[TDF] Add google tests for the callback feature

tree/treeplayer/test/CMakeLists.txt

@@ -14,5 +14,6 @@ ROOT_ADD_GTEST(dataframe_snapshot dataframe/dataframe_snapshot.cxx LIBRARIES Tre
 ROOT_ADD_GTEST(datasource_trivial dataframe/datasource_trivial.cxx LIBRARIES TreePlayer)
 ROOT_ADD_GTEST(datasource_root dataframe/datasource_root.cxx LIBRARIES TreePlayer)
 ROOT_ADD_GTEST(datasource_noncopiable dataframe/datasource_noncopiable.cxx LIBRARIES TreePlayer)
+ROOT_ADD_GTEST(dataframe_callbacks dataframe/dataframe_callbacks.cxx LIBRARIES TreePlayer)
 
 ROOT_ADD_PYUNITTEST(dataframe_histograms dataframe/dataframe_histograms.py)

tree/treeplayer/test/dataframe/dataframe_callbacks.cxx

+#include "ROOT/TDataFrame.hxx"
+#include "TRandom.h"
+#include "gtest/gtest.h"
+using namespace ROOT::Experimental;
+using namespace ROOT::Experimental::TDF;
+using namespace ROOT::Detail::TDF;
+
+// fixture that provides a TDF with no data-source and a single column "x" containing normal-distributed doubles
+class TDFCallbacks : public ::testing::Test {
+protected:
+   const ULong64_t nEvents = 8ull; // must be initialized before fLoopManager
+
+private:
+   TDataFrame fLoopManager;
+   TInterface<TLoopManager> DefineRandomCol()
+   {
+      TRandom r;
+      return fLoopManager.Define("x", [r]() mutable { return r.Gaus(); });
+   }
+
+protected:
+   TDFCallbacks() : fLoopManager(nEvents), tdf(DefineRandomCol()) {}
+   TInterface<TLoopManager> tdf;
+};
+
+TEST_F(TDFCallbacks, Histo1DWithFillTOHelper)
+{
+   auto h = tdf.Histo1D<double>({"", "", 128, -2., 2.}, "x");
+   using value_t = typename decltype(h)::Value_t;
+   ULong64_t i = 0ull;
+   ULong64_t everyN = 1ull;
+   h.RegisterCallback(everyN, [&i, &everyN](value_t &h_) {
+      i += everyN;
+      EXPECT_EQ(h_.GetEntries(), everyN * i);
+   });
+   *h;
+}
+
+TEST_F(TDFCallbacks, MultipleCallbacks)
+{
+   auto h = tdf.Histo1D<double>({"", "", 128, -2., 2.}, "x");
+   using value_t = typename decltype(h)::Value_t;
+   ULong64_t everyN = 1ull;
+   ULong64_t i = 0ull;
+   h.RegisterCallback(everyN, [&i, everyN](value_t &h_) {
+      i += everyN;
+      EXPECT_EQ(h_.GetEntries(), i);
+   });
+
+   everyN = 2ull;
+   ULong64_t i2 = 0ull;
+   h.RegisterCallback(everyN, [&i2, everyN](value_t &h_) {
+      i2 += everyN;
+      EXPECT_EQ(h_.GetEntries(), i2);
+   });
+
+   *h;
+}
+
+TEST_F(TDFCallbacks, MultipleEventLoops)
+{
+   auto h = tdf.Histo1D<double>({"", "", 128, -2., 2.}, "x");
+   using value_t = typename decltype(h)::Value_t;
+   ULong64_t i = 0ull;
+   h.RegisterCallback(1ull, [&i](value_t &) { ++i; });
+   *h;
+
+   auto h2 = tdf.Histo1D<double>({"", "", 128, -2., 2.}, "x");
+   *h2;
+
+   EXPECT_EQ(i, nEvents);
+}