Objective c / How do I call a user-implemented static function into the nsstring class?

Question

+ (NSString *)dictionaryToJson:(NSDictionary *)dic
{
    NSString *result = @"";
    NSError *err;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dic options:NSJSONWritingPrettyPrinted error:&err];
    
    if(! jsonData) {
        NSLog(@"Error: %@", err);
    } else {
        NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
        result = jsonString;
    }
    
    return result;
}

The functions that we define here are

[body appendData:[[NSString dictionaryToJson:jsonBody] dataUsingEncoding:NSUTF8StringEncoding]];

I don't think you understood the function call from objc yet. Please help me.

Answer 1

here you'd typically extend the class using a category. See e.g. https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html

// Header file
// Something like NSString+JSONAdditions.h
@interface NSString (JSONAdditions)

+ (NSString *)dictionaryToJson:(NSDictionary *)dic;

@end

// Implementation file
// Something like NSString+JSONAdditions.m
@implementation NSString (JSONAdditions)

+ (NSString *)dictionaryToJson:(NSDictionary *)dic
{
    NSString *result = @"";
    NSError *err;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dic options:NSJSONWritingPrettyPrinted error:&err];

    if(! jsonData) {
        NSLog(@"Error: %@", err);
    } else {
        NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
        result = jsonString;
    }

    return result;
}

@end

Now you can use the code as in the snippet you provided. FWIW in Xcode do this using

File -> New -> File ... -> Objective-C file