Binary return from a Struct

[danmoser]

Hi there,

I am trying to implement a feature I saw in a OPC-UA server that I could not find in python-asyncua. The goal is to have an object that returns the values of its children variables, so that the variable's values can be retrieved either directly, or grouped in this object.

I don't know the details of the OPC-UA server running that, but I could access it and got the following:

customObj = client.get_node(f"ns={idx_axis};s=customObj")

print("STRUCT", await customObj.read_value())
# Results:
STRUCT ExtensionObject(TypeId=NodeId(Identifier=101, NamespaceIndex=7, NodeIdType=<NodeIdType.Numeric: 2>), 
Body=b'\x00\x00\x00\x00\x00...')

print("STRUCT", await customObj.get_variables())
# Results:
STRUCT [Node(NodeId(Identifier='Var1', NamespaceIndex=7, NodeIdType=<NodeIdType.String: 3>)), 
Node(NodeId(Identifier='Var2', NamespaceIndex=7, NodeIdType=<NodeIdType.String: 3>)), ...] 

Any suggestions on how to mimic that?
Thanks in advance,
Daniel

[AndreasHeine]

sounds like ExposeItsArray https://reference.opcfoundation.org/v104/Core/docs/Part5/8.4.1/

[danmoser]

Thank you, Andreas! But do we have an example of it in python-asyncua repository?
I found a discussion here #1110 , and it does not appear to be implemented...

[AndreasHeine]

no

[danmoser]

What about Binary Encoding? https://reference.opcfoundation.org/v104/Core/docs/Part6/7.4.4/
Do we have it on asyncua?

[schroeder-]

import struct_to_binary from asyncua.ua.ua_binary.py
struct_to_binary(customObj)

[schroeder-]

Sry missed the read_value

import struct_to_binary from asyncua.ua.ua_binary.py
obj = await customObj.read_value()
struct_to_binary(obj)

[danmoser]

@schroeder- , that's great, thank you!
My final question: is there a way link the parent object struct with its children values (ie., a value update in a child will automatically be reflected as one of the struct entries)? Or is the proper way of doing that is using callbacks?

[schroeder-]

You have to link it your self and you can use callbacks to sync the values.

[danmoser]

Thank you!!