ImguiDirectoryTreeView.cpp

#include <filesystem>
#include <imgui.h>

#define BIT(x) (1 << x)

std::pair<bool, uint32_t> DirectoryTreeViewRecursive(const std::filesystem::path& path, uint32_t* count, int* selection_mask)
{
	ImGuiTreeNodeFlags base_flags = ImGuiTreeNodeFlags_OpenOnArrow | ImGuiTreeNodeFlags_OpenOnDoubleClick | ImGuiTreeNodeFlags_SpanAvailWidth | ImGuiTreeNodeFlags_SpanFullWidth;

	bool any_node_clicked = false;
	uint32_t node_clicked = 0;

	for (const auto& entry : std::filesystem::directory_iterator(path))
	{
		ImGuiTreeNodeFlags node_flags = base_flags;
		const bool is_selected = (*selection_mask & BIT(*count)) != 0;
		if (is_selected)
			node_flags |= ImGuiTreeNodeFlags_Selected;

		std::string name = entry.path().string();

		auto lastSlash = name.find_last_of("/\\");
		lastSlash = lastSlash == String::npos ? 0 : lastSlash + 1;
		name = name.substr(lastSlash, name.size() - lastSlash);

		bool entryIsFile = !std::filesystem::is_directory(entry.path());
		if (entryIsFile)
			node_flags |= ImGuiTreeNodeFlags_Leaf | ImGuiTreeNodeFlags_NoTreePushOnOpen;

		bool node_open = ImGui::TreeNodeEx((void*)(intptr_t)(*count), node_flags, name.c_str());

		if (ImGui::IsItemClicked())
		{
			node_clicked = *count;
			any_node_clicked = true;
		}

		(*count)--;

		if (!entryIsFile)
		{
			if (node_open)
			{

				auto clickState = DirectoryTreeViewRecursive(entry.path(), count, selection_mask);

				if (!any_node_clicked)
				{
					any_node_clicked = clickState.first;
					node_clicked = clickState.second;
				}

				ImGui::TreePop();
			}
			else
			{
				for (const auto& e : std::filesystem::recursive_directory_iterator(entry.path()))
					(*count)--;
			}
		}
	}

	return { any_node_clicked, node_clicked };
}

void OnImGui(std::string directoryPath)
{
	ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2{ 0.0f, 0.0f });

	ImGui::Begin("Assets");

	if (ImGui::CollapsingHeader("Assets"))
	{	
		uint32_t count = 0;
		for (const auto& entry : std::filesystem::recursive_directory_iterator(directoryPath))
			count++;

		static int selection_mask = 0;

		auto clickState = DirectoryTreeViewRecursive(directoryPath, &count, &selection_mask);

		if (clickState.first)
		{
			// Update selection state
			// (process outside of tree loop to avoid visual inconsistencies during the clicking frame)
			if (ImGui::GetIO().KeyCtrl)
				selection_mask ^= BIT(clickState.second);          // CTRL+click to toggle
			else //if (!(selection_mask & (1 << clickState.second))) // Depending on selection behavior you want, may want to preserve selection when clicking on item that is part of the selection
				selection_mask = BIT(clickState.second);           // Click to single-select
		}
	}
	ImGui::End();

	ImGui::PopStyleVar();
}

I think a good optimization trick from what you said about this with performance, is that you could cache what files and folders are showing, than refresh the cache everytime you maximize a folder / edit a file.

Edit*: When I get around to attempting to add this into my project, I can try to add a caching system and than hopefully share how I did it.