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

這裏的str.length = 8，因為字符串是unicode格式，一個字符是4個字節組合表示的。

- (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];
        // 判斷是否為英文和數字
        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]];
            // 不足位數補0 否則解碼不成功
            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];

將中文轉成unicode形式，strB = @"\ud83d\udc74\ud83c\udffb\ud83d\udc6e\ud83c\udffd";

那subStr=@"\ud83c\udffb\ud83d\udc6e\ud83c\udffd";但是因為被截斷後，打印出來變成了??????，而如果range.location從1開始，出現都是unicode形式的字符串，因為被截斷後沒有對應的組合字符串可以顯示。可以利用循環打印下結果

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
*/

結果可能不是我們想要的，我們如果想要截斷後的字符為顯示字符的子集，也就是????或????或者????????。那麽可以使用

rangeOfComposedCharacterSequencesForRange:調整range，防止有效的unicode字符被截斷成無效字符(無顯示意義)。請看下面

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 = ????
*/

避免unicode字符被截斷的方法