From 663db3b77e89b22be05dc9211789118094b90a3d Mon Sep 17 00:00:00 2001
From: Guillaume Giudicelli <guillaume.giudicelli@inl.gov>
Date: Wed, 27 Nov 2024 10:48:16 -0700
Subject: [PATCH] Add support for multi-NL and multi-system in Transient
 executioner refs #29158

---
 .../executioners/MultiSystemSolveObject.h       |  2 +-
 framework/src/executioners/Transient.C          | 17 +++++++++++++----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/framework/include/executioners/MultiSystemSolveObject.h b/framework/include/executioners/MultiSystemSolveObject.h
index 6118b034c448..d530d00ff5e8 100644
--- a/framework/include/executioners/MultiSystemSolveObject.h
+++ b/framework/include/executioners/MultiSystemSolveObject.h
@@ -27,7 +27,7 @@ class MultiSystemSolveObject : public SolveObject
    * Returns a reference to the vector of solver systems that this object is
    * supposed to solve
    */
-  const std::vector<SolverSystem *> & systemsToSolve() { return _systems; }
+  const std::vector<SolverSystem *> & systemsToSolve() const { return _systems; }
 
 protected:
   /// Vector of pointers to the systems
diff --git a/framework/src/executioners/Transient.C b/framework/src/executioners/Transient.C
index f8c4db327883..9c280b67bbae 100644
--- a/framework/src/executioners/Transient.C
+++ b/framework/src/executioners/Transient.C
@@ -37,9 +37,18 @@ Real
 Transient::relativeSolutionDifferenceNorm()
 {
   // TODO: add multi-system support
-  const NumericVector<Number> & current_solution =
-      _check_aux ? _aux.solution() : *_nl.currentSolution();
-  const NumericVector<Number> & old_solution = _check_aux ? _aux.solutionOld() : _nl.solutionOld();
+  if (_check_aux)
+    return _aux.solution().l2_norm_diff(_aux.solutionOld()) / _aux.solution().l2_norm();
+}
 
-  return current_solution.l2_norm_diff(old_solution) / current_solution.l2_norm();
+std::vector<TimeIntegrator *>
+Transient::getTimeIntegrators() const
+{
+  std::vector<TimeIntegrator *> tis;
+  tis.reserve(_feproblem_solve.systemsToSolve().size());
+  // Get all time integrators from the systems in the FEProblemSolve
+  for (const auto sys : _feproblem_solve.systemsToSolve())
+    for (const auto & ti : sys->getTimeIntegrators())
+      tis.push_back(ti.get());
+  return tis;
 }