Swift ambiguous methods with variadic parameter

Question

I have the following method

static func t(key: String, params: AnyObject...) -> String{
    let string = .......
    if (params.count == 0){
        return string
    } else {
        return String(format: string, params)
    }
}

The problem is that i need to make this method available in Objective C which is impossible with variadic params. What I tried to do is create another method for objective c where params is an array. But then I also need to have third one for when there is no params.

@objc static func t(key: String, params: [AnyObject]) -> String
@objc static func t(key: String) -> String

But now swift complains that method t is ambiguous because 1st and 3rd method can take just the key.

How to make this work? I know I could use different names, but I would like to keep things consistent.

UPDATE: Another problem is that I can't call String(format:...) correctly once inside the function since params is an array and not a group of params. Any nice way to solve this?

Answer 1

You can do the following:

@objc(C)
public class C : NSObject {

    public static func t(key: String, params firstParam: AnyObject, _ params: AnyObject...) -> String {
        return t(key, params: [firstParam] + params)
    }

    @objc
    public static func t(key: String, params: [AnyObject]) -> String {
        return "TODO"
    }

    @objc
    static func t(key: String) -> String {
        return t(key, params: [])
    }
}

... calling from Swift:

C.t("")
C.t("", params: 1)
C.t("", params: 1, 2)
C.t("", params: [1, 2, 3])

This is bridged as follows:

SWIFT_CLASS_NAMED("C")
@interface C : NSObject
+ (NSString * _Nonnull)t:(NSString * _Nonnull)key params:(NSArray * _Nonnull)params;
+ (NSString * _Nonnull)t:(NSString * _Nonnull)key;
- (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
@end

And can be messaged as follows:

#import "DarkSide.h"
#import "ObjCInterOp-Swift.h"

@implementation DarkSide

- (instancetype)init
{
    self = [super init];
    if (self) {
        [C t:@"" params:@[]];
        [C t:@""];
    }
    return self;
}
@end

Regarding the update to the question, there is a version of the formatted String initialiser that takes [CVarArgType] rather than CVarArgType...:

func f(format format: String, _ params: [CVarArgType]) -> String {
    return String(format: format, arguments: params)
}

f(format: "%d %d %d", [1, 2, 3]) // "1 2 3"

Only CVarArgType cannot be represented in Objective C, which really complicates things. In fact, if you really must use String.init(format: String, arguments: [CVarArgType]) and get to it from Objective C then I don't see at the moment how to avoid casting from AnyObjects to CVarArgTypes along the lines of the following murky code:

@objc(C)
public class C : NSObject {

    public static func t(key: String, params: CVarArgType...) -> String {
        return t(key, params: params)
    }

    public static func t(key: String, params: [CVarArgType]) -> String {
        return String(format: key, arguments: params)
    }

    @objc(t:params:)
    public static func t_objc(key: String, params: [AnyObject]) -> String {
        let args = params.map{
            // here you'll probably need to parse `key` so as to 
            // know what to cast into!
            ($0 as! NSNumber).integerValue as CVarArgType
        }
        return t(key, params: args)
    }

    @objc(t:)
    static func t_objc(key: String) -> String {
        return t(key)
    }
}

My suggestion is to either wean off things like String.init(format: String, arguments: [CVarArgType]), or to implement the base method in Objective C...