In general you can skip the interface definition and just convert the implementation.

One important area to note is with initializers. If you have an Objective-C class that has been modified to be used by Swift it may have had the NS_UNAVAILABLE or NS_DESIGNATED_INITIALIZER macros added to it. These correspond to the designated initializer concept in Swift so you should preserve them:

@interface ImagePathHelper : NSObject <ImagePathHelperProtocol>

- (instancetype)init NS_UNAVAILABLE;
- (instancetype)initWithDomain:(NSString*)domain NS_DESIGNATED_INITIALIZER;
- (NSURL*)buildUrlForImage:(id<ImageProtocol>)image sizeType:(ImageSize)sizeType;
- (NSURL*)buildUrlForImagePath:(NSString*)path withHash:(NSString*)hash sizeType:(ImageSize)sizeType;
- (NSString*)getSizeString:(ImageSize)sizeType;

@end

Becomes:

@objc class RSImagePathHelper : NSObject, RSImagePathHelperProtocol {
    internal init() { }
    init(domain: String)
    func buildUrl(image image: ImageProtocol, sizeType: RSImageSize) -> NSURL
    func buildUrl(imagePath imagePath path: String, withHash hash: String, sizeType: RSImageSize) -> NSURL
    func getSizeString(sizeType: RSImageSize) -> String
}

Note the following:

• That initializers are not functions, although they look them, so no need for the func statement. Also, you can make a designated initializer publicly unavailable by marking it private or internal.

• The ForImage and ForImagePath were replaced with named external parameters by specifying the