Hi, I've been cracking my head agains this for far too long.
I'm starting to think that what I'm trying to do is not possible or not
supported by Maya.
I'm writing a C++ plugin using the Maya API.
I have a custom data-type, that I want to expose to all Dag nodes as an
extension-attribute.
I coun't find a single example of doing this - anywhere.
Not in the documentation, not in the DevKit examples, and not anywhere on the
web.
(searched this forum and many others)
There are examples on how to add dynamic-attributes, but that is not what I'm
afte.
I've been able to expose it as a dynamic-attribute, but I then need to add it
to every node that is being queried/edited, if it doesn't exist, and this gets
complicated when supporting undo (even using MDGModifier).
Accodring to the Maya documentations, there should be 3 ways to add
extension-attributes:
1. Using 'addExtensios' MEL command
2. Using MNodeClass::addExtensionAttribute(<attr>)
3. Using MDGModifier::addExtensionAttribute(<node-class>,<attr>)
I put the code that adds the extension-attribute in the plugin's
-initialization function, and a corresponding code for deleting it in the
plugin's uninitialize function.
I am able to use the MEL option with:
MGlobal::executeCommand("addExtension -nt dagNode -sn S -ln L -dt T;");
(where: 'S' and 'L' are the attr-name, and 'T' is my datat-type's name)
The attribute even shows-up in the attribute-editor of all dag-nodes inthe
'Extended' section.
But then when I try to get the value of it later, it fails.
The code I'm using to get the data is taken from the blindComlexData example.
Within a MItSelectionList itereator, I do (satus-handling code excluded for
clarity):
MObject node;
iter.getDependNode(node);
MFnDependencyNode fnNode;
fnNode.setObject(node);
plug = fnNode.findPlug(S, false);
MObject sData;
plug.getValue(sData);
MFnPluginData fnData(sData);
data = (MyMPx*)fnData.constData();
(Where: 'S' is the short-name of the attribute, and MyMPx is my custom-data
class)
The last-line fails, when it tries to case the data back to my custom-class.
When using the other two options, I can't even initialize the plugin if I use
status-checking code - and there are absolutly no examples about how to do that
anywhere (not that I could find, and I think I can search pretty well).
I've tried everythin I can think of, and then some guess-work.. (a LOT
actually..)
I first have to define/create an attribute, of-course, before I pass it along
to any of the functions that would add it as an extension attribute.
I actually found out that it doesn't matter what kind of attribute I created,
using MFnTypedAttribute - even creating a built-in attribute-type, causes the
same kind of failure:
MFnTypedAttribute fnAttr;
MObject attr = fnAttr.create(L, S, MFnStringData::kString);
BTW, if I add this right after: MFnData::Type t = fnAttr.attrType();
Then 't' is 'kInvalid', which I find odd - can't I even create a normal
typed-attribute properly?
If I do this, though, then 't' is 'kString':
MFnTypedAttribute fnAttr;
MString defaultString("");
MFnStringData defaultTextData;
MObject defaultTextAttr = defaultTextData.create(defaultString);
MObject attr = fnAttr.create(L, S, MFnStringData::kString, defaultTextAttr,
&status);
MFnData::Type t = fnAttr.attrType();
Using MNodeClass, I've tried doing:
MTypeId mDagNodeTypeID(MFn::kDagNode);
const MNodeClass mnDagNodeClass(mDagNodeTypeID);
status = mnDagNodeClass.addExtensionAttribute(attr);
CHECK_MSTATUS_AND_RETURN_IT(status);
The status-check fails and returns immediately.
Using MDGModifier, I've tried doing:
MTypeId mDagNodeTypeID(MFn::kDagNode);
const MNodeClass mnDagNodeClass(mDagNodeTypeID);
MDGModifier mDgMod;
status = mDgMod.addExtensionAttribute(mnDagNodeClass, attr);
CHECK_MSTATUS_AND_RETURN_IT(status);
The status-check fails and returns immediately.
So it can't even be the way I try to create my custom-typed attribute.
But just for completion-sake, here is what I've tried:
MFnTypedAttribute fnAttr;
MObject attr = fnAttr.create(L, S, MyMPx::id);
MFnData::Type t = fnAttr.attrType();
Doesn't work...
't' is 'kInvalid', and trying to add the attribute later as an
extension-attribute (using either MNodeType or MDGModifier), fails the next
status-check.
I then though, maybe it needs a default-value, so I first try:
MObject attr = fnAttr.create(L, S, MyMPx::id, MObject::kNullObj, &status);
CHECK_MSTATUS_AND_RETURN_IT(status);
MFnData::Type t = fnAttr.attrType();
Status-check passes, but 't' is still 'kInavalid', and again, can't add it as
an extension-attribute later.
Lastly then tried:
MFnPluginData fnData;
MObject defaultData = fnData.create(MyMPx::id);
MTypeId tPluginData(MFnData::kPlugin);
MObject attr = fnAttr.create(L, S, tPluginData, defaultData, &status);
CHECK_MSTATUS_AND_RETURN_IT(status);
MFnData::Type t = fnAttr.attrType();
...
At this point, I am out of creative ideas...
Any help would be appreciated! :)
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