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 = ??*/