AFNetworking - AFURLRequestSerialization源码解析（二）

知识预备：

Content-Type：表示具体请求中或者返回的数据类型

application/x-www-urlencoded：form表单数据被编码为key/value格式发送到服务器（表单默认的提交数据的格式）
multipart/form-data：文件上传时
application/json ：JSON数据格式
application/octet-stream ：二进制流数据（如常见的文件下载）

form-data，multipart/form-data 主要用于 POST方法中传递多种格式和含义的数据，在 body 中引入 boundary 的概念，用分割线将多部分数据融合到一个 body 中发送给服务端。那么对于一个简单的 form-data，它发送的 body 内容可能如下

--Boundary+FD2E180F039993ED
Content-Disposition: form-data; name="myArray[]"

v1
--Boundary+FD2E180F039993ED
Content-Disposition: form-data; name="myArray[]"

v2
--Boundary+FD2E180F039993ED
Content-Disposition: form-data; name="myArray[]"

v3
--Boundary+FD2E180F039993ED
Content-Disposition: form-data; name="mydic[key1]"

value1
--Boundary+FD2E180F039993ED
Content-Disposition: form-data; name="mydic[key2]"

value2
--Boundary+FD2E180F039993ED
header: headerkey

BodyData
--Boundary+FD2E180F039993ED--

它的特点是

每一部分都可以包含 header，一般默认必须包含的标识 header 是 Content-Disposition
头部和每一部分需要以 --Boundary+{XXX} 格式分割
末尾以 --Boundary+{XXX}-- 结束
请求头中，要设置 Content-Type: multipart/form-data; boundary=Boundary+{XXX}
请求头要设置 Content-Length 为 body 总长度

参考链接

一般的请求创建方法：

- (NSMutableURLRequest *)requestWithMethod:(NSString *)method
                                 URLString:(NSString *)URLString
                                parameters:(id)parameters
                                     error:(NSError *__autoreleasing *)error
{
    NSParameterAssert(method);
    NSParameterAssert(URLString);

    NSURL *url = [NSURL URLWithString:URLString];

    NSParameterAssert(url);

    NSMutableURLRequest *mutableRequest = [[NSMutableURLRequest alloc] initWithURL:url];
    mutableRequest.HTTPMethod = method;

    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self.mutableObservedChangedKeyPaths containsObject:keyPath]) {
            [mutableRequest setValue:[self valueForKeyPath:keyPath] forKey:keyPath];
        }
    }

    mutableRequest = [[self requestBySerializingRequest:mutableRequest withParameters:parameters error:error] mutableCopy];

    return mutableRequest;
}

主要是调用requestBySerializingRequest方法，设置一个默认的序列化请求

- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
                               withParameters:(id)parameters
                                        error:(NSError *__autoreleasing *)error
{
    NSParameterAssert(request);

    NSMutableURLRequest *mutableRequest = [request mutableCopy];
    //设置请求头
    [self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
        if (![request valueForHTTPHeaderField:field]) {
            [mutableRequest setValue:value forHTTPHeaderField:field];
        }
    }];

    NSString *query = nil;
    if (parameters) {
        //自定义的序列化方法block
        if (self.queryStringSerialization) {
            NSError *serializationError;
            query = self.queryStringSerialization(request, parameters, &serializationError);

            if (serializationError) {
                if (error) {
                    *error = serializationError;
                }

                return nil;
            }
        } else {
            switch (self.queryStringSerializationStyle) {
                case AFHTTPRequestQueryStringDefaultStyle:
                    query = AFQueryStringFromParameters(parameters);
                    break;
            }
        }
    }
    //不过请求方式是@"GET", @"HEAD", @"DELETE"，就需要进行拼接
    if ([self.HTTPMethodsEncodingParametersInURI containsObject:[[request HTTPMethod] uppercaseString]]) {
        if (query && query.length > 0) {
            mutableRequest.URL = [NSURL URLWithString:[[mutableRequest.URL absoluteString] stringByAppendingFormat:mutableRequest.URL.query ? @"&%@" : @"?%@", query]];
        }
    } else {
        // #2864: an empty string is a valid x-www-form-urlencoded payload
        if (!query) {
            query = @"";
        }
        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {
            //默认就是将表单编码为键值对格式
            [mutableRequest setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
        }
        [mutableRequest setHTTPBody:[query dataUsingEncoding:self.stringEncoding]];
    }

    return mutableRequest;
}

如果是上传文件的请求，那就用multipartFormRequestWithMethod这个方法，它对头部的处理又会有一些不同，会设置上传文件所需要的一些头部信息，比如Content-Disposition，Content-Type， Content-Length，分隔符Boundary

- (NSMutableURLRequest *)multipartFormRequestWithMethod:(NSString *)method
                                              URLString:(NSString *)URLString
                                             parameters:(NSDictionary *)parameters
                              constructingBodyWithBlock:(void (^)(id <AFMultipartFormData> formData))block
                                                  error:(NSError *__autoreleasing *)error
{
    NSParameterAssert(method);
    NSParameterAssert(![method isEqualToString:@"GET"] && ![method isEqualToString:@"HEAD"]);

    NSMutableURLRequest *mutableRequest = [self requestWithMethod:method URLString:URLString parameters:nil error:error];

    //AFStreamingMultipartFormData做的事情就是设置文件上传需要的头部信息，如Content-Disposition，Content-Type， Content-Length，分隔符Boundary
    __block AFStreamingMultipartFormData *formData = [[AFStreamingMultipartFormData alloc] initWithURLRequest:mutableRequest stringEncoding:NSUTF8StringEncoding];

    if (parameters) {
        for (AFQueryStringPair *pair in AFQueryStringPairsFromDictionary(parameters)) {
            NSData *data = nil;
            if ([pair.value isKindOfClass:[NSData class]]) {
                data = pair.value;
            } else if ([pair.value isEqual:[NSNull null]]) {
                data = [NSData data];
            } else {
                data = [[pair.value description] dataUsingEncoding:self.stringEncoding];
            }

            if (data) {
                [formData appendPartWithFormData:data name:[pair.field description]];
            }
        }
    }

    if (block) {
        block(formData);
    }

    return [formData requestByFinalizingMultipartFormData];
}

如果请求数据是JSON格式数据，其实他的序列化工作只是把Content-Type改为application/json，把json序列化数据设置成HTTPbody，同理以属性列表的方式请求也是一样，把Content-Type改为application/x-plist，然后把属性列表序列化数据设置成HTTPbody

- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
                               withParameters:(id)parameters
                                        error:(NSError *__autoreleasing *)error
{
    NSParameterAssert(request);
    //对于`GET`,`HEAD`,`DELETE`等方法中。直接使用父类的处理方式
    if ([self.HTTPMethodsEncodingParametersInURI containsObject:[[request HTTPMethod] uppercaseString]]) {
        return [super requestBySerializingRequest:request withParameters:parameters error:error];
    }

    NSMutableURLRequest *mutableRequest = [request mutableCopy];

    [self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
        if (![request valueForHTTPHeaderField:field]) {
            [mutableRequest setValue:value forHTTPHeaderField:field];
        }
    }];

    if (parameters) {
        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {
            [mutableRequest setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
        }

        if (![NSJSONSerialization isValidJSONObject:parameters]) {
            if (error) {
                NSDictionary *userInfo = @{NSLocalizedFailureReasonErrorKey: NSLocalizedStringFromTable(@"The `parameters` argument is not valid JSON.", @"AFNetworking", nil)};
                *error = [[NSError alloc] initWithDomain:AFURLRequestSerializationErrorDomain code:NSURLErrorCannotDecodeContentData userInfo:userInfo];
            }
            return nil;
        }
        //把parameters转换为JSON序列化的data
        NSData *jsonData = [NSJSONSerialization dataWithJSONObject:parameters options:self.writingOptions error:error];
        
        if (!jsonData) {
            return nil;
        }
        
        [mutableRequest setHTTPBody:jsonData];
    }

    return mutableRequest;
}

总结

AFURLRequestSerialization核心就是根据不同的请求类型，设置不同的请求头信息，选择对应的方法序列化请求数据然后设置成HTTPbody