afnetworking中的URLConnectionOperation为什么要实现NSCoding, NSCopying协议

在看afnetworking的源码，
发现在AFURLConnectionOperation中实现了NSCoding, NSCopying这两个协议，实现这两个协议的作用是什么？
在什么情况下会用到这些方法？
版本是2.6.1

#pragma mark - NSCoding

- (id)initWithCoder:(NSCoder *)aDecoder {
    NSURLRequest *request = [aDecoder decodeObjectForKey:@"request"];
    
    self = [self initWithRequest:request];
    if (!self) {
        return nil;
    }

    self.state = [aDecoder decodeIntegerForKey:@"state"];
    self.cancelled = [aDecoder decodeBoolForKey:@"isCancelled"];
    self.response = [aDecoder decodeObjectForKey:@"response"];
    self.error = [aDecoder decodeObjectForKey:@"error"];
    self.responseData = [aDecoder decodeObjectForKey:@"responseData"];
    self.totalBytesRead = [[aDecoder decodeObjectForKey:@"totalBytesRead"] longLongValue];
    
    return self;
}

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [self pause];
    
    [aCoder encodeObject:self.request forKey:@"request"];

    switch (self.state) {
        case AFOperationExecutingState:
        case AFOperationPausedState:
            [aCoder encodeInteger:AFOperationReadyState forKey:@"state"];
            break;
        default:
            [aCoder encodeInteger:self.state forKey:@"state"];
            break;
    }
    
    [aCoder encodeBool:[self isCancelled] forKey:@"isCancelled"];
    [aCoder encodeObject:self.response forKey:@"response"];
    [aCoder encodeObject:self.error forKey:@"error"];
    [aCoder encodeObject:self.responseData forKey:@"responseData"];
    [aCoder encodeObject:[NSNumber numberWithLongLong:self.totalBytesRead] forKey:@"totalBytesRead"];
}

#pragma mark - NSCopying

- (id)copyWithZone:(NSZone *)zone {
    AFURLConnectionOperation *operation = [[[self class] allocWithZone:zone] initWithRequest:self.request];
            
    operation.uploadProgress = self.uploadProgress;
    operation.downloadProgress = self.downloadProgress;
    operation.authenticationAgainstProtectionSpace = self.authenticationAgainstProtectionSpace;
    operation.authenticationChallenge = self.authenticationChallenge;
    operation.cacheResponse = self.cacheResponse;
    operation.redirectResponse = self.redirectResponse;

    return operation;
}

即使是新的3.0版本
在AFHTTPSessionManager里也是实现了这两个协议

- (instancetype)initWithCoder:(NSCoder *)decoder {
    NSURL *baseURL = [decoder decodeObjectOfClass:[NSURL class] forKey:NSStringFromSelector(@selector(baseURL))];
    NSURLSessionConfiguration *configuration = [decoder decodeObjectOfClass:[NSURLSessionConfiguration class] forKey:@"sessionConfiguration"];
    if (!configuration) {
        NSString *configurationIdentifier = [decoder decodeObjectOfClass:[NSString class] forKey:@"identifier"];
        if (configurationIdentifier) {
#if (defined(__IPHONE_OS_VERSION_MIN_REQUIRED) && __IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1100)
            configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:configurationIdentifier];
#else
            configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:configurationIdentifier];
#endif
        }
    }

    self = [self initWithBaseURL:baseURL sessionConfiguration:configuration];
    if (!self) {
        return nil;
    }

    self.requestSerializer = [decoder decodeObjectOfClass:[AFHTTPRequestSerializer class] forKey:NSStringFromSelector(@selector(requestSerializer))];
    self.responseSerializer = [decoder decodeObjectOfClass:[AFHTTPResponseSerializer class] forKey:NSStringFromSelector(@selector(responseSerializer))];
    AFSecurityPolicy *decodedPolicy = [decoder decodeObjectOfClass:[AFSecurityPolicy class] forKey:NSStringFromSelector(@selector(securityPolicy))];
    if (decodedPolicy) {
        self.securityPolicy = decodedPolicy;
    }

    return self;
}

- (void)encodeWithCoder:(NSCoder *)coder {
    [super encodeWithCoder:coder];

    [coder encodeObject:self.baseURL forKey:NSStringFromSelector(@selector(baseURL))];
    if ([self.session.configuration conformsToProtocol:@protocol(NSCoding)]) {
        [coder encodeObject:self.session.configuration forKey:@"sessionConfiguration"];
    } else {
        [coder encodeObject:self.session.configuration.identifier forKey:@"identifier"];
    }
    [coder encodeObject:self.requestSerializer forKey:NSStringFromSelector(@selector(requestSerializer))];
    [coder encodeObject:self.responseSerializer forKey:NSStringFromSelector(@selector(responseSerializer))];
    [coder encodeObject:self.securityPolicy forKey:NSStringFromSelector(@selector(securityPolicy))];
}

#pragma mark - NSCopying

- (instancetype)copyWithZone:(NSZone *)zone {
    AFHTTPSessionManager *HTTPClient = [[[self class] allocWithZone:zone] initWithBaseURL:self.baseURL sessionConfiguration:self.session.configuration];

    HTTPClient.requestSerializer = [self.requestSerializer copyWithZone:zone];
    HTTPClient.responseSerializer = [self.responseSerializer copyWithZone:zone];
    HTTPClient.securityPolicy = [self.securityPolicy copyWithZone:zone];
    return HTTPClient;
}

---

NSCoding肯定是为了持久化的。
比如下载视频之类的大文件，没下载完的时候app被kill掉了，这个时候就可以把Operation持久化，下次再读出来直接发送就ok了。

---

NSCoding, NSCopying 协议，是整个Objective－C中比较重要的两个协议。

前者，是编码协议，后者是拷贝协议。

AFURLConnectionOperation实现NSCoding以后，可以通过NSKeyedArchiver类进行数据序列化，简单说就是将AFURLConnectionOperation对象转成NSData，方便写入数据库或者文件，下次使用使用NSUnKeyedArchiver进行反序列化，就可以得到相同的实例。

NSCopying就更常用了。实现拷贝用的，比如

AFURLConnectionOperation* c = [AFURLConnectionOperation new];

AFURLConnectionOperation* d = [c copy];

这个时候 c 和 d 是不同的对象，d是c的拷贝。