@Override
    public Attribute.Type getReturnType() {
        return returnType;
    }
The main execution method which will be called upon event arrival
when there are zero or one function parameter

@param data  null if the function parameter count is zero or
             runtime data value of the function parameter
@param state Function state
@return the function result
modules/siddhi-samples/quick-start-samples/src/main/java/io/siddhi/sample/util/CustomFunctionExtension.java