Add a per-model execution-enable property

Each scheduled model gains a simulation/models/<name>/enabled property (default true), bound by FGFDMExec using a stable canonical name per eModels. When the property is set false, FGModel::Run() skips the model's body while its last state and property bindings are preserved. This is a property-tree mechanism by which an external system can replace a model, for example an external propagator owning FGPropagate, or a host physics engine owning ground reactions, without removing it from the schedule.

Behaviour is unchanged unless a model is explicitly disabled. The canonical names are deliberately independent of FGModel::Name, since some models rename themselves when their config is loaded. A static_assert keeps the name table in step with the eModels enum.

Author: Zaretto

src/FGFDMExec.cpp

@@ -162,6 +162,23 @@ FGFDMExec::FGFDMExec(FGPropertyManager* root, std::shared_ptr<unsigned int> fdmc
   instance->Tie("simulation/trim-completed", &trim_completed);
   instance->Tie("forces/hold-down", this, &FGFDMExec::GetHoldDown, &FGFDMExec::SetHoldDown);
 
+  // Per-model execution-enable property: simulation/models/<name>/enabled
+  // (default true). Setting one false skips that model's Run() while preserving
+  // its state. This is the property-tree mechanism by which an external system
+  // can replace a model, for example external propagation or host-owned ground
+  // reactions. Names use the stable canonical eModels order, independent of
+  // FGModel::Name.
+  static const char* const modelEnableNames[] = {
+    "propagate", "input", "inertial", "atmosphere", "winds", "systems",
+    "massbalance", "auxiliary", "propulsion", "aerodynamics", "groundreactions",
+    "externalreactions", "buoyantforces", "aircraft", "accelerations", "output"
+  };
+  static_assert(sizeof(modelEnableNames)/sizeof(*modelEnableNames) == eNumStandardModels,
+                "modelEnableNames must stay in step with eModels");
+  for (unsigned int i = 0; i < Models.size(); ++i)
+    if (Models[i])
+      Models[i]->BindModelEnabled(modelEnableNames[i]);
+
   Constructing = false;
 }
 

src/models/FGModel.cpp

@@ -42,6 +42,7 @@ INCLUDES
 #include "FGFDMExec.h"
 #include "input_output/FGModelLoader.h"
 #include "input_output/FGLog.h"
+#include "input_output/FGPropertyManager.h"
 
 using namespace std;
 
@@ -87,10 +88,27 @@ bool FGModel::InitModel(void)
 
 //%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
+void FGModel::BindModelEnabled(const std::string& name)
+{
+  const string path = "simulation/models/" + name + "/enabled";
+  const bool existed = PropertyManager->HasNode(path);
+  ModelEnabled = PropertyManager->GetNode(path, true);
+  // Default to enabled; a freshly created node would otherwise read false.
+  if (!existed && ModelEnabled) ModelEnabled->setBoolValue(true);
+}
+
+//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
 bool FGModel::Run(bool Holding)
 {
   FGModel::Debug(2);
 
+  // A model whose simulation/models/<name>/enabled property is false is skipped
+  // entirely: its Run() body does not execute, but its last state and property
+  // bindings are preserved so an external system can replace it, for example
+  // external propagation or host-owned ground reactions.
+  if (ModelEnabled && !ModelEnabled->getBoolValue()) return true;
+
   if (rate == 1) return false; // Fast exit if nothing to do
 
   if (exe_ctr >= rate) exe_ctr = 0;

src/models/FGModel.h

@@ -48,6 +48,8 @@ INCLUDES
 FORWARD DECLARATIONS
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
 
+class SGPropertyNode;
+
 namespace JSBSim {
 
 class FGFDMExec;
@@ -93,6 +95,16 @@ class JSBSIM_API FGModel : public FGModelFunctions
   unsigned int GetRate(void) const { return rate; }
   FGFDMExec* GetExec(void) const { return FDMExec; }
 
+  /** Bind this model's execution-enable flag to the property
+      simulation/models/<name>/enabled (default true), using the stable
+      canonical name supplied by FGFDMExec. When that property is set false,
+      Run() is skipped while the model's last state and property bindings are
+      preserved. This is the property-tree mechanism by which an external system
+      can replace a model, for example external propagation owning FGPropagate,
+      or a host physics engine owning FGGroundReactions. Behaviour is unchanged
+      unless the property is explicitly set false. */
+  void BindModelEnabled(const std::string& name);
+
   void SetPropertyManager(std::shared_ptr<FGPropertyManager> fgpm) { PropertyManager=fgpm;}
   virtual SGPath FindFullPathName(const SGPath& path) const;
   const std::string& GetName(void) const { return Name; }
@@ -101,6 +113,7 @@ class JSBSIM_API FGModel : public FGModelFunctions
 protected:
   unsigned int exe_ctr;
   unsigned int rate;
+  SGPropertyNode* ModelEnabled = nullptr;
   std::string Name;
 
   /** Uploads this model in memory.