Ok, I figured it out:
I don't know what about the MEL approach (I didn't like it anyway).
But as for the 2 other approaches, the problem was the way I instantiated
MNodeClass.
I thought I was giving it a NoteTypeID, but I was actually trying to
instantiate is with an MFnTypeID which doesn't work...
I found that out by trying to get it's TypeName afer instanciating it, and
the AttributeCound, and I got invalid results...
Until I tried to just pass it a NoteTypeName, like "transform" (all
lower-case), and suddenly it started working.
Eventually, I used "dagNode" (with this exact casing), so I could add the
extension-attribute to all DAG-nodes.
It also works with my custom MPxData-type, just as well as with the
StringData.
I didn't end-up having to wrap it in MFnPluginData for this to work - I
just pass MyMPx::id to the attribute-creation function, and it works.
So, this is how it looks when it works:
MFnTypedAttribute fnAttr;
MObject attr = fnAttr.create(L, S, MyMPx::id); // or: MObject attr =
fnAttr.create(L, S, MyMPx::id, MObject::kNullObj, &status);
MNodeClass mnDagNodeClass("dagNode");
status = mnDagNodeClass.addExtensionAttribute(attr);
Realistically, I actually did have to use MFnPluginData in order to
pre-generate a default-value for my attribute:
MFnPluginData fnData;
MObject defaultValue = fnData.create(MyMPx::id, &status);
CHECK_MSTATUS_AND_RETURN_IT(status);
MFnTypedAttribute fnAttr;
MObject attr = fnAttr.create(L, S, MyMPx::id, defaultValue, &status);
MNodeClass mnDagNodeClass("dagNode");
status = mnDagNodeClass.addExtensionAttribute(attr);
I haven't tried to use the MDGModifier approach, but I would guess it would
work out just as well, with a proper MNodeClass instance..
I wish there was an easier way to find-out, when trying to instantiate
MNodeClass, when it doesn't actually work...
I also wish there was a list somewhere of node-names and type-ids...
Anyway, anyone reading this, who wants to write a plugin with this
use-case, now you know how.
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