Adds support for ISystemReset in test fixture (#2647)

* Adds support for Reset in test fixture

This PR adds support for the Reset API to the test fixture. As `TestFixture`
is one of the main ways one can get access to the ECM in python
when trying to write some scripts for Deep Reinforcement Learning I
realized that without `Reset` supported in the `TestFixture` API, end
users would have a very hard time using our python APIs (which are
actually quite nice). For reference I'm hacking a demo template here:

#2667
---------

Signed-off-by: Arjo Chakravarty <[email protected]>

include/gz/sim/TestFixture.hh

@@ -96,6 +96,12 @@ class GZ_SIM_VISIBLE TestFixture
   public: TestFixture &OnPostUpdate(std::function<void(
       const UpdateInfo &, const EntityComponentManager &)> _cb);
 
+  /// \brief Wrapper around a system's update callback
+  /// \param[in] _cb Function to be called every reset
+  /// \return Reference to self.
+  public: TestFixture &OnReset(std::function<void(
+      const UpdateInfo &, EntityComponentManager &)> _cb);
+
   /// \brief Finalize all the functions and add fixture to server.
   /// Finalize must be called before running the server, otherwise none of the
   /// `On*` functions will be called.

python/src/gz/sim/TestFixture.cc

@@ -83,6 +83,17 @@ defineSimTestFixture(pybind11::object module)
     ),
     pybind11::return_value_policy::reference,
     "Wrapper around a system's post-update callback"
+  )
+  .def(
+    "on_reset", WrapCallbacks(
+      [](TestFixture* self, std::function<void(
+          const UpdateInfo &, EntityComponentManager &)> _cb)
+      {
+        self->OnReset(_cb);
+      }
+    ),
+    pybind11::return_value_policy::reference,
+    "Wrapper around a system's reset callback"
   );
   // TODO(ahcorde): This method is not compiling for the following reason:
   // The EventManager class has an unordered_map which holds a unique_ptr

src/TestFixture.cc

@@ -29,7 +29,8 @@ class HelperSystem :
   public ISystemConfigure,
   public ISystemPreUpdate,
   public ISystemUpdate,
-  public ISystemPostUpdate
+  public ISystemPostUpdate,
+  public ISystemReset
 {
   // Documentation inherited
   public: void Configure(
@@ -50,6 +51,10 @@ class HelperSystem :
   public: void PostUpdate(const UpdateInfo &_info,
                 const EntityComponentManager &_ecm) override;
 
+  // Documentation inherited
+  public: void Reset(const UpdateInfo &_info,
+                            EntityComponentManager &_ecm) override;
+
   /// \brief Function to call every time  we configure a world
   public: std::function<void(const Entity &_entity,
                 const std::shared_ptr<const sdf::Element> &_sdf,
@@ -68,6 +73,10 @@ class HelperSystem :
   /// \brief Function to call every post-update
   public: std::function<void(const UpdateInfo &,
       const EntityComponentManager &)> postUpdateCallback;
+
+  /// \brief Reset callback
+  public: std::function<void(const UpdateInfo &,
+      EntityComponentManager &)> resetCallback;
 };
 
 /////////////////////////////////////////////////
@@ -105,6 +114,14 @@ void HelperSystem::PostUpdate(const UpdateInfo &_info,
     this->postUpdateCallback(_info, _ecm);
 }
 
+/////////////////////////////////////////////////
+void HelperSystem::Reset(const UpdateInfo &_info,
+    EntityComponentManager &_ecm)
+{
+  if (this->resetCallback)
+    this->resetCallback(_info, _ecm);
+}
+
 //////////////////////////////////////////////////
 class gz::sim::TestFixture::Implementation
 {
@@ -200,6 +217,15 @@ TestFixture &TestFixture::OnPostUpdate(std::function<void(
   return *this;
 }
 
+//////////////////////////////////////////////////
+TestFixture &TestFixture::OnReset(std::function<void(
+          const UpdateInfo &, EntityComponentManager &)> _cb)
+{
+  if (nullptr != this->dataPtr->helperSystem)
+    this->dataPtr->helperSystem->resetCallback = std::move(_cb);
+  return *this;
+}
+
 //////////////////////////////////////////////////
 std::shared_ptr<Server> TestFixture::Server() const
 {