Expression Evaluation Problem and possible Bug?

[bolek]

Right,

I have several problem while trying to make an expression evaluate when using the xform command.

Simple example:
Let's say I'm trying to "constrain" a cube (pCube1) to a vertex of another cube (pCube2) :

that doesn't work (for some obscure reason):
pCube1.translate = xform -q -t -ws pCube2.vtx[2];

but that does:
float $flot[] = xform -q -t -ws pCube2.vtx[2];
pCube1.translateX = $flot[0];
pCube1.translateY = $flot[1];
pCube1.translateZ = $flot[2];

Except that the expression only evaluate when time changes... does anyone have any idea why ?
Is there a workaround to this ?

Another little funny thing (is that a bug or am I being thick?) If you query an xform of a vertex it returns a double3, so (outside of an expression here), why can't you just do this :

setAttr pCube1.translate xform -q -t -ws pCube2.vtx[1];

why does Maya always give the following message ?
Error reading data element number 1
What is that supposed to mean ? Is that because the xform return value is a double3 and that the destination attribute is a float3 ? Isn't the conversion supposed to be automatic ?

NB :

setAttr pCube1.translate -type "double3" xform -q -t -ws pCube2.vtx[2];
or
setAttr pCube1.translate -type "float3" xform -q -t -ws pCube2.vtx[2];

return a different error message:
Error reading data element number 3

Please help, that is driving me mad.

[joojaa]

Use rivet mel and take alook how the connection could be made more optimnaly!

[nr-puckett]

any time you use query or get/setAttr in an expression it will only eval when time changes. To solve the problem in general I would create a pointOnSurface node for the vertex and then connect the other cube to the POS node. take a look at surfaceLocator script in modeling section for an example.

[joojaa]

Not true, getAttr just gets evalueated out of context. And it forces the graph to reavaluate casing maya to evaluate the entire node hiearchy under the value querried. It does evaluate whenever you need, but with a fenomenal overhead.

Easiest is just to querry the shapes point index dierctly for the value thereby causing a direct connection. No mel or utility node needed.