    }

    scrollState->storage->SetInt(cursorId, data->CursorPos);

    return 0;
}

INTERNAL bool ImGuiInputTextMultiline(const char* label, char* buf, size_t buf_size, float height, ImGuiInputTextFlags flags = 0)
{
    float scrollbarSize = ImGui::GetStyle().ScrollbarSize;
    float labelWidth = ImGui::CalcTextSize(label).x + scrollbarSize;
    float SCROLL_WIDTH = 2000.0f; // Very large scrolling width to allow for very long lines.
    MultilineScrollState scrollState = {};

    // Set up child region for horizontal scrolling of the text box.
    ImGui::BeginChild(label, ImVec2(-labelWidth, height), false, ImGuiWindowFlags_HorizontalScrollbar);
    scrollState.scrollRegionX = MAX(0.0f, ImGui::GetWindowWidth() - scrollbarSize);
    scrollState.scrollX = ImGui::GetScrollX();
    scrollState.storage = ImGui::GetStateStorage();
    bool changed = ImGui::InputTextMultiline(label, buf, buf_size, ImVec2(SCROLL_WIDTH, MAX(0.0f, height - scrollbarSize)),
                                             flags | ImGuiInputTextFlags_CallbackAlways, MultilineScrollCallback, &scrollState);

    if (scrollState.newScrollPositionAvailable)
    {
        ImGui::SetScrollX(scrollState.newScrollX);
    }

    ImGui::EndChild();
    ImGui::SameLine();
    ImGui::Text(label);

    return changed;
}