APIs that ease migration from classic

[chapulina]

Classes like ignition::gazebo::Model are meant to provide helpful wrappers around entities and components which are simpler to use. This is important for users that are not accustomed to C++. This should also ease migration from Gazebo-classic, because the APIs would be more similar.

Not all functions from Gazebo-classic make sense in the new paradigm though. For example, one could query time information like SimTime, RealTime, IsPaused from gazebo::physics::World, but on Ignition, these properties don't belong to the world entity and should be obtained from UpdateInfo instead.

Here's a preliminary list of some APIs that we could offer on Ignition to mimic Gazebo-classic. The underlying implementation would use calls like ecm.Component, ecm.EntityByComponents, etc.

• We should create a class for each entity type: World, Model, Link, Joint, Light, Actor, Sensor, etc.
• We should have accessors for most properties, such as name, pose...
• Child entities should be accessible by name, such as World::LightByName()
• Each class will also have accessors specific to that entity type. For example, World::Gravity(), World::MagneticField(), etc.
• We should also add generic APIs that apply to many entity types to Util.hh

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Status:

• World
• Model
• Link
• Joint
• Light
• Actor
• Sensor