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 > Which layer does async code belong?

Short answer: not in the gui, data or domain layer.

Long answer:

I would put it into a service-layer that sits between presenter- and domain-layer resulting in these layers

gui - presenter - service - domain - data

In the domainlayer (or the servicelayer) you have a synchronios version of the logic that needs to consume the data of the external service. Lets call it mySyncMethod.

In the servicelayer you create a thin async wrapper around mySyncMethod.

Reason: It is very difficuilt to unittest async methods. sync methods are much easier to test.

If there is only one reference to the async functionality you can put into the presenter.

 > Which layer does async code belong?

Short answer: into presenter or into an extra service-layer but not in the gui, data or domain layer.

Long answer:

I would put it into a service-layer that sits between presenter- and domain-layer resulting in these layers

gui - presenter - service - domain - data

In the domainlayer (or the servicelayer) you have a synchronios version of the logic that needs to consume the data of the external service. Lets call it mySyncMethod.

In the servicelayer you create a thin async wrapper around mySyncMethod.

Reason: It is very difficuilt to unittest async methods. sync methods are much easier to test.

If there is only one reference to the async functionality you can put into the presenter.

 > Which layer does async code belong?

Short answer: not in the gui, data or domain layer.

Long answer:

I would put it into a service-layer that sits between presenter- and domain-layer: gui-presenter-service-domain-data

In the domainlayer (or the servicelayer) you have a syschronios version of the logic that needs to consume the data of the external service. Lets call it mySyncMethod.

In the servicelayer you create a thin async wrapper around mySyncMethod.

Reason: It is very difficuilt to unittest async methods. sync methods are much easier to test.

If there is only one reference to the async functionality you can put into the presenter.

 > Which layer does async code belong?

Short answer: not in the gui, data or domain layer.

Long answer:

I would put it into a service-layer that sits between presenter- and domain-layer resulting in these layers

gui - presenter - service - domain - data

In the domainlayer (or the servicelayer) you have a synchronios version of the logic that needs to consume the data of the external service. Lets call it mySyncMethod.

In the servicelayer you create a thin async wrapper around mySyncMethod.

Reason: It is very difficuilt to unittest async methods. sync methods are much easier to test.

If there is only one reference to the async functionality you can put into the presenter.

 > Which layer does async code belong?

Short answer: not into data or domain layer.

Long answer:

I would put it into a service-layer that sits between presenter- and domain-layer: gui-presenter-service-domain-data

In the domainlayer (or the servicelayer) you have a syschronios version of the logic that needs to consume the data of the external service. Lets call it mySyncMethod.

In the servicelayer you create a thin async wrapper around mySyncMethod.

Reason: It is very difficuilt to unittest async methods. sync methods are much easier to test.

If there is only one reference to the async functionality you can put into the presenter.

 > Which layer does async code belong?

Short answer: not in the gui, data or domain layer.

Long answer:

I would put it into a service-layer that sits between presenter- and domain-layer: gui-presenter-service-domain-data

In the domainlayer (or the servicelayer) you have a syschronios version of the logic that needs to consume the data of the external service. Lets call it mySyncMethod.

In the servicelayer you create a thin async wrapper around mySyncMethod.

Reason: It is very difficuilt to unittest async methods. sync methods are much easier to test.

If there is only one reference to the async functionality you can put into the presenter.