Strange behaviour of adding dynamic methods on Groovy's NodeChild - groovy

Question

I'm using Grails XML Parser to parse an XML string and after getting the parsed NodeChild instance, I'm adding dynamic methods on that instance like below:
import grails.converters.XML
import groovy.util.slurpersupport.NodeChild
NodeChild result = XML.parse("<root></root>")
result.getMetaClass().methodA = { return "a" }
result.getMetaClass().methodB = { return "b" }
println rootNode.methodA()
println rootNode.methodB()
Now the line where I'm calling methodA() and expecting "a" to be printed, I'm getting MissingMethodException that methodA() not found.
I investigated on this for some time and found that the all dynamic methods getting replaced with the last dynamic method we add, i.e. in this case: methodB() is replacing (or doing something) methodA(), so I call & print methodB() first, it prints "b" properly.
This strikes me to another test as following:
import grails.converters.XML
import groovy.util.slurpersupport.NodeChild
String result = "any-other-data-type-instance-here-to-inject-dynamic-methods"
result.getMetaClass().methodA = { return "a" }
result.getMetaClass().methodB = { return "b" }
println rootNode.methodA()
println rootNode.methodB()
In this case, both statement prints fine. So the problem is only with the classNodeChild. I'm using exando metaclass feature for long time and I faced such kind of problem. Any idea, that why this is happening?

Answer 1

You need to assign the metaclass methods before the instance is returned, otherwise it will have the old metaclass and not the new one with the new methods. Also, assign to the class, not the instance - I'm not sure if you wanted to only affect this instance's metaclass but that's not the syntax.
This works:
import grails.converters.XML
import groovy.util.slurpersupport.NodeChild
NodeChild.metaClass.methodA = { return "a" }
NodeChild.metaClass.methodB = { return "b" }
NodeChild result = XML.parse("<root></root>")
println result.methodA()
println result.methodB()
Note that you're defining methods with an implicit it argument, but passing nothing, so Groovy passes a null. If you intend for the methods to have no arguments, use this syntax:
NodeChild.metaClass.methodA = { -> return "a" }
NodeChild.metaClass.methodB = { -> return "b" }