Multi-window application with each window being handled by a sub-process

If you strike out here, you might want to burn a support ticket w/DTS to see if they can help.

Also...textdog salutes your AG skills
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               o  (oo)\_______
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It sounds like a given XPC service can only be run once.

That’s correct. Internally, XPC Services support the notion of multiple instances, but that support is not available for third-party use. If you think that’d be helpful in your situation, please file a bug report describing your requirements.

I’d appreciate your posting your bug number, just for the record.

It sounds like XPC services are really intended for processes that live a very short time …

I don’t think that’s a valid characterisation. For example, the XPC Services used by WebKit are very long-lived.

If XPC isn't the right choice, what is?

It’s a tradeoff. To implement this with XPC you have to jump through some hoops (more on that below) but you do get one key benefit, namely that the XPC API is much easier to use than most other IPC APIs. That’s especially important in your case, because it’s likely that you’ll want to pass an

IOSurface

With regards your specific setup, here’s how I’d implement this:

1. You set up an XPC Service for managing the rendezvous.

2. The app talks to the XPC Service to get it to start a new worker.

3. In response, the XPC Service allocates a UUID and associates that with the worker instance.

4. It then launches the worker (using

   NSTask

   , say), passing it the UUID as an argument.

5. The worker allocates an anonymous XPC listener.

6. It then creates an endpoint for that.

7. Finally, it checks in with the XPC Service, using the UUID to identify itself and passing it the endpoint.

8. The XPC Service then responds to the app, passing it the endpoint.

9. The app can then use that endpoint to open a connection directly to the worker’s anonymous listener.

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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

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[1] The only other IPC mechanism that allows you to transfer an

IOSurface

Yeah, that's one of the approaches that has come up. I'm trying to make sure I find the least complex and most sturdy approach before I die in and start coding.

I have filed a bug report suggesting the notion of allowing multiple instances of an XPC service (https://feedbackassistant.apple.com/feedback/6142396). In there, I explain my situation and what I'm suggesting, and point out that for people like me trying to update old 32-bit Carbon apps to modern, secure 64-bit code before Catalina ships, this would potentially save an enormous amount of time and effort.

I've actually already put in a DTS request

Yeah, I thought this sounded familiar.

I have filed a bug report suggesting the notion of allowing multiple instances of an XPC service

Thanks. My recommendation is that you plan to implement your solution on the currently shipping infrastructure. While it’s hard to predict the future with 100% accuracy, past experience is that it’s quite unusual for us to add significant APIs between WWDC and GM.

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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

Yes, that seemed obvious enough. Hopefully in the future I can simplify my code by just using multiple XPC service instances, but for now I'll have to create a service that creates instances, basically. OK. I'll see if I can implement something along those lines. FWIW it's something that would be really nice to see a DTS sample for, since upon searching around more I see this has been a topic of discussion in the past.

With that sorted out, the specifics of what the DTS request is about are clearer, hopefully. 🙂

Just a clarifying question before I start ripping apart my own app:

The XPC service you refer to in the first step, that has to be a login item/Helper/SMJobBless style service? I've tried to few simple experiments with an on-demand XPCService, but if I launch an NSTask from it the launched task cannot communicate with the XPCService. Unless there's a non-obvious trick I'm missing?

if I launch an

NSTask

from it the launched task cannot communicate with the XPC Service.

I just assumed that an embedded XPC Service would work, but I must admit to having not tested that case. Alas, I don’t have time to do that in the context of DevForums; if you’d like a definitive answer, you should open a DTS tech support incident and we can pick things up in that context.

Share and Enjoy
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"