How Can I tell if a plug is dirty during "prepareForDraw"?

[jmreinhart]

So I have a custom locator that draws a subset of faces to make a deformable control using two inputs (a mesh, and a intArray of faces IDs). Like this https://vimeo.com/311008901 

Ultimately I'm using this method to draw it:

drawManager.mesh(MHWRender::MUIDrawManager::kTriangles, draw_trianglePoints, NULL,NULL, &triangleIds);

Because I have to convert from facesIDs to triangle and vertex ids, I have stuff set up so that I cache the vertex ids I need to requery the positions of on each redraw, and I also put those positions into a non-repeating array and the "triangleIds" attribute contains the indices within that short list of positions that define each triangle I draw.

So "draw_trianglePoints" updates every time the mesh deforms, but "triangleIds" only updates if the topology changes or I change the value of the faceIds inputs. However during the "prepareForDraw" step I can't think of a way to avoid always re-querying "triangleIds" in case the value has changed, since I only have access to the DAG node being drawn. Does anyone have experience with optimizing custom locators for stuff like this?

[muir.j]

Not sure if this helps, but might be worth checking out the source for footPrintNode_GeometryOverride - it implements a variable called 'geometryChanging' that is updated in the compute node and then propagated through to the draw override class.

On a related note, if you are doing manipulators, the draw method also has limited access to variables so you have to pass the parent deformer class as a variable so you can get things while in the manip class.

[jmreinhart]

Unfortunately that's different than what I'm looking for. I found workaround where the user can manually reset a seprate attribute to tell the prepareForDraw method that the list of points I'm passing has changed. Unfortunately the speed increase was basically nothing, so I think I'm gonna have to shelf this MPxLocator and try a different approach entirely.

[jmreinhart · Accepted solution]

Thanks to Matt Lefevre's method of using a forced compute and MObject.userNode to access the custom locators internal data I came up with a solution using setDependentsDirty. I've attached the .cpp with the solution. To summarize:

when the input to the MPxLocator changes it triggers setDependentsDirty which sets an internal attribute_dirty variable to true. During the prepareForDraw method we use MObject.userNode to access that attribute_dirty value to determine if we need to requery the MPlug or if the value already in the MUserData is still valid.