实际开发当中存在一种情况：NSData转UTF-8的NSString有的时候会返回nil，原因就是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
那么我们怎么解决这一现象呢？以下是其代码实现过程：

NSData+LGJExtension.h

#import <Foundation/Foundation.h>

@interface NSData (LGJExtension)

/**
 NSData转化成string

 @return 返回nil的解决方案
 */
-(NSString *)convertedToUtf8String;

@end

NSData+LGJExtension.m

#import "NSData+LGJExtension.h"
@implementation NSData (LGJExtension)

- (NSString *)convertedToUtf8String{
    NSString *string = [[NSString alloc] initWithData:self encoding:NSUTF8StringEncoding];
    if (string == nil) {
        string = [[NSString alloc] initWithData:[self UTF8Data] encoding:NSUTF8StringEncoding];
    }
    return string;
}
- (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