# Bridging ObjC Categories on Types with Struct Swift Overlay

**URL:** <https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382>\
**Category:** Using Swift\
**Tags:** bridging\
**Created:** [March 26, 2018, 9:54am UTC](https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382 "2018-03-26T09:54:52Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![fabb](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/fabb/32/2230_2.png) [@fabb](https://forums.swift.org/u/fabb)\
**Post date:** [March 26, 2018, 9:54am UTC](https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382/1 "2018-03-26T09:54:52Z")

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I have a bridged projects with ObjC categories on types like NSURL or NSError. Swift does not see the category methods on the overlay structs (URL, Error), The only way to I found to work around this is by casting to the ObjC types in Swift (e.g. `(url as NSURL).extensionMethod()`). What is the recommended way to access the category methods from both ObjC and Swift?

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**Author:** ![anthonylatsis](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/anthonylatsis/32/39232_2.png) [@anthonylatsis](https://forums.swift.org/u/anthonylatsis)\
**Post date:** [March 26, 2018, 4:47pm UTC](https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382/2 "2018-03-26T16:47:19Z")

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That's because `NSError` conforms to `Error`. Other examples, like `URL` and `NSURL`, `Data` and `NSData`, `String` and `NSString` do not even have a subtyping relation.

**But** , they are designed to be easily convertible to each other either by casting or using initializers.

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<div class="post-metadata">

**Author:** ![fabb](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/fabb/32/2230_2.png) [@fabb](https://forums.swift.org/u/fabb)\
**Post date:** [March 26, 2018, 4:59pm UTC](https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382/3 "2018-03-26T16:59:58Z")

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I know they are not related, just overlays. I just wanted to know how to write the extensions without duplicating code and without burdening the caller with casting. I guess for making a method available on both the Swift and ObjC types, I have to write the extension on both, and if I want to avoid duplicated code, delegate one implementation to the other.

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**Author:** ![anthonylatsis](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/anthonylatsis/32/39232_2.png) [@anthonylatsis](https://forums.swift.org/u/anthonylatsis)\
**Post date:** [March 26, 2018, 5:12pm UTC](https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382/4 "2018-03-26T17:12:06Z")

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Yes, if they aren't related, you will have to extend both and delegate one method to the other if you want some _new_ equal functionality, or extend one of them if you want to delegate to an _existing_ method of the 'pair type'. However, If it's some existing functionality, I would recommend simply casting or converting rather than spending time on extending and delegating. It's OK since you only need to cast at a certain moment when bridging. Besides, it would be as well semantically incorrect to extend. But it's up to you of course.

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**Author:** ![jrose](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jrose/32/408_2.png) [@jrose](https://forums.swift.org/u/jrose)\
**Post date:** [March 26, 2018, 5:40pm UTC](https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382/5 "2018-03-26T17:40:28Z")

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Right, Swift doesn't automatically copy over category methods because it's possible they rely on the reference-type-ness of the `self` type. Unfortunately, the best answer right now is to redeclare such methods on the value types.

```swift
extension URL {
  public func extensionMethod() {
    (self as NSURL).extensionMethod()
  }
}

```

I hesitate to say this because it will make more work for us and because we're unlikely to get to it any time soon, but it would be a reasonable feature request to have a special annotation for Objective-C methods that can safely be copied over to a bridged value type. Please file a feature request at [https://bugs.swift.org](https://bugs.swift.org) if that's something you're interested in, and please include the sorts of category methods you're defining so that we have some examples of real-world use cases.

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<div class="post-metadata">

**Author:** ![fabb](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/fabb/32/2230_2.png) [@fabb](https://forums.swift.org/u/fabb)\
**Post date:** [March 26, 2018, 5:43pm UTC](https://forums.swift.org/t/bridging-objc-categories-on-types-with-struct-swift-overlay/11382/6 "2018-03-26T17:43:37Z")

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Thanks! I‘ll write down some of our uses. Mostly it‘s for custom error domain handling and url/string tools.