I am trying to validate my custom MPxEmitterNode attributes.
I have force_min and force_max attributes that are double3 typed in maya parlance, basically two objects containing double[3] data.
I want to ensure the force_min is less than force_max for each of its 3 components. I'd like to do this by just swapping the min and max around if someone enters a value on the attribute in the attribute editor, or calls mels setAttr for those attributes, which then fails the "min < max" check.
I have tried setting up ATTRIBUTE_AFFECTS relationships between force_min, force_max and their individual component x,y,z objects. That seems to cause a cyclic issue leading to Maya crashing. I have also tried editing the custom compute function for the derived MPxEmitterNode, so it sets the force_min and force_max values to swap. The force_* attributes are seemingly never computed in this case.
Any help would be much appreciated.
Generally the 'Maya' way to do this would be to let the output look wrong if the min and max are set incorrectly. You don't know who is going to set those attributes -- it could be as connection or a script, and it could even get reset in between frames of an animation -- and so it's better to let the dag evaluation flow through even if the result is nonsense. It's like setting a radius of zero on a sphere node --it's 'correct' even thought it's wrong.
You can however swap the values inside your compute() method to get the same effect as swapping the values without resetting the plug values themselves. Setting an input plug from inside compute is a bad idea, because it introduces a loop into the flow of the dag evaluation. Dag nodes must be acyclical (that's the "a" in dag: Directed Acyclic Graph)