ImGuiDynamicCombo.cpp

int BeginCombo(const char* pLabel, const char* pValue)
{
	ImGuiWindow* pWindow = ImGui::GetCurrentWindow();
	ImGuiState& oState = *GImGui;
	const ImGuiStyle& oStyle = oState.Style;

	const ImGuiID oId = pWindow->GetID(pLabel);
	const float fWidth = ImGui::CalcItemWidth();

	const ImVec2 oLabelSize = ImGui::CalcTextSize(pLabel, NULL, true);
	const ImRect oFrameRect(pWindow->DC.CursorPos, pWindow->DC.CursorPos + ImVec2(fWidth, oLabelSize.y) + oStyle.FramePadding * 2.0f);
	const ImRect oTotalRect(oFrameRect.Min, oFrameRect.Max + ImVec2(oLabelSize.x > 0.0f ? oStyle.ItemInnerSpacing.x + oLabelSize.x : 0.0f, 0.0f));

	ImGui::ItemSize(oTotalRect, oStyle.FramePadding.y);
	if (!ImGui::ItemAdd(oTotalRect, &oId))
		return false;

	const float fArrowSize = (oState.FontSize + oStyle.FramePadding.x * 2.0f);
	const bool bHovered = ImGui::IsHovered(oFrameRect, oId);

	const ImRect oValueRect(oFrameRect.Min, oFrameRect.Max - ImVec2(fArrowSize, 0.0f));
	ImGui::RenderFrame(oFrameRect.Min, oFrameRect.Max, pWindow->Color(ImGuiCol_FrameBg), true, oStyle.FrameRounding);
	ImGui::RenderFrame(ImVec2(oFrameRect.Max.x-fArrowSize, oFrameRect.Min.y), oFrameRect.Max, pWindow->Color(bHovered ? ImGuiCol_ButtonHovered : ImGuiCol_Button), true, oStyle.FrameRounding); // FIXME-ROUNDING
	ImGui::RenderCollapseTriangle(ImVec2(oFrameRect.Max.x-fArrowSize, oFrameRect.Min.y) + oStyle.FramePadding, true);

	ImGui::RenderTextClipped(oFrameRect.Min + oStyle.FramePadding, oValueRect.Max, pValue, NULL, NULL);

	if (oLabelSize.x > 0)
		ImGui::RenderText(ImVec2(oFrameRect.Max.x + oStyle.ItemInnerSpacing.x, oFrameRect.Min.y + oStyle.FramePadding.y), pLabel);

	bool bJustOpened = false;
	if (bHovered)
	{
		ImGui::SetHoveredID(oId);
		if (oState.IO.MouseClicked[0])
		{
			ImGui::SetActiveID(0, pWindow);

			ImGui::OpenPopup(pLabel);
			ImGui::SetNextWindowPos(ImVec2(oTotalRect.Min.x, oTotalRect.Max.y));
			bJustOpened = true;
		}
	}

	//TODO: Use BeginPopupEx but in next version of ImGui
	return ImGui::BeginPopup(pLabel) ? bJustOpened ? 2 : 1 : 0;
}

void EndCombo()
{
	ImGui::EndPopup();
}

ImGuiDynamicCombo.h

/* Exemple

if (BeginCombo("My combobox", "Value"))
{
  ImGui::MenuItem("Value");
  ImGui::MenuItem("Value2");
  ImGui::MenuItem("Value3");
  EndCombo();
}

*/

static int		BeginCombo(const char* pLabel, const char* pValue);
static void		EndCombo();