Undoit Works But It Breaks Redo Capability

[lnorri]

Hello everyone,

From what I have read this appears to be a great place to discuss the Maya API.
I normally wouldn't bother anybody with my questions, but I have searched high
and low and cannot seem to find the answer. Maybe someone will know this offhand...

Sorry this is a long one but I like to be detailed. =)

I have a very simple plug-in derived from MPxCommand.

It's only purpose is to generate a mesh from pre-existing data loaded by a different plug-in.

The structure of the plug-in is as follows :

 doIt()
{
      simply parses arguments (none right now)
      then does the actual work
      return redoIt();
 }
 redoIt()
{
      creates a new transform node using MFnTransfom.create()
      records the object hande of my new transform as a class member
      Uses the function MFnMesh.create() to generate a new mesh parented under the above transform
      // I plan to use a MDGModifier to create a shading network via MEL once I have squashed this bug 
 }
 undoIt()
{
      I use a MDGModifier coupled with a recursive function to delete the root transform I saved + any children
      return MDGModifier.doIt();
 }

Well the good news is that the plug-in works wonderfully despite one very annoying condition.
I can call the plug-in as many times as I want, creating geometry and translating/editing said geometry.
I can even UNDO all the way back to my very first command call!

Here is the problem... redo works UNTIL it hits one of my command calls.
The strange thing is, Redo actually performs correctly on my command ONE TIME!
The problem is any further attempt to redo ANYTHING from that point on...
Will give me the error "Error: There are no more commands to redo."

It's like my redo code decided to do it's job and flush the REDO stack.

I know the MDGModifier is handy with undo and redo, however I don't see how I can utilize it to create geometry with
MFnMesh... That is why I manually track and destroy my nodes in undo.

I am at a loss... what am I missing? I simply destroyed the nodes that I created...

Is the undo chain embedded in the nodes that I created? Is that why deleting them broke my redo?
I can't find documentation on this anywhere...

I know that shape modifying tools use the whole inMesh outMesh system for their construction history.
I didn't think brand new meshes required that... was I wrong?

I noticed that the basic shape(cube,plane,sphere, etc) commands in Maya do not have this problem.
What could they be doing differently? I would very much like to peek in and see... unfortunately that is impossible.

I thought It might be a Selection related problem, so I saved and restored selection states respectively. Alas that did absolutely nothing besides making my command a bit more user friendly by auto selecting the generated meshes.

If anyone has a clue as to what this might be, please do not hesitate to respond.

A HUGE thank you in advance to anyone who is willing to take the time to read this.

[jonasa]

It sounds like when you redo your command Maya interprets this as a new modification to the graph -> the undo stack is starting a new "branch" and your commands are lost.

You can create transform nodes using
transObj = dagMod->createNode("transform",parentObj, &stat);

then create the mesh like you do now.

Also try do all the creation in urCmd->doIt() and then only call dagMod->doIt() in urCmd->redoIt()

In urCmd->undoIt() you simply call dagMod->undoIt().

[lnorri]

Thanks Jonas,

I guessed that one could create nodes without an MFn class. I wasn't sure how though. From your explanation, I am guessing that if I use a "MFn::create" function with no "parentObj" that will cause Maya to branch. I assume that because the parent node will be registered with MDGModifier, It will not cause Maya to branch when MFnMesh.create is called. I am unsure about one thing however... when MDGModifier.undoIt() is called, will it account for the attached Mesh node I created with MFnMesh? Do I still have to manually delete this node? If so... wouldn't that force me to create the new Mesh node in the redoIt call?

I'm guessing I could also create the Mesh without MFnMesh. And use the MDGModifier instead, however that would force me to use MFnMesh.addpolygon(). I am hesitant to use that method because I read "MFnMesh::addpolygon" is only meant to be used in an "MPxNode::compute" function call.

In any case, this is all speculation that stems from my unfamiliarity with Maya. I need to go get my hands dirty again. I appreciate the branching insight! I am going to try what you said.

[lnorri]

Update:

Undo & Redo both work great now!(Thanks JonasA!!!) Strange thing is I only had to create and track my root parent node with MDagModifier. Everything else is just a child of this node & did not need to be created or edited with MDagModifier.

I have a better sense now of the purpose of MDagModifier. The only change I needed to track was the one that specifically modified the current state of the Dependency Graph. The edits and child nodes I generated underneath my root parent node did not need to be tracked for undo/redo. The fact that these changes were still available for redoIt() even though I did not track them, seems to say something.

In that sense I am guessing that "MPxCommand/undoIt/redoIt" paired with "MDagModifier/doIt/undoIt" is quite different than the names led me to believe. It seems to be more of a Dependency Graph "disconnect/reconnect" system rather than "undo/redo" in the truest sense. Although I am happy it is implemented in this fashion(very optimized), It has led to considerable confusion. Am I correct in thinking this way? It seems it must be so... How else could Maya track changes/creation of multiple child nodes which I never registered with the "undo/redo" system.

Can anyone with a deeper understanding of Maya's architecture shed some light on this? I can get by without knowing, but I am extremely curious.