Hopefully this is a simple question, that I've just bone-headedly missed in the documentation.
In process builder how can I access the return value of an Apex InvocableMethod?
I dont think we can access the return value in Process Builder .Process Builder just takes record on which an action is invoked and uses as parameter to InvocableMethod and process logic .
The return values from Invocable methods makes sense for flows and REST API .Return values can be fed back to flows as variables .
If the return type is not Null, the data type returned by the method must be one of the following:
Create a custom global or public Apex class to implement your data type, and make sure your class contains at least one member variable with the invocable variable annotation. https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_annotation_InvocableMethod.htm