iOS-Emoji转换成NSString

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Wechat的Emoji消息在iOS系统上显示。

Wechat里面对Emoji的消息处理为：

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<span class=\"emoji emoji1f319\"></span>
<span class=\"emoji emoji2b50\"></span>

为了在iOS上显示，必须将其转换为字符串：

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"\U0001f319"
"\U00002b50"

转换方式如下：

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+ (NSString*)replaceEmojiChars:(NSString*)string
{
    while(string != nil && string.length > 0){
        NSString *base = @"\U0001F000";
        NSString *prefix = @"<span class=\"emoji emoji", *postfix = @"\"></span>";
        NSRange preRange = [string rangeOfString:prefix], postRange = [string rangeOfString:postfix];
        if(preRange.length != 0 && postRange.length != 0){
            unsigned int low = 0;
            unichar chars[2];
            [base getCharacters:chars];
            NSString *replaceString = nil;
            if(postRange.location - preRange.location - preRange.length > 4){
                NSString *lowString = [string substringWithRange:NSMakeRange(postRange.location - 3, 3)];
                [[NSScanner scannerWithString:[@"0x" stringByAppendingString:lowString]] scanHexInt:&low];
                chars[0] += low / 1024;
                chars[1] += low % 1024;
                replaceString = [[NSString alloc] initWithCharacters:chars length:2];
            }else{
                NSString *lowString = [string substringWithRange:NSMakeRange(preRange.location + preRange.length, postRange.location - preRange.location - preRange.length)];
                [[NSScanner scannerWithString:[@"0x" stringByAppendingString:lowString]] scanHexInt:&low];
                chars[0] = low;
                replaceString = [[NSString alloc] initWithCharacters:chars length:1];
            }
            string = [string stringByReplacingCharactersInRange:NSMakeRange(preRange.location, postRange.location - preRange.location + postRange.length) withString:replaceString];
        }else{
            break;
        }
    }
    return string;
}

核心思想为：一个Emoji表情如果是5位的，例如”1F400”，则用两个unichar表示，如果是4位以下的，则用一个unichar表示。对于5位的，以@”\U0001F000”为基准，先转为十进制，上位加上1024的除数，下位加上1024的余数，对于4位的，直接加上数字即可。