iOS NSData与NSString相互转换

在平常开发中，经常会遇到过NSData与NSString相关转换的问题，一般都是用：

NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
NSString *string = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

但是在实际开发过程中会遇到至少如下三种NSData与NSString转换

一、字符编码转换

NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
NSString *string = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

上述是最常用的UTF8编码转换，实际上有很多编码，但在绝大多数情况下我们都是使用UTF8编码格式。
具体有哪些编码可以参考：NSStringEncoding 这个枚举值。

二、Base64编码转换

NSString *string = [data base64EncodedStringWithOptions:NSDataBase64Encoding64CharacterLineLength];
NSData *data = [[NSData alloc] initWithBase64EncodedString:string options:NSDataBase64DecodingIgnoreUnknownCharacters];

三、Hex编码

- (NSString *)stringFromData:(NSData *)data {
    Byte *bytes = (Byte *)[data bytes];
    NSString *string = @"";
    for (NSInteger i = 0; i<data.length; i++) {
        NSString *newHexStr = [NSString stringWithFormat:@"%x", bytes[i] & 0xff]; //16进制数
        newHexStr = [newHexStr uppercaseString];
        if ([newHexStr length] == 1) {
            newHexStr = [NSString stringWithFormat:@"0%@",newHexStr];
        }
        string = [string stringByAppendingString:newHexStr];
    }
    return string;
}

- (NSData *)dataFromString:(NSString *)string {
    const char *ch = [[string 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;
}