add UFE contextOps for working set management (loading and unloading)

[mattyjams]

This adds contextOps items for "Load", "Load with Descendants", and "Unload" based on the load set of the prim's stage, the loadable paths at or below that prim's path, and whether the prim has a payload.

These items map to undoable commands that call UsdStage::Load() (with the appropriate UsdLoadPolicy) or UsdStage::Unload() with the prim's path.

See the "Working Set Management" section of the USD docs on UsdStage for more discussion of load and unload rules:
https://graphics.pixar.com/usd/docs/api/class_usd_stage.html

[wtelford]

This is great to see. Could you attach an image showing how this appears in the context menu. This is specific to payload loading only correct?

[mattyjams]

This is great to see. Could you attach an image showing how this appears in the context menu. This is specific to payload loading only correct?

Sure thing. Here are a few screenshots using the Maya scene from the unit test:

• Everything loaded, showing items for Ball_1:
  

• Ball_1 was unloaded, showing items for Ball_1, which has its own payload, and could potentially introduce child prims with payloads, so we show both flavors of "Load":
  

• Ball_1 still unloaded, showing items for Props, which does not have its own payload. There are both loaded and unloaded things below it though, so we show "Load with Descendants" and "Unload":
  

• Props was unloaded, which unloaded the payloads for all the Balls, so only their top-level prims appear. Props does not have its own payload, so we only show "Load with Descendants" for it:
  

And as far as being payload-only, I intentionally didn't explicitly use the word "Payload" in any user-visible strings, but that is effectively what's being managed here. The context ops map to UsdStage::Load(<primPath>) and UsdStage::Unload(<primPath>) calls which currently manage payload inclusion rules. If those were extended in the future to encompass some other concept of loading in addition to payloads, we'd pick that up here as well.

[wtelford]

Looking at the load/Unload rules in the docs, I'm curious how you are handling the following two rules.
Are you preventing the menu entries from showing?

• Specifying a path to an inactive prim is an error.
• Specifying a path to a master prim or a prim within a master is an
  error.

Also, do you present "unload" and "load with descendants" items on the proxyShape right-click as well to apply to all root level prims?

[mattyjams]

Looking at the load/Unload rules in the docs, I'm curious how you are handling the following two rules.
Are you preventing the menu entries from showing?

• Specifying a path to an inactive prim is an error.
• Specifying a path to a master prim or a prim within a master is an
  error.

Yes, those cases are handled. We don't present any context items for inactive prims or prims within a prototype. The code for that is here:
https://github.com/Autodesk/maya-usd/pull/823/files#diff-3fb4a94e0a781c0c1ee67a66b8d69519R337

Also, do you present "unload" and "load with descendants" items on the proxyShape right-click as well to apply to all root level prims?

I did not add context items for the proxy shape itself, since that functionality was already provided by the "Load Payloads" attribute on the proxy shape. When you make it back up to the proxy shape, you're back in Maya data model land, so there's a lot of Maya-side stuff in the right-click context menu. I didn't want to clutter that up further with "Load with Descendants" and "Unload".

[ppt-adsk]

Hey Matt, this looks wonderful, will have a look at it ASAP. However, the next Maya Preview Release code complete deadline is breathing down my neck, and we have Canadian Thanksgiving on Monday, so I might not be as responsive as I'd like to be. Hope that's O.K.

[mattyjams]

@ppt-adsk: No problem, thanks for the heads up! There's no rush with this one.

Enjoy the holiday!

[wtelford]

I did not review the code, but I did review the imagery submitted along with our design team. Looks good from a UI and workflow perspective. I'll leave it to Pierre to do the code-review.

[seando-adsk]

I reviewed the code changes and everything looks good. The new test cases are great. Thanks.