Passing an Objective-C protocol to an objective-C function in Swift?

JetForMe · 2023-05-11T10:41:51.200Z

I'm trying to convert an old Obj-C project to Swift. I'm using an esoteric serialization protocol called Hessian that gets configured with calls like this:

[CWHessianUnarchiver setProtocol: @protocol(MissionProtocol) forClassName: @"com.some.java.server.class.ClientV1Mission"];

…

@protocol MissionProtocol
// Properties…
@end

In Swift, the calls look like this, but this isn’t quite right, and I'm not sure how to write it:

CWHessianUnarchiver.setProtocol(MissionProtocol.Type, forClassName: "com.some.java.server.class.ClientV1Mission")

But I get {Cannot convert value of type '(any MissionProtocol.Type).Type' to expected argument type 'Protocol?'" for the setProtocol call. How should I pass this?

bbrk24 · 2023-05-11T11:09:13.084Z

Objective-C Protocol objects, like Swift metatypes, are spelled P.self, not P.Type.

Kyle-Ye · 2023-05-11T12:16:07.506Z

The 2 posts may help solve your problem.

Swift metatype and ObjectiveC Protocol question Using Swift

We have an ObjC API which receive a Protocol instance and return Any?. // Imported ObjectiveC API func resolve(_ p: Protocol) -> Any? // Swift Side Code @objc protocol P: NSObjectProtocol { func hello() } func test() { let p = resolve(P.self) as? P p?.hello() } In Swift side, we'd like to wrap the API to make it more swift-friendly. func resolve2<T>(_ t: T.Type) -> T? { return resolve(t) as? T // return resolve(t.self) as? T // return resolve(T.self) as? T // …

swiftrocks.com

What's .self, .Type and .Protocol? Understanding Swift Metatypes

Metatypes are pretty useful in Swift, and you have certainly used it in multiple occasions. Unfortunately they look pretty weird in code, which can cause some confusion when trying to understand what they really are.

JetForMe · 2023-05-11T22:42:24.442Z

bbrk24:

Objective-C Protocol objects, like Swift metatypes, are spelled P.self, not P.Type.

I tried that, but it didn't work. However, I just tried it again, and it works. I must've not tried. But now I'm questioning my sanity. Thanks!