避免unicode字符被截断的方法

NSString *str = @"👴🏻👮🏽";
NSRange range = NSMakeRange(2, str.length - 2);
NSString *subStr = [str substringWithRange:range];

这里的str.length = 8，因为字符串是unicode格式，一个字符是4个字节组合表示的。

- (NSString *)utf8ToUnicode:(NSString *)string{
    
    NSUInteger length = [string length];
    NSMutableString *str = [NSMutableString stringWithCapacity:0];
    for (int i = 0;i < length; i++){
        NSMutableString *s = [NSMutableString stringWithCapacity:0];
        unichar _char = [string characterAtIndex:i];
        // 判断是否为英文和数字
        if (_char <= '9' && _char >='0'){
            [s appendFormat:@"%@",[string substringWithRange:NSMakeRange(i,1)]];
        }else if(_char >='a' && _char <= 'z'){
            [s appendFormat:@"%@",[string substringWithRange:NSMakeRange(i,1)]];
        }else if(_char >='A' && _char <= 'Z')
        {
            [s appendFormat:@"%@",[string substringWithRange:NSMakeRange(i,1)]];
        }else{
            // 中文和字符
            [s appendFormat:@"\\u%x",[string characterAtIndex:i]];
            // 不足位数补0 否则解码不成功
            if (s.length == 4) {
                [s insertString:@"00" atIndex:2];
            } else if (s.length == 5) {
                [s insertString:@"0" atIndex:2];
            }
        }
        [str appendFormat:@"%@", s];
    }
    return str;
}

NSString *strB = [self utf8ToUnicode:str];

将中文转成unicode形式，strB = @"\ud83d\udc74\ud83c\udffb\ud83d\udc6e\ud83c\udffd";

那subStr=@"\ud83c\udffb\ud83d\udc6e\ud83c\udffd";但是因为被截断后，打印出来变成了🏻👮🏽，而如果range.location从1开始，出现都是unicode形式的字符串，因为被截断后没有对应的组合字符串可以显示。可以利用循环打印下结果

for (int i = 0; i < str.length; i++) {
        NSRange range = NSMakeRange(i, str.length - i);
        NSString *temp = [str substringWithRange:range];
        NSLog(@"temp = %@", temp);
}

/*
     temp = 👴🏻👮🏽
     temp = \udc74\ud83c\udffb\ud83d\udc6e\ud83c\udffd
     temp = 🏻👮🏽
     temp = \udffb\ud83d\udc6e\ud83c\udffd
     temp = 👮🏽
     temp = \udc6e\ud83c\udffd
     temp = 🏽
     temp = \udffd
*/

结果可能不是我们想要的，我们如果想要截断后的字符为显示字符的子集，也就是👴🏻或👮🏽或者👴🏻👮🏽。那么可以使用

rangeOfComposedCharacterSequencesForRange:调整range，防止有效的unicode字符被截断成无效字符(无显示意义)。请看下面

for (int i = 0; i < str.length; i++) {
        NSRange range = NSMakeRange(i, str.length - i);
        range = [str rangeOfComposedCharacterSequencesForRange:range];
        NSString *temp = [str substringWithRange:range];
        NSLog(@"temp = %@", temp);
}

/*
     temp = 👴🏻👮🏽
     temp = 👴🏻👮🏽
     temp = 👴🏻👮🏽
     temp = 👴🏻👮🏽
     temp = 👮🏽
     temp = 👮🏽
     temp = 👮🏽
     temp = 👮🏽
*/