Connecting Expression Nodes

[jackinoob]

Hi all,

I have a custom node which holds a string attribute. Precisely:

CODE
"object3.translateX = object1.translateX + object2.translateX;"

I am simply trying to connect this string attribute to the expression attribute of an expression node by:

CODE
createNode ("expression");
connectAttr customNode.xformRule[0] expression1.expression;

This does not work! I don't understand why. When I use setAttr to set that exact string to the expression node, it works fine, but when it is coming from another attribute, it fails.

The connection is first made in the hypergraph, then it is immediately followed by an '"expression -edit ..." command, and a series of errors, and finally by the error: "Expression invalid after edit". The connection remains in the hypergraph, but the expression node does not contain the string. This is the exact error after using connectAttr as above. I have even tried setting the input[] connections to the expression node manually, but the expression attribute does not hold! Thanks in advance.

CODE
connectAttr customNode.xformRule[0] expression1.expression;
// Result: Connected customNode.xformRule to expression1.expression //
expression -e -ae 1 -o "" -a "" -s "object3.translateX = object1.translateX + object2.translateX;" expression1;
// Error: line 1: Connection not made: 'object1.translateX' -> 'expression1.input[0]'. Connections cannot be modified during evaluation. //
// Error: Connection failed from object1.translateX to expression1.input[0] //
// Error: line 1: Connection not made: 'object1.translateX' -> 'expression1.input[0]'. Connections cannot be modified during evaluation. //
// Error: Connection failed from object2.translateX to expression1.input[1] //
// Error: line 1: Connection not made: 'object1.translateX' -> 'expression1.input[0]'. Connections cannot be modified during evaluation. //
// Error: Connection failed from expression5.output[0] to object3.translateX //
// Error: line 1: Connection not made: 'object1.translateX' -> 'expression1.input[0]'. Connections cannot be modified during evaluation. //
// Error: Expression invalid after edit. //

[rob-bateman]

have you tried the expression mel command?

[jackinoob]

QUOTE(Rob Bateman @ 07/25/06, 08:37 AM) [snapback]243348[/snapback]
have you tried the expression mel command?

Well the expression command is static. What I am trying to do is read the expression from a custom node, which reads the expression from file. This way, if the user changes the expression in the file, the custom node will autoatically update, and the expression node will automatically contain the most up to date expression rule. I don't believe this can be done with the expression command.

[joojaa]

Exxpressions are terrible candidates for connectiin is all cases since the do NOTevaluate things like most other maya nodes DO. but rathar create themselvves each time anew. TOTALy form scratch. So doing something like this is a very batd idea indeed.

Sice a the exporession synatx wouldnnt be dymaic in firstplace, (expressions are compiled before they make it into the node). Secondly theers no real practical use for this.

And you need a node for this for what exact reason? (this would be a GUI thing. Gui helppers dont make good candidates for nodes use a event trigger that uses mel on the event instead of a node.)

Se a helpper does not contain scenetime computation. Maya has a celar differentiation of gui and node networks. This clarly falls into gui/ initialisation phase things. So what youd do is a scriptnode that refreshes this on load and register a event that triggers this on file change!

HOWEVER! the expression node MAY be refernced so edtting the referece would make it work.

[jackinoob]

Well, I do have a practical use for it.

The expressiona are a set of transformation rules, which are user defined. The user can change such rules, and there needs to be a way to reflect this change in the expression node, without the user being aware of the existence of an expression node or the topology of the DG. I think this is very practical.

My question simply is this: when I make the connection, the connection is made, no problem there. But then Maya automatically runs expression command, which creates errors. Running this expression command indepenedently is no problem either. I understand it runs after the connection is made to update the expression string, but how can I stop the automatic execution of this expression? Or is there another way to this whole problem? Thanks

EDIT: ----

Joojaa, I'm not sure if I understood clearly. You're saying to run a script node upon changes in the custom node's expression string...and the purpose of the script node is to setAttr the expression1.expression attribute to the new custom node value? Did I get that right?

hmm..if so, how do you specify a script to execute as a result of a change in attribute of a node?

[joojaa]

Yes this is a GUI thing dont confuse the purpose of DG versus purpose of GUI. wich in maya ideology are VERY different entities.

No dont use sattar to manage expressions bacyuse they are a bit special nodes. use expression intead. Expression is a VERY specialized maya node that has very much leeway in hwo it operates versus other stiuff in maya.

Anyway if you need realtime filewatching isntead of making a node that monitors it make a evnet that you can trigger to trough mel wich is the gGUI side of things NOT the dg evaluator part.

Edit and no you cabnt sstop the expression from compiling itself. If you would nothing would be driven by the expression.

PS theres very little intelligent use for string array connections in maya except MAYVBE for set controll.

[jackinoob]

Thanks man
Well, I have a whole lot of newbie questions in response to your post, but I'll put them off until I play around with the idea of events a bit and see what happens...I'll probably be back here soon. (sigh)

[joojaa]

Naah just write yoru expressions into .ma files and reference them works even better! Then you have a external file thet will update woithout breaking anything! Then just make the expression connections manualy in the target svvcene thisway same node can be used at several placec at once.

If you now still need the event make it refresh references instead.