How to call an Objective-C category method in Swift

Question

How would you call an Objective-C category method like this in Swift?

+(UIColor*)colorWithHexString:(NSString*)hex alpha:(float)alpha;

Answer 1

The compiler automatically looks for common ObjC naming patterns and substitutes Swift patterns in their place. An ObjC class method that returns an instance of the class (and is named a certain way, it looks like) gets turned into a Swift convenience initializer.

If you have the ObjC method (defined by a custom category):

 + (UIColor *)colorWithHexString:(NSString *)hex alpha:(float)alpha;

The compiler generates the Swift declaration:

convenience init(hexString: String?, alpha: CFloat)

And you call it like this:

let color = UIColor(hexString: "#ffffff", alpha: 1.0)

---

And in Swift 2.0 or later, you can use the NS_SWIFT_NAME macro to make ObjC factory methods that don't match the naming pattern import to Swift as initializers. e.g.:

@interface UIColor(Hex)
+ (UIColor *)hexColorWithString:(NSString *)string
NS_SWIFT_NAME(init(hexString:));
@end

// imports as
extension UIColor {
    init(hexString: String)
}