Update Expression automatically when value changes in another attribute

[Anonymous]

Hi All, 

What I'd like to do is control the translation limits for a control based on the twist attribute of a second control. I've created an expression within the Expression Editor and it works once when I hit Edit. However, I want it to evaluate every time the twist value changes. I'm pretty sure I've seen it done before but I'm drawing a blank on how to do it. 

float $twistValue = `getAttr("torso_CTL.twist")`;

if ($twistValue < -10) 
	transformLimits -tz 0 200 -etz 1 1 CTRL_throttleHandle;
else if ($twistValue >= -10 || $twistValue <= 10) 
	transformLimits -tz 50 250 -etz 1 1 CTRL_throttleHandle;
else 
	transformLimits -tz 100 300 -etz 1 1 CTRL_throttleHandle;

[lee.dunham]

Expressions are different to scripts, and you usually try to avoid using things like getAttr and setAttr.

Instead, you access the attribute directly, and I think it will provide the behaviour you are looking for.

if torso_CTL.twist < -10 {

...

[Anonymous]

Thanks @lee.dunham 

Accessing the attribute directly does help with setting up the expression, but it still doesn't apply the changes automatically when I change the value of torso_CTL.twist. I thought perhaps there was a way to do this with expressions, but I'm starting to consider that there isn't.

[lee.dunham]

I don't have access to Maya atm, but you could look at a scriptJob tied to attributeChanged instead?

[Anonymous]

Hi @lee.dunham 

I've been on a big project with a looming deadline so I haven't had a chance to try your suggestion yet but I'll let you know how it goes when I do.

Thanks!

[olarn · Accepted solution]

If you want it to play nice with maya scene graph, implement the expression as nodes.

http://help.autodesk.com/cloudhelp/2018/ENU/Maya-Tech-Docs/Nodes/condition.html
http://help.autodesk.com/cloudhelp/2018/ENU/Maya-Tech-Docs/Nodes/clamp.html
etc

Or even better, as a single DIY node.

http://help.autodesk.com/view/MAYAUL/2018/ENU/?guid=__files_GUID_4F1D2DC9_D544_432B_A103_7A32D47757C...

If you really hated nodes you can try using this to monitor upstream attribute changes yourself.

http://help.autodesk.com/cloudhelp/2018/ENU/Maya-SDK/cpp_ref/class_m_node_message.html#add4ee645b2ce...

[Anonymous]

@olarn I ended up just using the node editor to get the results that I wanted. Since I was dealing with 3 scenarios rather than a simple if / else situation, I had to get creative with the nodes. 

[olarn]

Nice.

I'd like to interject for some more that implement as nodes has one more hidden benefits -- free parallelization.