Add tutorial on migrating the Joint class from gazebo classic

include/ignition/gazebo/Joint.hh

@@ -260,7 +260,7 @@ namespace ignition
       public: std::optional<std::vector<double>> Position(
           const EntityComponentManager &_ecm) const;
 
-      /// \brief Get the position of the joint
+      /// \brief Get the transmitted wrench of the joint
       /// \param[in] _ecm Entity-component manager.
       /// \return Transmitted wrench of the joint or nullopt if transmitted
       /// wrench check is not enabled.

tutorials/migration_joint_api.md

@@ -0,0 +1,192 @@
+\page migrationjointapi
+
+# Migration from Gazebo-classic: Joint API
+
+When migrating plugins from Gazebo-classic to Gazebo, developers will
+notice that the C++ APIs for both simulators are quite different. Be sure to
+check the [plugin migration tutorial](migrationplugins.html) to get a high-level
+view of the architecture differences before using this guide.
+
+This tutorial is meant to serve as a reference guide for developers migrating
+functions from the
+[gazebo::phyiscs::Joint](http://osrf-distributions.s3.amazonaws.com/gazebo/api/11.0.0/classgazebo_1_1physics_1_1Joint.html)
+class.
+
+If you're trying to use some API which doesn't have an equivalent on Gazebo
+yet, feel free to
+[ticket an issue](https://github.com/gazebosim/gz-sim/issues/).
+
+## Joint API
+
+Gazebo-classic's `gazebo::physics::Joint` provides lots of functionality, which
+can be divided in these categories:
+
+* **Properties**: Setting / getting properties
+    * Example: [Joint::Anchor](http://osrf-distributions.s3.amazonaws.com/gazebo/api/11.0.0/classgazebo_1_1physics_1_1Joint.html#a73e5d867a0cbe1dff76957425a58c9d0) / [Joint::SetAnchor](http://osrf-distributions.s3.amazonaws.com/gazebo/api/11.0.0/classgazebo_1_1physics_1_1Joint.html#a9698c1294d75ae76026842a35750afa5)
+* **Read family**: Getting children and parent
+    * Example: [Joint::GetChild](http://osrf-distributions.s3.amazonaws.com/gazebo/api/11.0.0/classgazebo_1_1physics_1_1Base.html#a3e8e42332400d79fbeb7f721350ab980)
+* **Write family**: Adding children, changing parent
+    * Example: [Joint::RemoveChildren](http://osrf-distributions.s3.amazonaws.com/gazebo/api/11.0.0/classgazebo_1_1physics_1_1Base.html#aa85d2386e6fb02bdbec060a74b63238a)
+* **Lifecycle**: Functions to control the joint's lifecycle
+    * Example: [Joint::Init](http://osrf-distributions.s3.amazonaws.com/gazebo/api/11.0.0/classgazebo_1_1physics_1_1Joint.html#ae048ef824aaf614707c1496a2aefd415)
+
+You'll find the Gazebo APIs below on the following headers:
+
+* [ignition/gazebo/Joint.hh](https://gazebosim.org/api/gazebo/6.15/Joint_8hh.html)
+* [ignition/gazebo/Util.hh](https://gazebosim.org/api/gazebo/6.14/Util_8hh.html)
+* [ignition/gazebo/SdfEntityCreator.hh](https://gazebosim.org/api/gazebo/6.14/SdfEntityCreator_8hh.html)
+* [ignition/gazebo/EntityComponentManager.hh](https://gazebosim.org/api/gazebo/6.14/classignition_1_1gazebo_1_1EntityComponentManager.html)
+
+It's worth remembering that most of this functionality can be performed using
+the
+[EntityComponentManager](https://gazebosim.org/api/gazebo/6.14/classignition_1_1gazebo_1_1EntityComponentManager.html)
+directly. The functions presented here exist for convenience and readability.
+
+### Properties
+
+Most of Gazebo-classic's Joint API is related to setting and getting
+properties. These functions are great candidates to have equivalents on
+Gazebo, because the Entity-Component-System architecture is perfect for setting
+components (properties) into entities such as joints.
+
+---
+
+Classic | Gazebo
+-- | --
+AddType | `ecm.CreateComponent<Type>(entity, Type())`
+Anchor | TODO
+AnchorErrorPose | TODO
+ApplyStiffnessDamping | TODO
+AreConnected | TODO
+AxisFrame | TODO
+AxisFrameOffset | TODO
+CacheForceTorque | TODO
+CheckAndTruncateForce | TODO
+DOF | TODO
+FillMsg | TODO
+GetDamping | TODO
+GetEffortLimit | TODO
+GetForce | `ignition::gazebo::Joint::TransmittedWrench`
+GetForceTorque |  `ignition::gazebo::Joint::TransmittedWrench`
+GetId | `ignition::gazebo::Joint::Entity`
+GetInertiaRatio | TODO
+GetJointLink | use `ignition::gazebo::Joint::*LinkName`
+GetMsgType | TODO
+GetName | `ignition::gazebo::Joint::Name`
+GetParam | TODO
+GetSaveable | Not supported
+GetScopedName | `ignition::gazebo::scopedName`
+GetSDF | TODO
+GetSDFDom | TODO
+GetSpringReferencePosition | TODO
+GetStiffness | TODO
+GetStopDissipation | TODO
+GetStopStiffness | TODO
+GetType | `ignition::gazebo::Joint::Type`
+GetVelocity | `ignition::gazebo::Joint::Velocity`
+GetVelocityLimit | TODO
+GetWorldEnergyPotentialSpring | TODO
+GlobalAxis | TODO
+HasType | `gazebo::components::Joint::typeId == entityTypeId(entity, ecm)`
+InertiaRatio | TODO
+InitialAnchorPose | TODO
+IsSelected | Selection is client-specific, not porting
+LinkForce | TODO
+LinkTorque | TODO
+LocalAxis | `ignition::gazebo::Joint::Axis`
+LowerLimit | TODO
+ParentWorldPose | TODO
+Position | `ignition::gazebo::Joint::Position`
+Print | TODO
+ResolveAxisXyz | TODO
+SDFPoseRelativeToParent | `ignition::gazebo::Joint::Pose`
+SDFSemanticPose | `ignition::gazebo::Joint::Pose`
+SetAnchor | TODO
+SetAxis | TODO
+SetDamping | TODO
+SetEffortLimit | `ignition::gazebo::Joint::SetEffortLimits`
+SetForce | `ignition::gazebo::Joint::SetForce`
+SetLowerLimit | `ignition::gazebo::Joint::SetPositionLimits`
+SetName | TODO
+SeParam | TODO
+SetPosition | `ignition::gazebo::Joint::ResetPosition`
+SetProvideFeedback | `ignition::gazebo::Joint::EnableTransmittedWrenchCheck`
+SetSaveable | Not supported
+SetSelected |  Selection is client-specific, not porting
+SetState | TODO
+SetStiffness | TODO
+SetStiffnessDamping | TODO
+SetStopDissipation | TODO
+SetStopStiffness | TODO
+SetUpperLimit | `ignition::gazebo::Joint::SetPositionLimits`
+SetVelocity | `ignition::gazebo::Joint::SetVelocity`
+SetVelocityLimit | `ignition::gazebo::Joint::SetVelocityLimits`
+TypeStr | `ignition::gazebo::Joint::Type`
+UpdateParameters | TODO
+UpperLimit | TODO
+URI | TODO
+WorldPose | TODO
+
+---
+
+## Read family
+
+These APIs deal with reading information related to child / parent
+relationships.
+
+The main difference in these APIs across Gazebo generations is that
+on classic, they deal with shared pointers to entities, while on Gazebo,
+they deal with entity IDs.
+
+---
+
+Classic | Gazebo
+-- | --
+GetByName | Use type-specific `ignition::gazebo::Joint::*ByName`
+GetChild | Use type-specific `ignition::gazebo::Joint::*ByName`
+GetChild (Child link) | `ignition::gazebo::Joint::ChildLinkName`
+GetChildCount | Use type-specific `ignition::gazebo::Joint::*Count`
+GetParent | `ignition::gazebo::Joint::ParentModel`
+GetParent (Parent link) | `ignition::gazebo::Joint::ParentLinkName`
+GetParentId | `ignition::gazebo::EntiyComponentManager::ParentEntity`
+GetWorld |  `ignition::gazebo::worldEntity`
+
+---
+
+## Write family
+
+These functions deal with modifying the entity tree, attaching children to new
+parents.
+
+---
+
+Classic | Gazebo
+-- | --
+Attach | TODO
+Detach | TODO
+RemoveChild | TODO
+RemoveChildren | TODO
+SetModel | TODO
+SetParent | TODO
+SetWorld | TODO
+
+---
+
+## Lifecycle
+
+These functions aren't related to the state of a joint, but perform some
+processing related to the joint's lifecycle, like initializing, updating or
+terminating it.
+
+---
+
+Classic | Gazebo
+-- | --
+ConnectJointUpdate | TODO
+Fini | N/A
+Init | N/A
+Load | `ignition::gazebo::SdfEntityCreator::CreateEntities`
+Reset | `ignition::gazebo::Joint::ResetPosition` / `ignition::gazebo::Joint::ResetVelocity`
+Update | Entities are updated by systems
+
+---

tutorials/migration_link_api.md

@@ -261,7 +261,7 @@ logic that should be performed from within a system.
 Classic | Ignition
 -- | --
 GetNearestEntityBelow | Requires a system
-PlaceOnEntity | Involves Requires a system
+PlaceOnEntity | Requires a system
 PlaceOnNearestEntityBelow | Requires a system
 
 ---

tutorials/migration_model_api.md

@@ -218,7 +218,7 @@ Classic | Ignition
 -- | --
 GetJointController | Use this system: `ignition::gazebo::systems::JointController`
 GetNearestEntityBelow | Requires a system
-PlaceOnEntity | Involves Requires a system
+PlaceOnEntity | Requires a system
 PlaceOnNearestEntityBelow | Requires a system
 
 ---