Add support for no gravity link

include/sdf/Link.hh

@@ -321,12 +321,24 @@ namespace sdf
     /// \sa bool Model::EnableWind
     public: bool EnableWind() const;
 
+    /// \brief Check if this link should be subject to gravity.
+    /// If true, this link should be affected by gravity.
+    /// \return true if the model should be subject to gravity, false otherwise.
+    /// \sa bool Model::EnableGravity
+    public: bool EnableGravity() const;
+
     /// \brief Set whether this link should be subject to wind.
     /// \param[in] _enableWind True or false depending on whether the link
     /// should be subject to wind.
     /// \sa Model::SetEnableWind(bool)
     public: void SetEnableWind(bool _enableWind);
 
+    /// \brief Set whether this link should be subject to gravity.
+    /// \param[in] _enableGravity True or false depending on whether the link
+    /// should be subject to gravity.
+    /// \sa Model::SetEnableGravity(bool)
+    public: void SetEnableGravity(bool _enableGravity);
+
     /// \brief Add a collision to the link.
     /// \param[in] _collision Collision to add.
     /// \return True if successful, false if a collision with the name already

src/Link.cc

@@ -74,6 +74,9 @@ class sdf::Link::Implementation
   /// \brief True if this link should be subject to wind, false otherwise.
   public: bool enableWind = false;
 
+  /// \brief True if this link should be subject to gravity, false otherwise.
+  public: bool enableGravity = true;
+
   /// \brief Scoped Pose Relative-To graph at the parent model scope.
   public: sdf::ScopedGraph<sdf::PoseRelativeToGraph> poseRelativeToGraph;
 };
@@ -189,6 +192,9 @@ Errors Link::Load(ElementPtr _sdf)
   this->dataPtr->enableWind = _sdf->Get<bool>("enable_wind",
       this->dataPtr->enableWind).first;
 
+  this->dataPtr->enableGravity = _sdf->Get<bool>("gravity",
+      this->dataPtr->enableGravity).first;
+
   return errors;
 }
 
@@ -578,12 +584,24 @@ bool Link::EnableWind() const
   return this->dataPtr->enableWind;
 }
 
+/////////////////////////////////////////////////
+bool Link::EnableGravity() const
+{
+  return this->dataPtr->enableGravity;
+}
+
 /////////////////////////////////////////////////
 void Link::SetEnableWind(const bool _enableWind)
 {
   this->dataPtr->enableWind = _enableWind;
 }
 
+/////////////////////////////////////////////////
+void Link::SetEnableGravity(const bool _enableGravity)
+{
+  this->dataPtr->enableGravity = _enableGravity;
+}
+
 //////////////////////////////////////////////////
 bool Link::AddCollision(const Collision &_collision)
 {

src/Link_TEST.cc

@@ -60,6 +60,10 @@ TEST(DOMLink, Construction)
   link.SetEnableWind(true);
   EXPECT_TRUE(link.EnableWind());
 
+  EXPECT_TRUE(link.EnableGravity());
+  link.SetEnableGravity(false);
+  EXPECT_FALSE(link.EnableGravity());
+
   EXPECT_EQ(0u, link.SensorCount());
   EXPECT_EQ(nullptr, link.SensorByIndex(0));
   EXPECT_EQ(nullptr, link.SensorByIndex(1));