iOS开发中，比较蛋疼的一个问题，NSData转UTF-8格式的NSString，有时候会返回nil。

究其原因，无非就是：UTF-8的字符中混进了其他编码格式的字符，这样NSData转NSString的时候，就是返回nil

而现在网上的方法基本就这几个：

http://blog.csdn.net/cuibo1123/article/details/40938225
http://blog.csdn.net/xocom/article/details/50905578
http://www.cnblogs.com/xiao-love-meng/p/5757564.html

这几个方法都不完美，都不能完全解决我遇到的问题。

我的解决方法：

查了UTF-8的wiki，utf-8的编码格式如下，理论上可以到6个字节，但之用到了4个字节。

utf-8的编码规则

其中，有一些特殊字节是不会出现在utf-8中的：

utf-8中不会出现的字节

其中，还指出了判断utf-8的方法：

UTF-8字符串可以由一个简单的算法可靠地识别出来。就是，一个字符串在任何其它编码中表现为合法的UTF-8的可能性很低，并随字符串长度增长而减小。举例说，字符值C0,C1,F5至FF从来没有出现。为了更好的可靠性，可以使用正则表达式来统计非法过长和替代值（可以查看W3 FAQ: Multilingual Forms上的验证UTF-8字符串的正则表达式）。

$field =~
  m/\A(
     [\x09\x0A\x0D\x20-\x7E]            # ASCII
   | [\xC2-\xDF][\x80-\xBF]             # non-overlong 2-byte
   |  \xE0[\xA0-\xBF][\x80-\xBF]        # excluding overlongs
   | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}  # straight 3-byte
   |  \xED[\x80-\x9F][\x80-\xBF]        # excluding surrogates
   |  \xF0[\x90-\xBF][\x80-\xBF]{2}     # planes 1-3
   | [\xF1-\xF3][\x80-\xBF]{3}          # planes 4-15
   |  \xF4[\x80-\x8F][\x80-\xBF]{2}     # plane 16
  )*\z/x;

因此，我们只要把上面的代码，转换成OC语言就行了：
这里我直接写了一个Category：

//
// Created by WDY on 2016/11/24.
// Copyright (c) 2016 andforce. All rights reserved.
//

#import "NSData+UTF8.h"

@implementation NSData (UTF8)

- (NSString *)utf8String {
    NSString *string = [[NSString alloc] initWithData:self encoding:NSUTF8StringEncoding];
    if (string == nil) {
        string = [[NSString alloc] initWithData:[self UTF8Data] encoding:NSUTF8StringEncoding];
    }
    return string;
}

//              https://zh.wikipedia.org/wiki/UTF-8
//              https://www.w3.org/International/questions/qa-forms-utf-8
//
//            $field =~
//                    m/\A(
//            [\x09\x0A\x0D\x20-\x7E]            # ASCII
//            | [\xC2-\xDF][\x80-\xBF]             # non-overlong 2-byte
//            |  \xE0[\xA0-\xBF][\x80-\xBF]        # excluding overlongs
//            | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}  # straight 3-byte
//            |  \xED[\x80-\x9F][\x80-\xBF]        # excluding surrogates
//            |  \xF0[\x90-\xBF][\x80-\xBF]{2}     # planes 1-3
//            | [\xF1-\xF3][\x80-\xBF]{3}          # planes 4-15
//            |  \xF4[\x80-\x8F][\x80-\xBF]{2}     # plane 16
//            )*\z/x;

- (NSData *)UTF8Data {
    //保存结果
    NSMutableData *resData = [[NSMutableData alloc] initWithCapacity:self.length];

    NSData *replacement = [@"�" dataUsingEncoding:NSUTF8StringEncoding];

    uint64_t index = 0;
    const uint8_t *bytes = self.bytes;

    long dataLength = (long) self.length;

    while (index < dataLength) {
        uint8_t len = 0;
        uint8_t firstChar = bytes[index];

            // 1个字节
        if ((firstChar & 0x80) == 0 && (firstChar == 0x09 || firstChar == 0x0A || firstChar == 0x0D || (0x20 <= firstChar && firstChar <= 0x7E))) {
            len = 1;
        }
            // 2字节
        else if ((firstChar & 0xE0) == 0xC0 && (0xC2 <= firstChar && firstChar <= 0xDF)) {
            if (index + 1 < dataLength) {
                uint8_t secondChar = bytes[index + 1];
                if (0x80 <= secondChar && secondChar <= 0xBF) {
                    len = 2;
                }
            }
        }
            // 3字节
        else if ((firstChar & 0xF0) == 0xE0) {
            if (index + 2 < dataLength) {
                uint8_t secondChar = bytes[index + 1];
                uint8_t thirdChar = bytes[index + 2];
                
                if (firstChar == 0xE0 && (0xA0 <= secondChar && secondChar <= 0xBF) && (0x80 <= thirdChar && thirdChar <= 0xBF)) {
                    len = 3;
                } else if (((0xE1 <= firstChar && firstChar <= 0xEC) || firstChar == 0xEE || firstChar == 0xEF) && (0x80 <= secondChar && secondChar <= 0xBF) && (0x80 <= thirdChar && thirdChar <= 0xBF)) {
                    len = 3;
                } else if (firstChar == 0xED && (0x80 <= secondChar && secondChar <= 0x9F) && (0x80 <= thirdChar && thirdChar <= 0xBF)) {
                    len = 3;
                }
            }
        }
            // 4字节
        else if ((firstChar & 0xF8) == 0xF0) {
            if (index + 3 < dataLength) {
                uint8_t secondChar = bytes[index + 1];
                uint8_t thirdChar = bytes[index + 2];
                uint8_t fourthChar = bytes[index + 3];
                
                if (firstChar == 0xF0) {
                    if ((0x90 <= secondChar & secondChar <= 0xBF) && (0x80 <= thirdChar && thirdChar <= 0xBF) && (0x80 <= fourthChar && fourthChar <= 0xBF)) {
                        len = 4;
                    }
                } else if ((0xF1 <= firstChar && firstChar <= 0xF3)) {
                    if ((0x80 <= secondChar && secondChar <= 0xBF) && (0x80 <= thirdChar && thirdChar <= 0xBF) && (0x80 <= fourthChar && fourthChar <= 0xBF)) {
                        len = 4;
                    }
                } else if (firstChar == 0xF3) {
                    if ((0x80 <= secondChar && secondChar <= 0x8F) && (0x80 <= thirdChar && thirdChar <= 0xBF) && (0x80 <= fourthChar && fourthChar <= 0xBF)) {
                        len = 4;
                    }
                }
            }
        }
            // 5个字节
        else if ((firstChar & 0xFC) == 0xF8) {
            len = 0;
        }
            // 6个字节
        else if ((firstChar & 0xFE) == 0xFC) {
            len = 0;
        }

        if (len == 0) {
            index++;
            [resData appendData:replacement];
        } else {
            [resData appendBytes:bytes + index length:len];
            index += len;
        }
    }

    return resData;
}
@end