fixed particle emitter forward playback (#745)

Signed-off-by: Jenn Nguyen <jenn@openrobotics.org>

src/systems/log/LogPlayback.cc

@@ -44,6 +44,7 @@
 #include "ignition/gazebo/components/Geometry.hh"
 #include "ignition/gazebo/components/LogPlaybackStatistics.hh"
 #include "ignition/gazebo/components/Material.hh"
+#include "ignition/gazebo/components/ParticleEmitter.hh"
 #include "ignition/gazebo/components/Pose.hh"
 #include "ignition/gazebo/components/World.hh"
 
@@ -124,6 +125,9 @@ class ignition::gazebo::systems::LogPlaybackPrivate
   /// \brief Saves which entity poses have changed according to the latest
   /// LogPlaybackPrivate::Parse call.
   public: std::unordered_map<Entity, msgs::Pose> recentEntityPoseUpdates;
+
+  // \brief Saves which particle emitter emitting components have changed
+  public: std::unordered_map<Entity, bool> prevParticleEmitterCmds;
 };
 
 bool LogPlaybackPrivate::started{false};
@@ -636,6 +640,31 @@ void LogPlayback::Update(const UpdateInfo &_info, EntityComponentManager &_ecm)
     return true;
   });
 
+    // particle emitters
+  _ecm.Each<components::ParticleEmitterCmd>(
+      [&](const Entity &_entity,
+          const components::ParticleEmitterCmd *_emitter) -> bool
+  {
+    if (this->dataPtr->prevParticleEmitterCmds.find(_entity) ==
+        this->dataPtr->prevParticleEmitterCmds.end())
+    {
+      this->dataPtr->prevParticleEmitterCmds[_entity]
+          = _emitter->Data().emitting().data();
+      return true;
+    }
+
+    if (this->dataPtr->prevParticleEmitterCmds[_entity] !=
+        _emitter->Data().emitting().data())
+    {
+      this->dataPtr->prevParticleEmitterCmds[_entity]
+          = _emitter->Data().emitting().data();
+      _ecm.SetChanged(_entity, components::ParticleEmitterCmd::typeId,
+          ComponentState::OneTimeChange);
+    }
+
+    return true;
+  });
+
   // for seek back in time only
   // remove entities that should not be present in the current time step
   for (auto entity : entitiesToRemove)