将字符串转换为二进制data类型

//将字符串转换成data

• (NSData)dataForHexString:(NSString)hexString{
  if (hexString == nil) {
  return nil;
  }
  const char* ch = [[hexString lowercaseString] cStringUsingEncoding:NSUTF8StringEncoding];
  NSMutableData* data = [NSMutableData data];

  while (ch) {
  if (ch == ' ') {
  continue;
  }
  char byte = 0;

    if ('0' <= *ch && *ch <= '9') {
        byte = *ch - '0';
    }
    else if ('a' <= *ch && *ch <= 'f') {
        
        byte = *ch - 'a' + 10;
    }
    else if ('A' <= *ch && *ch <= 'F') {
        
        byte = *ch - 'A' + 10;
    }
    ch++;
    byte = byte << 4;
    
    if (*ch) {
        
        if ('0' <= *ch && *ch <= '9') {
            
            byte += *ch - '0';
            
        } else if ('a' <= *ch && *ch <= 'f') {
            
            byte += *ch - 'a' + 10;
        }
        else if('A' <= *ch && *ch <= 'F')
        {
            byte += *ch - 'A' + 10;
        }
        ch++;
    }
    [data appendBytes:&byte length:1];
  

  }
  return data;

}