Swift class does not conform to Objective-C protocol with error handling

Question

I have Objective-C Protocol

@protocol SomeObjCProtocol
- (BOOL) doSomethingWithError: (NSError **)error;
@end

And Swift class

class SwiftClass : SomeObjCProtocol
{
    func doSomething() throws {    
    }
}

Compilers gives me an error

Type 'SwiftClass' does not conform to protocol 'SomeObjCProtocol'"

Is there any solution how to get rid of this error? I'm using XCode 7 Beta 4

No. Function in Swift class was autocompleted by xcode. Also see documentation — Joey
– Joey, Commented Aug 5, 2015 at 12:18
func buyFavoriteSnack(exception: bool) throws In the implementing class func buyFavoriteSnack(exception: bool) throws { let snackName = favoriteSnacks[person] ?? "Candy Bar" try vend(itemNamed: snackName) } Or simply refer developer.apple.com/library/prerelease/ios/documentation/Swift/… — Arun
– Arun, Commented Aug 5, 2015 at 12:25

Martin R · Accepted Answer · 2015-08-05 13:05:13Z

6

There are two problems:

Swift 2 maps func doSomething() throws to the Objective-C method - (BOOL) doSomethingAndReturnError: (NSError **)error;, which is different from your protocol method.
The protocol method must be marked as "Objective-C compatible" with the @objc attribute.

There are two possible solutions:

Solution 1: Rename the Objective-C protocol method to

@protocol SomeObjCProtocol
- (BOOL) doSomethingAndReturnError: (NSError **)error;
@end

Solution 2: Leave the Objective-C protocol method as it is, and specify the Objective-C mapping for the Swift method explicitly:

@objc(doSomethingWithError:) func doSomething() throws {
    // Do stuff
}

edited Aug 5, 2015 at 13:05

answered Aug 5, 2015 at 12:54

Martin R

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3 Comments

Joey Over a year ago

Both solutions works. Thank you. Although Solution 1 is a bit weird, because according to documentation suffix WithError should work too.

Martin R Over a year ago

@Joey: I assume that the rules are slightly different for protocol methods. Here the Swift method is mapped to Objective-C (to check the conformance), not the other way around.

kalafun Over a year ago

The problem for me was that I misspelled the name of the method in terms of lower and uppercase. Something like: - (void) thisClass:(Class *)class DidSendStuff:(Stuff *)stuff and I was trying to work with - (void) thisClass:(Class *)class didSendStuff:(Stuff *)stuff notice the lower and uppercase of DID SEND STUFF

Arcanelab · Accepted Answer · 2017-05-05 08:35:17Z

0

When encountering that error message, one source of the problem might be that the Swift class conforming to the Objetive C protocol was not inherited from NSObject.

answered May 5, 2017 at 8:35

Arcanelab

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