Add support for no gravity link (#2398)

Signed-off-by: youhy <haoyuan2019@outlook.com>
Signed-off-by: Ian Chen <ichen@openrobotics.org>
Co-authored-by: Ian Chen <ichen@openrobotics.org>

src/SdfEntityCreator.cc

@@ -588,6 +588,14 @@ Entity SdfEntityCreator::CreateEntities(const sdf::Link *_link)
         linkEntity, components::WindMode(_link->EnableWind()));
   }
 
+  if (!_link->EnableGravity())
+  {
+    // If disable gravity, create a gravity component to the entity
+    // This gravity will have value 0,0,0
+    this->dataPtr->ecm->CreateComponent(
+        linkEntity, components::Gravity());
+  }
+
   // Visuals
   for (uint64_t visualIndex = 0; visualIndex < _link->VisualCount();
       ++visualIndex)

src/systems/physics/Physics.cc

@@ -1160,6 +1160,24 @@ void PhysicsPrivate::CreateLinkEntities(const EntityComponentManager &_ecm)
           link.SetInertial(inertial->Data());
         }
 
+        // get link gravity
+        const components::Gravity *gravity =
+            _ecm.Component<components::Gravity>(_entity);
+        if (nullptr != gravity)
+        {
+          // Entity has a gravity component that is all zeros when
+          // <gravity> is set to false
+          // See SdfEntityCreator::CreateEntities()
+          if (gravity->Data() == math::Vector3d::Zero)
+          {
+            link.SetEnableGravity(false);
+          }
+          else
+          {
+            link.SetEnableGravity(true);
+          }
+        }
+
         auto linkPtrPhys = modelPtrPhys->ConstructLink(link);
         this->entityLinkMap.AddEntity(_entity, linkPtrPhys);
         this->topLevelModelMap.insert(std::make_pair(_entity,