Objective C - Category No Visible selector @removeWhiteSpaceStringWithString

so im trying to add new method for testing using Category from NSString, but some how i must declared like this with following step:

1. Create Category from NSString with name StringExtension so it will be NSString+StringExtension, after that i declared my own methos that return type is String

2. so after i define in NSString+StringExtension @interface and @implementation, i tried in my viewController to called it, but first i import the class NSString+StringExtension

3. after that i do like this

   NSString *testString = @"as d a s d";
   NSLog(@"===== %@", [testString removeWhiteSpaceStringWithString:testString]);
   

4. and it says

   No visible @interface for 'NSString' declares the selector 'removeWhiteSpaceStringWithString:'

the question is, why it cannot use like that? i already search and see tutorial doing like that and its possible, but why i'm not able to do that?

so i found this way, but i don't know is this the correct code to use?

NSLog(@"===== %@", [[testString class] removeWhiteSpaceStringWithString:testString]);

anyone have the same case like i am?

Based upon what you have shared with us, it would appear that you defined a class method (with +). It should be an instance method (with -) and then you don’t need the parameter, either. You can simply reference self.

For example:

//  NSString+Whitespace.h

@import Foundation;

NS_ASSUME_NONNULL_BEGIN

@interface NSString (Whitespace)

- (NSString *)stringByRemovingWhitespace;

@end

NS_ASSUME_NONNULL_END

And

//  NSString+Whitespace.m

#import "NSString+Whitespace.h"

@implementation NSString (Whitespace)

- (NSString *)stringByRemovingWhitespace {
    return [self stringByReplacingOccurrencesOfString:@"\\s+"
                                           withString:@""
                                              options:NSRegularExpressionSearch
                                                range:NSMakeRange(0, self.length)];
}

@end

Then you can do:

NSString *testString = @"as d a s d";
NSLog(@"===== %@", [testString stringByRemovingWhitespace]);  // ===== asdasd

Obviously, do whatever you want in your implementation, but it illustrates the idea, that you want an instance method and you do not need to pass the string again as a parameter.