using namespace ROOT; // RDataFrame lives in here

ROOT::EnableImplicitMT();
const auto poolSize = ROOT::GetThreadPoolSize();
const auto nSlots = 0 == poolSize ? 1 : poolSize;

const auto nEvents = nSlots * 10000ull;
RDataFrame d(nEvents);

TRandom r;
auto heavyWork = [&r]() {
   for (volatile int i = 0; i < 1000000; ++i)
      ;
   return r.Gaus();
};

auto df = d.Define("x", heavyWork);

auto h = df.Histo1D<double>({"browserHisto", "", 100, -2., 2.}, "x");

auto dfDirectory = new TMemFile("RDFResults", "RECREATE");
auto browser = new TBrowser("b", dfDirectory);

auto browserPad = gPad;

h.OnPartialResult(h.kOnce, [dfDirectory](TH1D &h_) { dfDirectory->Add(&h_); });

h.OnPartialResult(50, [&browserPad](TH1D &hist) {
   if (!browserPad)
      return; // in case root -b was invoked
   browserPad->cd();
   hist.Draw();
   browserPad->Update();
   // This call tells ROOT to process all pending GUI events
   // It allows users to use the TBrowser as usual while the event-loop is running
   gSystem->ProcessEvents();
});

std::string progressBar;
std::mutex barMutex; // Only one thread at a time can lock a mutex. Let's use this to avoid concurrent printing.

const auto everyN = nSlots == 8 ? 1000 : 100ull * nSlots;
const auto barWidth = nEvents / everyN;
h.OnPartialResultSlot(everyN, [&barWidth, &progressBar, &barMutex](unsigned int /*slot*/, TH1D & /*partialHist*/) {
   std::lock_guard<std::mutex> l(barMutex); // lock_guard locks the mutex at construction, releases it at destruction
   progressBar.push_back('#');
   // re-print the line with the progress bar
   std::cout << "\r[" << std::left << std::setw(barWidth) << progressBar << ']' << std::flush;
});

std::cout << "Analysis running..." << std::endl;
h->Draw(); // the final, complete result will be drawn after the event-loop has completed.
std::cout << "\nDone!" << std::endl;

dfDirectory->Clear();
auto clone = static_cast<TH1D *>(h->Clone());
clone->SetDirectory(nullptr);
dfDirectory->Add(clone);
if (!browserPad)
   return; // in case root -b was invoked
browserPad->cd();
clone->Draw();
browserPad->Update();

gROOT->GetListOfCanvases()->Draw()