Add render event (#70)

Signed-off-by: Sarathkrishnan Ramesh <sarathkrishnan99@gmail.com>
Co-authored-by: Louise Poubel <louise@openrobotics.org>

include/ignition/gui/GuiEvents.hh

@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2020 Open Source Robotics Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+*/
+#ifndef IGNITION_GUI_GUIEVENTS_HH_
+#define IGNITION_GUI_GUIEVENTS_HH_
+
+#include <QEvent>
+#include <utility>
+#include <vector>
+#include <ignition/math/Vector3.hh>
+
+namespace ignition
+{
+  namespace gui
+  {
+    /// \brief Namespace for all events.
+    namespace events
+    {
+      /// User defined events should start from QEvent::MaxUser and
+      /// count down to avoid collision with ign-gazebo events
+
+      /// \brief Event called in the render thread of a 3D scene.
+      /// It's safe to make rendering calls in this event's callback.
+      class Render : public QEvent
+      {
+        public: Render()
+            : QEvent(kType)
+        {
+        }
+        /// \brief Unique type for this event.
+        static const QEvent::Type kType = QEvent::Type(QEvent::MaxUser);
+      };
+    }
+  }
+}
+
+#endif  // IGNITION_GUI_GUIEVENTS_HH_

src/plugins/scene3d/Scene3D.cc

@@ -49,7 +49,11 @@
 
 #include <ignition/transport/Node.hh>
 
+#include "ignition/gui/Application.hh"
 #include "ignition/gui/Conversions.hh"
+#include "ignition/gui/GuiEvents.hh"
+#include "ignition/gui/MainWindow.hh"
+
 #include "Scene3D.hh"
 
 namespace ignition
@@ -846,6 +850,13 @@ void IgnRenderer::Render()
 
   // update and render to texture
   this->dataPtr->camera->Update();
+
+  if (ignition::gui::App())
+  {
+    ignition::gui::App()->sendEvent(
+        ignition::gui::App()->findChild<ignition::gui::MainWindow *>(),
+        new gui::events::Render());
+  }
 }
 
 /////////////////////////////////////////////////