Method BeginMultiSelect

imgui_tiny_app/imgui/imgui_widgets.cpp:7928–8047

Return ImGuiMultiSelectIO structure. Lifetime: don't hold on ImGuiMultiSelectIO* pointers over multiple frames or past any subsequent call to BeginMultiSelect() or EndMultiSelect(). Passing 'selection_size' and 'items_count' parameters is currently optional. - 'selection_size' is useful to disable some shortcut routing: e.g. ImGuiMultiSelectFlags_ClearOnEscape won't claim Escape key when selection

// - If they are costly for you to compute (e.g. external intrusive selection without maintaining size), you may avoid them and pass -1.
//   - If you can easily tell if your selection is empty or not, you may pass 0/1, or you may enable ImGuiMultiSelectFlags_ClearOnEscape flag dynamically.
ImGuiMultiSelectIO* ImGui::BeginMultiSelect(ImGuiMultiSelectFlags flags, int selection_size, int items_count)
{
    ImGuiContext& g = *GImGui;
    ImGuiWindow* window = g.CurrentWindow;

    if (++g.MultiSelectTempDataStacked > g.MultiSelectTempData.Size)
        g.MultiSelectTempData.resize(g.MultiSelectTempDataStacked, ImGuiMultiSelectTempData());
    ImGuiMultiSelectTempData* ms = &g.MultiSelectTempData[g.MultiSelectTempDataStacked - 1];
    IM_STATIC_ASSERT(offsetof(ImGuiMultiSelectTempData, IO) == 0); // Clear() relies on that.
    g.CurrentMultiSelect = ms;
    if ((flags & (ImGuiMultiSelectFlags_ScopeWindow | ImGuiMultiSelectFlags_ScopeRect)) == 0)
        flags |= ImGuiMultiSelectFlags_ScopeWindow;
    if (flags & ImGuiMultiSelectFlags_SingleSelect)
        flags &= ~(ImGuiMultiSelectFlags_BoxSelect2d | ImGuiMultiSelectFlags_BoxSelect1d);
    if (flags & ImGuiMultiSelectFlags_BoxSelect2d)
        flags &= ~ImGuiMultiSelectFlags_BoxSelect1d;

    // FIXME: Workaround to the fact we override CursorMaxPos, meaning size measurement are lost. (#8250)
    // They should perhaps be stacked properly?
    if (ImGuiTable* table = g.CurrentTable)
        if (table->CurrentColumn != -1)
            TableEndCell(table); // This is currently safe to call multiple time. If that properly is lost we can extract the "save measurement" part of it.

    // FIXME: BeginFocusScope()
    const ImGuiID id = window->IDStack.back();
    ms->Clear();
    ms->FocusScopeId = id;
    ms->Flags = flags;
    ms->IsFocused = (ms->FocusScopeId == g.NavFocusScopeId);
    ms->BackupCursorMaxPos = window->DC.CursorMaxPos;
    ms->ScopeRectMin = window->DC.CursorMaxPos = window->DC.CursorPos;
    PushFocusScope(ms->FocusScopeId);
    if (flags & ImGuiMultiSelectFlags_ScopeWindow) // Mark parent child window as navigable into, with highlight. Assume user will always submit interactive items.
        window->DC.NavLayersActiveMask |= 1 << ImGuiNavLayer_Main;

    // Use copy of keyboard mods at the time of the request, otherwise we would requires mods to be held for an extra frame.
    ms->KeyMods = g.NavJustMovedToId ? (g.NavJustMovedToIsTabbing ? 0 : g.NavJustMovedToKeyMods) : g.IO.KeyMods;
    if (flags & ImGuiMultiSelectFlags_NoRangeSelect)
        ms->KeyMods &= ~ImGuiMod_Shift;

    // Bind storage
    ImGuiMultiSelectState* storage = g.MultiSelectStorage.GetOrAddByKey(id);
    storage->ID = id;
    storage->LastFrameActive = g.FrameCount;
    storage->LastSelectionSize = selection_size;
    storage->Window = window;
    ms->Storage = storage;

    // Output to user
    ms->IO.Requests.resize(0);
    ms->IO.RangeSrcItem = storage->RangeSrcItem;
    ms->IO.NavIdItem = storage->NavIdItem;
    ms->IO.NavIdSelected = (storage->NavIdSelected == 1) ? true : false;
    ms->IO.ItemsCount = items_count;

    // Clear when using Navigation to move within the scope
    // (we compare FocusScopeId so it possible to use multiple selections inside a same window)
    bool request_clear = false;