Add variadic-generic Service.init overload

[bbrk24]

Checklist

• I searched the existing GitHub issues and this hasn't already been requested.

Problem

When a single type has a lot of dependencies, the registration gets long and repetitive, and is fragile to changing the init:

Service(.transient, FooProtocol.self) { r in
  Foo(
    bar: r.resolve(),
    baz: r.resolve(),
    qux: r.resolve(),
    // ...
  )
}

Feature Request

As of Swift 5.9 (Xcode 15), we can add an overload like this:

#if swift(>=5.9)
extension Service {
    @available(swift 5.9)
    public init<TRegistration, each TDependency>(
        _ scope: Scope,
        _ type: TRegistration.Type,
        name: String? = nil,
        _ callback: @escaping (repeat each TDependency) -> TRegistration
    ) {
        self.init(scope, type, name: name) { (r: Resolver) in
            callback(repeat r.resolve((each TDependency).self))
        }
    }
}
#endif

each TDependency represents a "parameter pack"; the number of generic arguments to this initializer is variable. With this overload, one can say:

Service(.transient, FooProtocol.self, Foo.init)

I tested this locally and it works without confusing the compiler on existing registrations. However, I tested this on Linux, so I had to disable everything related to SwiftUI/Combine. I don't have a Mac to test those parts myself.

Value

The Factory.register blocks would become shorter, making them easier to read and write. Most callbacks passed to the Service are just an initializer.

Something similar could be done for MultitypeService.

[bbrk24]

@wboyd600 Have you updated to Xcode 15 yet? I could create the PR, but as I said I can't fully test it myself.

[wboyd600]

Hey @bbrk24, hope you're doing well.

I just read up on all of the new features in 5.9. As far as the feature request goes, this definitely seems like an ideal use case for variadic generics and gets rid of a lot of boiler plate code.

I haven't updated to Xcode 15 yet! I'm always a little hesitant to update to X.0 releases, but I'm sure I'll end up updating soon enough (whether that's on purpose or not...).

I'll let you know when I do so I can fully test everything related to SwiftUI/Combine.

[wboyd600]

Sorry that took forever -- but we did finally get around to updating!