Evaluate a node in a Maya MEL script - maya

Question

I have a Maya MEL script, which inserts some nodes. The evaluation seems to be deferred until the script finishes. I guess this comes from the pipeline evaluating when the shape is requested by the renderer, so the dirty propagation starts.
Now I want to run commands if the inserted node calculated a certain output like this:
$node = `insertMyNode`;
dgdirty ($node+".outputAttr");
if(`getAttr ($node+".outputAttr")` == 1) {
print("true");
} else {
print("false");
}
This always prints false. When I insert the node and then run getAttr ($node+".outputAttr") in the MEL editor, the node is computed and I get 1.
I tried dgeval as well and it didn't work either. I think in principle neither dgeval nor dgdirty should be needed but getAttr should start the dirty propagation.
But it always returns the default value of the node, not the evaluated one.
dgdirty $node works for me, but I still would rather only dirty the output I am using (and if possible automatically, not with a command which is documented to be for debugging purposeses), so the node does not need to recompute all outputs.
myNode has defined and inputMesh parameter which affects an output bool outputAttr value using attributeAffects in its C++ code. the insertMyNode command connects an input mesh. It is correct that the node is not computed without a connection to an output plug, but when reading the plug it should be computed. When I open the node editor and hover over the output plug, the node is computed correctly. I would expect getAttr to do the same in the example code above.

Answer 1

The dirty propagation usually works if you are dirtying input attributes and request output attributes. To it seems you are dirtying an input attribute and try to get the same input attribute. Usually in a node this is coded with "affects" like: affects(attrA, attrB). and if attrB is requested, the compute() method is called.
So I suppose it could work better if you try to get an output attribute.