That I could probably look at the source and figure things out.

No, what kills me is

public Session getSession(Foo, Bar, Baz)

where Foo, Bar, and Baz are themselves interfaces, and I have no idea what implementing class I need, so I go searching, and after an hour of pain piece together that I need to instantiate a NotAtAllAFooButNeverthelessImplementsFoo, then pass that into the BuildingFactory class' static factory method to get an instance of Bar, and then I can just pass in a null for Baz, and it -seems- to work.

On the SwiftUI side, you might go digging in these docs for Previews, which are one of its biggest selling points: https://developer.apple.com/documentation/swiftui/previews

Take 30 seconds to read through that and the linked pages (that's all the time it will take) and see if you could figure out how to use a PreviewDevice to make your preview show a particular device.

It will tell you all the ways to initialize a PreviewDevice struct (from many different varieties of String), but you'll have no fucking idea what to do with that object.

And here's an actual code sample, via Paul Hudson:

  struct ContentView_Previews: PreviewProvider {
     static var previews: some View {
        Group {
           ContentView()
              .previewDevice(PreviewDevice(rawValue: "iPhone SE"))
              .previewDisplayName("iPhone SE")

           ContentView()
              .previewDevice(PreviewDevice(rawValue: "iPhone XS Max"))
              .previewDisplayName("iPhone XS Max")
        }
     }
  }

It turns out what you need to do is pass the PreviewDevice instance into a previewDevice modifier that you've applied to your view inside a struct adhering to the PreviewProvider protocol. I don't think this docs page even mentions that the previewDevice modifier exists, let alone how to put any of these pieces together to configure and display a preview.

You can find how to do this in tutorials elsewhere on Apple's site. But you can't find it in their documentation for Previews, because it's a bunch of automatically generated pages of function signatures with no explanation.

We tried to buy some mechanical equipment which can be steered by a computer API. I asked for the doc beforehand. Got something like your example (10 pages of C header). I asked any better docs? Answer:'a real programmer would know how to use this' - well somehow they didnt manage to sell the device to us. Later they managed to email the comprehensive docs (which existed!), but we alread spent a much larger amount (without regrets) to the competition.