MAYA-129059 - Add command to create stages.

[frohnej-adsk]

Add an undoable command that creates a stage with a new layer.

The new command is a C++ implementation of the existing mayaUsd_createStageWithNewLayer.py Python script. The command enables the creation of stages from within the C++ code, which will be required by future work (MAYA-128151) and might also be generally useful.

This commit also adds a Python binding for the stage creation command, which is used in mayaUsd_createStageWithNewLayer.py to replace the current implementation.

[ppt-adsk]

Hi @frohnej-adsk, I won't be reviewing this one. I will say that it needs a unit test :)

[seando-adsk]

@frohnej-adsk Before I review this code I just have a general question. In the comments you say that this will allow creating a new stage from C++ code. Is that all this is needed for? If so then you can do that now by using MGlobal::executePythonCommand() and import and call the existing python script.

[frohnej-adsk]

Hi @seando-adsk , I was told a while ago that it would be preferable to reproduce what the Python script is doing in C++. However, we can definitely challenge that.

The new C++ code allows for:

1. Creating stages from within the C++ code. This could also be done now by using the existing Python script and MGlobal::executePythonCommand().
2. Creating stages deep in the Maya hierarchy. This could probably be done by modifying the existing Python script and using MGlobal::executePythonCommand().
3. Wrapping stage creation into an Ufe::UndoableCommand so that it can be undone by UFE clients. I'm not sure if this is possible with MGlobal::executePythonCommand(). According to my current understanding, MGlobal::executePythonCommand() will add an item to the Maya undo queue directly. A Ufe::UndoableCommand on the other hand can be undone by Ufe::UndoableCommand::undo() and can be added to the Maya undo queue manually if desired.

These three points are required for a UFE interface I'm working on.

Is it possible to wrap MGlobal::executePythonCommand() in an Ufe::UndoableCommand? If so, we could consider calling the existing Python script (and modify it to allow for point 2). If not, I think it's necessary to go through C++.

[seando-adsk]

Overall the changes are good. The ability to create the stage anywhere in Maya hier is nice.
But the execute() method needs to be reworked.

[frohnej-adsk]

Rebase on dev to fix formating.

[seando-adsk]

Do we really need this check? Your create() method below and checks for and returns nullptr if input parent is empty.

[frohnej-adsk]

Good point, see 3e744ca.

[frohnej-adsk]

Squash commits for a clean history.