Swift:

import Foundation

extension String{
    
    func emojiStr() -> String {
        //1.在一段字符串中查找十六进制的字符串
        let scanner = NSScanner(string: self)
        //2.将查找的字符串转换为十六进制的数字
        var value: UInt32 = 0
        scanner.scanHexInt(&value)
        //3.将十六进制的数字转化为 unicode字符
        let charCode = Character(UnicodeScalar(value))
        //4.将uniconde字符转换 字符串
        
        return "\(charCode)"
        
    }
}

Objective-C:

.h

#import <Foundation/Foundation.h>

@interface NSString (Emoji)
/**
 *  将十六进制的编码转为emoji字符
 */
+ (NSString *)emojiWithIntCode:(long)intCode;

/**
 *  将十六进制的编码转为emoji字符
 */
+ (NSString *)emojiWithStringCode:(NSString *)stringCode;
- (NSString *)emoji;

/**
 *  是否为emoji字符
 */
- (BOOL)isEmoji;
@end

.m

#import "NSString+Emoji.h"
#define EmojiCodeToSymbol(c) ((((0x808080F0 | (c & 0x3F000) >> 4) | (c & 0xFC0) << 10) | (c & 0x1C0000) << 18) | (c & 0x3F) << 24)

@implementation NSString (Emoji)

+ (NSString *)emojiWithIntCode:(long)intCode {
    NSString * s = [NSString stringWithFormat:@"%ld",intCode];
    int symbol = EmojiCodeToSymbol([s intValue]);
    NSString *string = [[NSString alloc] initWithBytes:&symbol length:sizeof(symbol) encoding:NSUTF8StringEncoding];
    if (string == nil) { // 新版Emoji
        string = [NSString stringWithFormat:@"%C", (unichar)intCode];
    }
    return string;
}

- (NSString *)emoji
{
    return [NSString emojiWithStringCode:self];
}

+ (NSString *)emojiWithStringCode:(NSString *)stringCode
{
    char *charCode = (char *)stringCode.UTF8String;
    long intCode = strtol(charCode, NULL, 16);
    return [self emojiWithIntCode:intCode];
}

// 判断是否是 emoji表情
- (BOOL)isEmoji
{
     BOOL returnValue = NO;
         
     const unichar hs = [self characterAtIndex:0];
     // surrogate pair
     if (0xd800 <= hs && hs <= 0xdbff) {
         if (self.length > 1) {
             const unichar ls = [self characterAtIndex:1];
             const int uc = ((hs - 0xd800) * 0x400) + (ls - 0xdc00) + 0x10000;
             if (0x1d000 <= uc && uc <= 0x1f77f) {
                 returnValue = YES;
             }
         }
     } else if (self.length > 1) {
         const unichar ls = [self characterAtIndex:1];
         if (ls == 0x20e3) {
             returnValue = YES;
         }
     } else {
         // non surrogate
         if (0x2100 <= hs && hs <= 0x27ff) {
             returnValue = YES;
         } else if (0x2B05 <= hs && hs <= 0x2b07) {
             returnValue = YES;
         } else if (0x2934 <= hs && hs <= 0x2935) {
             returnValue = YES;
         } else if (0x3297 <= hs && hs <= 0x3299) {
             returnValue = YES;
         } else if (hs == 0xa9 || hs == 0xae || hs == 0x303d || hs == 0x3030 || hs == 0x2b55 || hs == 0x2b1c || hs == 0x2b1b || hs == 0x2b50) {
             returnValue = YES;
         }
     }
    
    return returnValue;
}
@end