how to use a static method in objective-c defined in extension

Question

I have a static method created on extension of Date. I am trying to call this method in Objective-C file. I am getting error as "No Known class for selector method name".

More detail:

In SWIFT 3.0

extension Date {
     static func  getDefaultFormattedDateStringFromDate(_ validToDate:Date) -> String? {

    }
}

In Objective-C

NSString *dateString = [NSDate getDefaultFormattedDateTimeStringFromDate:[NSDate date]];

Answer 1

struct Date is the "Swift overlay type" to the Foundation type NSDate (compare Swift 3: Date vs NSDate?) and unknown to Objective-C.

You'll have to define an extension to NSDate in order to be useable from Objective-C:

extension NSDate {
    static func  getDefaultFormattedDateStringFromDate(_ validToDate: Date) -> String? {
        // ...
    }
}

This is imported to Objective-C as

 @interface NSDate
 + (NSString * _Nullable)getDefaultFormattedDateStringFromDate:(NSDate * _Nonnull)validToDate;
 @end

Note that the parameter and return types (Date, String) are automatically bridged to the Foundation types (NSDate, NSString).

---

All this is unrelated to static (type) vs instance methods, and in your case it might be more "natural" to define an instance method:

extension NSDate {
    func defaultFormattedString() -> String? {
        // ...
    }
}

which is imported to Objective-C as

@interface NSDate
- (NSString * _Nullable)defaultFormattedString;
@end

and called as

NSString *dateString = [[NSDate date] defaultFormattedString];