Emoji是我们常见的在文字中插入的图形符号，在iOS中可以实现程序自动实现的方式实现此功能；即可在需要展示的文字中添加Emoji表情或者在应用通知中添加。

按照现在理解，其实Emoij也就是一种编码而已。使用一般会按照苹果的使用规则和编码方式进行相应的Emoij对应。

实现代码(.h/.m)如下：

NSString+Emoji.m实现代码:
#import "NSString+Emoji.h"
@implementation NSString (Emoji)

static NSDictionary * s_unicodeToCheatCodes = nil;
static NSDictionary * s_cheatCodesToUnicode = nil;

+ (void)initializeEmojiCheatCodes
{
    // 下面的NSDictionary展示的部分Emoji代码
    // 详细Emoji代码可见，已封装完成的自定义表情NSString扩展类中
    NSDictionary *forwardMap = @{
                                 @"😄": @":smile:",
                                 @"😆": @[@":laughing:", @":D"],
                                 @"😊": @":blush:",
                                 @"😃": @[@":smiley:", @":)", @":-)"],
                                 @"☺": @":relaxed:",
                                 @"😏": @":smirk:",
                                 @"😞": @[@":disappointed:", @":("],
                                 @"😍": @":heart_eyes:"
                                 };
NSMutableDictionary *reversedMap = [NSMutableDictionary dictionaryWithCapacity:[forwardMap count]];
    [forwardMap enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
        if ([obj isKindOfClass:[NSArray class]]) {
            for (NSString *object in obj) {
                [reversedMap setObject:key forKey:object];
            }
        } else {
            [reversedMap setObject:key forKey:obj];
        }
    }];

    @synchronized(self) {
        s_unicodeToCheatCodes = forwardMap;
        s_cheatCodesToUnicode = [reversedMap copy];
    }
}

/*  方法使用示例：对照Emoji表情代码
*   Example: 
*    "This is a smiley face :smiley:"
*   Will be replaced with:
*    "This is a smiley face \U0001F604"
*/

- (NSString *)stringByReplacingEmojiCheatCodesWithUnicode
{
    if (!s_cheatCodesToUnicode) {
        [NSString initializeEmojiCheatCodes];
    }
    
    if ([self rangeOfString:@":"].location != NSNotFound) {
        __block NSMutableString *newText = [NSMutableString stringWithString:self];
        [s_cheatCodesToUnicode enumerateKeysAndObjectsUsingBlock:^(NSString *key, id obj, BOOL *stop) {
            [newText replaceOccurrencesOfString:key withString:obj options:NSLiteralSearch range:NSMakeRange(0, newText.length)];
        }];
        return newText;
    }
    
    return self;
}

/*  Example:
     "This is a smiley face \U0001F604"
    Will be replaced with:
     "This is a smiley face :smiley:"
 */
- (NSString *)stringByReplacingEmojiUnicodeWithCheatCodes
{
    if (!s_cheatCodesToUnicode) {
        [NSString initializeEmojiCheatCodes];
    }
    
    if (self.length) {
        __block NSMutableString *newText = [NSMutableString stringWithString:self];
        [s_unicodeToCheatCodes enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
            NSString *string = ([obj isKindOfClass:[NSArray class]] ? [obj firstObject] : obj);
            [newText replaceOccurrencesOfString:key withString:string options:NSLiteralSearch range:NSMakeRange(0, newText.length)];
        }];
        return newText;
    }
    return self;
}
@end
NSString+Emoji.h实现调用代码:
    Example: 
    // 加入Emoji表情
    NSString *smileEmojiStr = @":blush:"; // 写出此代码对照表情
    NSString *str = @"后接表情";
    NSString *emojiStr = [str stringByReplacingEmojiCheatCodesWithUnicode];