Synchronize ECM between GUI/Server

[adlarkin]

Desired behavior

Add functionality so that when simulation is paused, users can perform GUI actions that update the GUI's ECM directly. Then, when simulation is unpaused, the GUI shares its updated ECM state with the server, and the server does whatever is needed to reflect the changes made in the GUI.

Alternatives considered

N/A (new feature). For more context, we currently update the server from the GUI via gui events.

Implementation suggestion

See the design proposed in gazebosim/design#8

Additional context

Here is a checklist of things that need to be done to add this functionality. The design document linked above can be referenced for more context about each checklist item. I will add relevant PRs by each item as they're created so that we can track the progress of this new feature:

• Create a new message type that contains updated ECM information for when users press "play" on the GUI (see the conversation in Design document for syncing GUI ECM with server ECM design#8 (comment) for more context)
  • Add WorldControlState msg type gz-msgs#190
• Update the interaction between GUI/server to make use of the new message type above when "play" is clicked in the GUI. Consider the case of multiple clients attached to the same server (see Design document for syncing GUI ECM with server ECM design#8 (comment) for a solution)
  • Support emitting an event on play/pause/step gz-gui#306
  • Get updated GUI ECM info when a user presses 'play' #1109
• Create a GUI system plugin that tracks ECM changes from GUI user actions when simulation is paused (this is the GuiUserCommands plugin in the design document) - I may also be able to add this functionality to the existing ign-gui WorldControl plugin, since this is the plugin that interacts with the server when "play" is pressed
  • Support emitting an event on play/pause/step gz-gui#306 (I took the approach of adding to the existing ign-gui's WorldControl plugin instead of creating a new one)
• Update the server to not only update its ECM based on the ECM-changing GUI actions, but to notify things like server systems of changes as well (notifying server systems of updates may require a lot of work). For the case of server/client running in the same process, the server's ECM wouldn't need to be updated since the server/client share the same ECM, but server systems would still need to be notified of changes
  • TODO note left in Get updated GUI ECM info when a user presses 'play' #1109

[adlarkin]

I'm closing this, since #1231 has been merged.