Objective-C 类别导致无法识别的选择器

Objective-C 类别是一种非常强大的特性，它允许我们为已有的类增加方法，从而满足特定需求。然而，有时候在使用类别时可能会遇到一些问题，其中之一就是无法识别的选择器。

objective-c
#import 
@interface NSString (CustomCategory)
- (NSInteger)length;
@end
@implementation NSString (CustomCategory)
- (NSInteger)length {
    return 10;
}
@end
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSString *str = @"Hello, World!";
        NSInteger len = [str length];
        NSLog(@"Length: %ld", len);
    }
    return 0;
}

objective-c
#import 
@interface NSString (CustomCategory)
- (NSString *)substringFromIndex:(NSInteger)fromIndex;
@end
@implementation NSString (CustomCategory)
- (NSString *)substringFromIndex:(NSInteger)fromIndex {
    return @"Custom Category";
}
@end
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSString *str = @"Hello, World!";
        NSString *subStr = [str substringFromIndex:7];
        NSLog(@"Substring: %@", subStr);
    }
    return 0;
}

objective-c
#import 
@interface NSString (CustomCategory)
- (NSString *)cc_substringFromIndex:(NSInteger)fromIndex;
@end
@implementation NSString (CustomCategory)
- (NSString *)cc_substringFromIndex:(NSInteger)fromIndex {
    return @"Custom Category";
}
@end
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSString *str = @"Hello, World!";
        NSString *subStr = [str cc_substringFromIndex:7];
        NSLog(@"Substring: %@", subStr);
    }
    return 0;
}