Add GravityEnabled boolean component (#2451)

This adds a boolean GravityEnabled component to the existing gz/sim/components/Gravity.hh header file, which should be used with Link entities instead of the Vector3 gravity component.

---------

Signed-off-by: Steve Peters <scpeters@openrobotics.org>
Signed-off-by: youhy <haoyuan2019@outlook.com>
Co-authored-by: youhy <haoyuan2019@outlook.com>

include/gz/sim/components/Gravity.hh

@@ -36,6 +36,11 @@ namespace components
   /// \brief Store the gravity acceleration.
   using Gravity = Component<math::Vector3d, class GravityTag>;
   IGN_GAZEBO_REGISTER_COMPONENT("ign_gazebo_components.Gravity", Gravity)
+
+  /// \brief Store the gravity acceleration.
+  using GravityEnabled = Component<bool, class GravityEnabledTag>;
+  IGN_GAZEBO_REGISTER_COMPONENT(
+      "ign_gazebo_components.GravityEnabled", GravityEnabled)
 }
 }
 }

src/SdfEntityCreator.cc

@@ -590,10 +590,9 @@ Entity SdfEntityCreator::CreateEntities(const sdf::Link *_link)
 
   if (!_link->EnableGravity())
   {
-    // If disable gravity, create a gravity component to the entity
-    // This gravity will have value 0,0,0
+    // If disable gravity, create a GravityEnabled component to the entity
     this->dataPtr->ecm->CreateComponent(
-        linkEntity, components::Gravity());
+        linkEntity, components::GravityEnabled(false));
   }
 
   // Visuals

src/systems/physics/Physics.cc

@@ -1161,21 +1161,12 @@ void PhysicsPrivate::CreateLinkEntities(const EntityComponentManager &_ecm)
         }
 
         // 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);
-          }
+        const components::GravityEnabled *gravityEnabled =
+            _ecm.Component<components::GravityEnabled>(_entity);
+        if (nullptr != gravityEnabled)
+        {
+          // gravityEnabled set in SdfEntityCreator::CreateEntities()
+          link.SetEnableGravity(gravityEnabled->Data());
         }
 
         auto linkPtrPhys = modelPtrPhys->ConstructLink(link);