Add null checks when loading example render engine

examples/hello_world_plugin/HelloWorldPlugin.cc

@@ -38,40 +38,55 @@ namespace mock
     // Documentation Inherited.
     public: virtual std::string Name() const override
     {
-      return "test2";
+      return "HelloWorldPlugin";
     }
 
+    // Documentation Inherited.
     protected: virtual bool LoadImpl(const std::map<std::string, std::string> &_params) override
     {
       return true;
     }
 
+    /// \brief Initialize the render engine.
+    /// \return True if the operation is successful
     protected: virtual bool InitImpl() override
     {
       return true;
     }
 
+    /// \brief Get a pointer to the list of scenes managed by the render
+    /// engine.
+    /// \return list of scenes
     protected: virtual ignition::rendering::SceneStorePtr Scenes() const override
     {
       return nullptr;
     }
 
+    /// \brief Create a scene.
+    /// \param[in] _id Unique scene Id
+    /// \parampin] _name Name of scene
     protected: virtual ignition::rendering::ScenePtr CreateSceneImpl(unsigned int _id,
                 const std::string &_name) override
     {
       return nullptr;
     }
 
+    /// \brief Singelton setup.
     private: friend class ignition::common::SingletonT<HelloWorldRenderEngine>;
   };
 
   class HelloWorldPlugin :
     public ignition::rendering::RenderEnginePlugin
   {
+    /// \brief Get the name of the render engine loaded by this plugin.
+    /// \return Name of render engine
     public: std::string Name() const
     {
-      return "test";
+      return HelloWorldRenderEngine::Instance()->Name();
     }
+
+    /// \brief Get a pointer to the render engine loaded by this plugin.
+    /// \return Render engine instance
     public: ignition::rendering::RenderEngine *Engine() const
     {
       return HelloWorldRenderEngine::Instance();

examples/hello_world_plugin/README.md

@@ -19,3 +19,25 @@ This will generate the `HelloWorldPlugin` library under `build`.
 The exact name of the library file depends on the operating system
 such as `libHelloWorldPlugin.so` on Linux, `libHelloWorldPlugin.dylib` on macOS,
 and `HelloWorldPlugin.dll` on Windows.
+
+## Run
+
+Be sure to have the `IGN_GAZEBO_RENDER_ENGINE_PATH` environment variable set to the path
+where your plugin is located.  From within the `build` directory of this example, you can run
+
+~~~
+export IGN_GAZEBO_RENDER_ENGINE_PATH=$PWD
+~~~
+
+to set the environment variable accordingly.
+
+
+Now you can run `ign gazebo` with the name of the resultant library file (without the `lib` prefix
+or the file extension, i.e., libHelloWorldPlugin.so -> HelloWorldPlugin):
+
+~~~
+ign gazebo --render-engine HelloWorldPlugin
+~~~
+
+You should see a blank screen within the Ignition GUI, as this mocked plugin provides no implementation
+for the scene.

src/base/BaseRenderEngine.cc

@@ -94,72 +94,108 @@ bool BaseRenderEngine::IsEnabled() const
 //////////////////////////////////////////////////
 unsigned int BaseRenderEngine::SceneCount() const
 {
-  return this->Scenes()->Size();
+  auto scenes = this->Scenes();
+  if (scenes)
+    return scenes->Size();
+  return 0u;
 }
 
 //////////////////////////////////////////////////
 bool BaseRenderEngine::HasScene(ConstScenePtr _scene) const
 {
-  return this->Scenes()->Contains(_scene);
+  auto scenes = this->Scenes();
+  if (scenes)
+    return scenes->Contains(_scene);
+  return false;
 }
 
 //////////////////////////////////////////////////
 bool BaseRenderEngine::HasSceneId(unsigned int _id) const
 {
-  return this->Scenes()->ContainsId(_id);
+  auto scenes = this->Scenes();
+  if (scenes)
+    return scenes->ContainsId(_id);
+  return false;
 }
 
 //////////////////////////////////////////////////
 bool BaseRenderEngine::HasSceneName(const std::string &_name) const
 {
-  return this->Scenes()->ContainsName(_name);
+  auto scenes = this->Scenes();
+  if (scenes)
+    return scenes->ContainsName(_name);
+  return false;
 }
 
 //////////////////////////////////////////////////
 ScenePtr BaseRenderEngine::SceneById(unsigned int _id) const
 {
-  return this->Scenes()->GetById(_id);
+  auto scenes = this->Scenes();
+  if (scenes)
+    return scenes->GetById(_id);
+  return ScenePtr();
 }
 
 //////////////////////////////////////////////////
 ScenePtr BaseRenderEngine::SceneByName(const std::string &_name) const
 {
-  return this->Scenes()->GetByName(_name);
+  auto scenes = this->Scenes();
+  if (scenes)
+    return scenes->GetByName(_name);
+  return ScenePtr();
 }
 
 //////////////////////////////////////////////////
 ScenePtr BaseRenderEngine::SceneByIndex(unsigned int _index) const
 {
-  return this->Scenes()->GetByIndex(_index);
+  auto scenes = this->Scenes();
+  if (scenes)
+    return scenes->GetByIndex(_index);
+  return ScenePtr();
 }
 
 //////////////////////////////////////////////////
 void BaseRenderEngine::DestroyScene(ScenePtr _scene)
 {
+  auto scenes = this->Scenes();
+  if (!scenes)
+    return;
   this->Scenes()->Destroy(_scene);
 }
 
 //////////////////////////////////////////////////
 void BaseRenderEngine::DestroySceneById(unsigned int _id)
 {
+  auto scenes = this->Scenes();
+  if (!scenes)
+    return;
   this->Scenes()->DestroyById(_id);
 }
 
 //////////////////////////////////////////////////
 void BaseRenderEngine::DestroySceneByName(const std::string &_name)
 {
+  auto scenes = this->Scenes();
+  if (!scenes)
+    return;
   this->Scenes()->DestroyByName(_name);
 }
 
 //////////////////////////////////////////////////
 void BaseRenderEngine::DestroySceneByIndex(unsigned int _index)
 {
+  auto scenes = this->Scenes();
+  if (!scenes)
+    return;
   this->Scenes()->DestroyByIndex(_index);
 }
 
 //////////////////////////////////////////////////
 void BaseRenderEngine::DestroyScenes()
 {
+  auto scenes = this->Scenes();
+  if (!scenes)
+    return;
   this->Scenes()->DestroyAll();
 }