前言

序列化，百度百科的解釋摘抄如下：

序列化 (Serialization)將對(duì)象的狀態(tài)信息轉(zhuǎn)換為可以存儲(chǔ)或傳輸?shù)男问降倪^程。在序列化期間挽霉，對(duì)象將其當(dāng)前狀態(tài)寫入到臨時(shí)或持久性存儲(chǔ)區(qū)写穴。以后，可以通過從存儲(chǔ)區(qū)中讀取或反序列化對(duì)象的狀態(tài)判没，重新創(chuàng)建該對(duì)象。
1隅茎、以某種存儲(chǔ)形式使自定義對(duì)象持久化澄峰；
2、將對(duì)象從一個(gè)地方傳遞到另一個(gè)地方辟犀。
3俏竞、使程序更具維護(hù)性。

簡(jiǎn)單點(diǎn)說就是將對(duì)象轉(zhuǎn)換為二進(jìn)制流堂竟，以便存儲(chǔ)或傳輸魂毁，且日后也能從二進(jìn)制流轉(zhuǎn)換回對(duì)象。 所以我們今天要說的請(qǐng)求序列化跃捣，說到底就是造一個(gè)可以在網(wǎng)絡(luò)上傳遞（被序列化）的漱牵、合法可用的請(qǐng)求。這其中就包括了參數(shù)的處理和請(qǐng)求頭的設(shè)置等疚漆。不管怎樣的代碼酣胀，萬變不離其宗，它的核心就是這個(gè)知識(shí)點(diǎn)娶聘。所以理解了這個(gè)闻镶，再去看代碼就容易多了。
我們知道丸升，GET請(qǐng)求和POST請(qǐng)求對(duì)于參數(shù)是不同的處理铆农。GET請(qǐng)求是多個(gè)參數(shù)之間以&相連，且單個(gè)參數(shù)的鍵值間以=連接狡耻，并將參數(shù)以墩剖？開頭猴凹，經(jīng)過編碼再追加在url后面，而POST卻是將其放入請(qǐng)求體HTTPBody的岭皂。
另外一個(gè)HTTP連接的請(qǐng)求頭HTTPHeaderField設(shè)置也是非常重要的郊霎，它提供了很多字段，用于不同場(chǎng)景爷绘，不同特征下的使用书劝。

生成請(qǐng)求request的地方是在AFHTTPSessionManager中，當(dāng)時(shí)我們說了該類有名為序列化器的屬性requestSerializer土至，然后調(diào)用該序列化器的方法生成了一個(gè)request购对。當(dāng)時(shí)，因?yàn)檫@是AFHTTPRequestSerializer類中的方法陶因，所以對(duì)于具體實(shí)現(xiàn)沒有研究骡苞。今天我們的工作就是這個(gè)。

源碼

先來看頭文件AFHTTPRequestSerializer.h坑赡。它繼承自NSObject烙如，實(shí)現(xiàn)了AFURLRequestSerialization協(xié)議么抗。

首先看頭文件中定義的屬性：

@property (nonatomic, assign) NSStringEncoding stringEncoding;

@property (nonatomic, assign) BOOL allowsCellularAccess;  // 是否允許使用蜂窩網(wǎng)絡(luò)毅否，默認(rèn)YES

@property (nonatomic, assign) NSURLRequestCachePolicy cachePolicy; // 緩存策略

@property (nonatomic, assign) BOOL HTTPShouldHandleCookies;  // 是否處理Cookie

@property (nonatomic, assign) BOOL HTTPShouldUsePipelining; // 是否開啟管線化

@property (nonatomic, assign) NSURLRequestNetworkServiceType networkServiceType;

@property (nonatomic, assign) NSTimeInterval timeoutInterval;

@property (readonly, nonatomic, strong) NSDictionary <NSString *, NSString *> *HTTPRequestHeaders;

@property (nonatomic, strong) NSSet <NSString *> *HTTPMethodsEncodingParametersInURI;

stringEncoding意為序列化參數(shù)時(shí)的字符串編碼，默認(rèn)是NSUTF8StringEncoding蝇刀；
HTTPRequestHeaders是個(gè)字典螟加，意為即將被序列化的HTTP的請(qǐng)求頭。默認(rèn)包括Accept-Language和User-Agent等字段吞琐；
HTTPMethodsEncodingParametersInURI是個(gè)集合對(duì)象捆探，它表示序列化時(shí)參數(shù)被追加在url里的請(qǐng)求方式的集合，想想也知道站粟，它的元素應(yīng)該是GET黍图、Head。
其他幾個(gè)屬性都比較好理解奴烙，而且注釋寫得很清楚了助被，不解釋了。

接著看在頭文件中暴露的前幾個(gè)方法：

+ (instancetype)serializer;

- (void)setValue:(nullable NSString *)value
forHTTPHeaderField:(NSString *)field;

- (nullable NSString *)valueForHTTPHeaderField:(NSString *)field;

- (void)setAuthorizationHeaderFieldWithUsername:(NSString *)username
                                       password:(NSString *)password;
- (void)clearAuthorizationHeader;

- (void)setQueryStringSerializationWithStyle:(AFHTTPRequestQueryStringSerializationStyle)style;

- (void)setQueryStringSerializationWithBlock:(nullable NSString * (^)(NSURLRequest *request, id parameters, NSError * __autoreleasing *error))block;

首先是初始化方法切诀，return一個(gè)該類的實(shí)例對(duì)象揩环；然后是兩個(gè)對(duì)請(qǐng)求頭字典操作的兩個(gè)方法；再接著是兩個(gè)關(guān)于登錄認(rèn)證的方法幅虑；然后是一個(gè)設(shè)置序列化類型的方法丰滑，代表遵循什么樣的規(guī)則進(jìn)行queryString轉(zhuǎn)換。參數(shù)是個(gè)枚舉倒庵，但是這個(gè)枚舉只有一個(gè)值AFHTTPRequestQueryStringDefaultStyle褒墨。最后一個(gè)方法提供了以block形式自定義queryString轉(zhuǎn)換的接口炫刷，也就是說可以通過block回調(diào)的方式讓調(diào)用者以自己的方式完成queryString的轉(zhuǎn)換。

最后就剩下三個(gè)核心方法了郁妈。其中第一個(gè)方法便是我們?cè)谇懊嬉呀?jīng)接觸過的柬唯，由HTTP method、URLString圃庭、parameters返回一個(gè)請(qǐng)求request锄奢。

下面代碼注釋非常占篇幅，但是注釋的很好剧腻，舍不得刪拘央。

Creates an NSMutableURLRequest object with the specified HTTP method and URL string.

If the HTTP method is GET, HEAD, or DELETE, the parameters will be used to construct a url-encoded query string that is appended to the request's URL. Otherwise, the parameters will be encoded according to the value of the parameterEncoding property, and set as the request body.

/**
 Creates an `NSMutableURLRequest` object with the specified HTTP method and URL string.

 If the HTTP method is `GET`, `HEAD`, or `DELETE`, the parameters will be used to construct a url-encoded query string that is appended to the request's URL. Otherwise, the parameters will be encoded according to the value of the `parameterEncoding` property, and set as the request body.

 @param method The HTTP method for the request, such as `GET`, `POST`, `PUT`, or `DELETE`. This parameter must not be `nil`.
 @param URLString The URL string used to create the request URL.
 @param parameters The parameters to be either set as a query string for `GET` requests, or the request HTTP body.
 @param error The error that occurred while constructing the request.

 @return An `NSMutableURLRequest` object.
 */
- (NSMutableURLRequest *)requestWithMethod:(NSString *)method
                                 URLString:(NSString *)URLString
                                parameters:(nullable id)parameters
                                     error:(NSError * _Nullable __autoreleasing *)error;

/**
 Creates an `NSMutableURLRequest` object with the specified HTTP method and URLString, and constructs a `multipart/form-data` HTTP body, using the specified parameters and multipart form data block. See http://www.w3.org/TR/html4/interact/forms.html#h-17.13.4.2

 Multipart form requests are automatically streamed, reading files directly from disk along with in-memory data in a single HTTP body. The resulting `NSMutableURLRequest` object has an `HTTPBodyStream` property, so refrain from setting `HTTPBodyStream` or `HTTPBody` on this request object, as it will clear out the multipart form body stream.

 @param method The HTTP method for the request. This parameter must not be `GET` or `HEAD`, or `nil`.
 @param URLString The URL string used to create the request URL.
 @param parameters The parameters to be encoded and set in the request HTTP body.
 @param block A block that takes a single argument and appends data to the HTTP body. The block argument is an object adopting the `AFMultipartFormData` protocol.
 @param error The error that occurred while constructing the request.

 @return An `NSMutableURLRequest` object
 */
- (NSMutableURLRequest *)multipartFormRequestWithMethod:(NSString *)method
                                              URLString:(NSString *)URLString
                                             parameters:(nullable NSDictionary <NSString *, id> *)parameters
                              constructingBodyWithBlock:(nullable void (^)(id <AFMultipartFormData> formData))block
                                                  error:(NSError * _Nullable __autoreleasing *)error;

/**
 Creates an `NSMutableURLRequest` by removing the `HTTPBodyStream` from a request, and asynchronously writing its contents into the specified file, invoking the completion handler when finished.

 @param request The multipart form request. The `HTTPBodyStream` property of `request` must not be `nil`.
 @param fileURL The file URL to write multipart form contents to.
 @param handler A handler block to execute.

 @discussion There is a bug in `NSURLSessionTask` that causes requests to not send a `Content-Length` header when streaming contents from an HTTP body, which is notably problematic when interacting with the Amazon S3 webservice. As a workaround, this method takes a request constructed with `multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:error:`, or any other request with an `HTTPBodyStream`, writes the contents to the specified file and returns a copy of the original request with the `HTTPBodyStream` property set to `nil`. From here, the file can either be passed to `AFURLSessionManager -uploadTaskWithRequest:fromFile:progress:completionHandler:`, or have its contents read into an `NSData` that's assigned to the `HTTPBody` property of the request.

 @see https://github.com/AFNetworking/AFNetworking/issues/1398
 */
- (NSMutableURLRequest *)requestWithMultipartFormRequest:(NSURLRequest *)request
                             writingStreamContentsToFile:(NSURL *)fileURL
                                       completionHandler:(nullable void (^)(NSError * _Nullable error))handler;

頭文件中基本就是以上東西，現(xiàn)在我們應(yīng)該如饑似渴书在，迫不及待的開始看.m文件了灰伟。

@interface AFHTTPRequestSerializer ()

@property (readwrite, nonatomic, strong) NSMutableSet *mutableObservedChangedKeyPaths;
@property (readwrite, nonatomic, strong) NSMutableDictionary *mutableHTTPRequestHeaders;
@property (readwrite, nonatomic, strong) dispatch_queue_t requestHeaderModificationQueue;
@property (readwrite, nonatomic, assign) AFHTTPRequestQueryStringSerializationStyle queryStringSerializationStyle;
@property (readwrite, nonatomic, copy) AFQueryStringSerializationBlock queryStringSerialization;

@end

@implementation AFHTTPRequestSerializer

+ (instancetype)serializer {
    return [[self alloc] init];
}

- (instancetype)init {
    self = [super init];
    if (!self) {
        return nil;
    }

    self.stringEncoding = NSUTF8StringEncoding;

    self.mutableHTTPRequestHeaders = [NSMutableDictionary dictionary];
    self.requestHeaderModificationQueue = dispatch_queue_create("requestHeaderModificationQueue", DISPATCH_QUEUE_CONCURRENT);

    // Accept-Language HTTP Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
    NSMutableArray *acceptLanguagesComponents = [NSMutableArray array];
    [[NSLocale preferredLanguages] enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        float q = 1.0f - (idx * 0.1f);
        [acceptLanguagesComponents addObject:[NSString stringWithFormat:@"%@;q=%0.1g", obj, q]];
        *stop = q <= 0.5f;
    }];
    [self setValue:[acceptLanguagesComponents componentsJoinedByString:@", "] forHTTPHeaderField:@"Accept-Language"];

    NSString *userAgent = nil;
#if TARGET_OS_IOS
    // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
    userAgent = [NSString stringWithFormat:@"%@/%@ (%@; iOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[UIDevice currentDevice] model], [[UIDevice currentDevice] systemVersion], [[UIScreen mainScreen] scale]];
#elif TARGET_OS_WATCH
    // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
    userAgent = [NSString stringWithFormat:@"%@/%@ (%@; watchOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[WKInterfaceDevice currentDevice] model], [[WKInterfaceDevice currentDevice] systemVersion], [[WKInterfaceDevice currentDevice] screenScale]];
#elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
    userAgent = [NSString stringWithFormat:@"%@/%@ (Mac OS X %@)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[NSProcessInfo processInfo] operatingSystemVersionString]];
#endif
    if (userAgent) {
        if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
            NSMutableString *mutableUserAgent = [userAgent mutableCopy];
            if (CFStringTransform((__bridge CFMutableStringRef)(mutableUserAgent), NULL, (__bridge CFStringRef)@"Any-Latin; Latin-ASCII; [:^ASCII:] Remove", false)) {
                userAgent = mutableUserAgent;
            }
        }
        [self setValue:userAgent forHTTPHeaderField:@"User-Agent"];
    }

    // HTTP Method Definitions; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
    self.HTTPMethodsEncodingParametersInURI = [NSSet setWithObjects:@"GET", @"HEAD", @"DELETE", nil];

    // 在AFHTTPRequestSerializer的初始化方法init中就初始化了集合mutableObservedChangedKeyPaths。并且遍歷AFHTTPRequestSerializerObservedKeyPaths數(shù)組儒旬，為每一項(xiàng)添加觀察
    self.mutableObservedChangedKeyPaths = [NSMutableSet set];
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
            [self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];
        }
    }

    return self;
}

- (void)dealloc {
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
            [self removeObserver:self forKeyPath:keyPath context:AFHTTPRequestSerializerObserverContext];
        }
    }
}

首先是初始化方法栏账，乍看這里感覺好復(fù)雜，其實(shí)一點(diǎn)不復(fù)雜栈源。初始化方法說到底就是初始化挡爵。它首先初始化了幾個(gè)屬性。stringEncoding屬性初始化默認(rèn)為NSUTF8StringEncoding甚垦；然后初始化了用于裝HTTP請(qǐng)求頭屬性的字典mutableHTTPRequestHeaders茶鹃；還初始化了修改請(qǐng)求頭時(shí)而專門創(chuàng)建的隊(duì)列requestHeaderModificationQueue。
接下來是一大串亂糟糟的代碼艰亮，仔細(xì)看就明白了：此時(shí)既然已初始化了裝置請(qǐng)求頭屬性的字典闭翩，那不就可以先設(shè)置一些可以設(shè)置的請(qǐng)求頭屬性了。即Accept-Language和User-Agent迄埃。

初始化方法的最后為序列化需要觀察的屬性添加了監(jiān)聽疗韵。這里是指哪些需要觀察的屬性字段呢？從上面代碼可以看到侄非，它是由一個(gè)C函數(shù)獲取的AFHTTPRequestSerializerObservedKeyPaths()蕉汪，返回了一個(gè)數(shù)組，數(shù)組便是需要觀察的屬性字段的數(shù)組彩库。

而當(dāng)我們?cè)O(shè)置了這些HTTP配置屬性的值時(shí)肤无，就會(huì)觸發(fā)觀察回調(diào)的方法，在此方面里將該屬性字符串放入了mutableObservedChangedKeyPaths數(shù)組骇钦。代碼如下：

// 當(dāng)我們?cè)O(shè)置了這些HTTP配置屬性的值時(shí)宛渐，就會(huì)觸發(fā)觀察回調(diào)方法。并將該屬性字符串放入mutableObservedChangedKeyPaths集合
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(__unused id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if (context == AFHTTPRequestSerializerObserverContext) {
        if ([change[NSKeyValueChangeNewKey] isEqual:[NSNull null]]) {
            [self.mutableObservedChangedKeyPaths removeObject:keyPath];
        } else {
            [self.mutableObservedChangedKeyPaths addObject:keyPath];
        }
    }
}

接著往下看：

- (NSDictionary *)HTTPRequestHeaders {
    NSDictionary __block *value;
    dispatch_sync(self.requestHeaderModificationQueue, ^{
        value = [NSDictionary dictionaryWithDictionary:self.mutableHTTPRequestHeaders];
    });
    return value;
}

- (void)setValue:(NSString *)value
forHTTPHeaderField:(NSString *)field
{
    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
        [self.mutableHTTPRequestHeaders setValue:value forKey:field];
    });
}

- (NSString *)valueForHTTPHeaderField:(NSString *)field {
    NSString __block *value;
    dispatch_sync(self.requestHeaderModificationQueue, ^{
        value = [self.mutableHTTPRequestHeaders valueForKey:field];
    });
    return value;
}

- (void)setAuthorizationHeaderFieldWithUsername:(NSString *)username
                                       password:(NSString *)password
{
    NSData *basicAuthCredentials = [[NSString stringWithFormat:@"%@:%@", username, password] dataUsingEncoding:NSUTF8StringEncoding];
    NSString *base64AuthCredentials = [basicAuthCredentials base64EncodedStringWithOptions:(NSDataBase64EncodingOptions)0];
    [self setValue:[NSString stringWithFormat:@"Basic %@", base64AuthCredentials] forHTTPHeaderField:@"Authorization"];
}

- (void)clearAuthorizationHeader {
    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
        [self.mutableHTTPRequestHeaders removeObjectForKey:@"Authorization"];
    });
}

#pragma mark -

- (void)setQueryStringSerializationWithStyle:(AFHTTPRequestQueryStringSerializationStyle)style {
    self.queryStringSerializationStyle = style;
    self.queryStringSerialization = nil;
}

- (void)setQueryStringSerializationWithBlock:(NSString *(^)(NSURLRequest *, id, NSError *__autoreleasing *))block {
    self.queryStringSerialization = block;
}

上面這些代碼沒什么可說的，接著往下看窥翩，就到了最核心的方法了：

- (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;

    /*
     為該HTTP的request設(shè)置配置屬性

     方法AFHTTPRequestSerializerObservedKeyPaths()返回一個(gè)數(shù)組业岁，代表我們需要關(guān)注的HTTP配置屬性。
     而mutableObservedChangedKeyPaths集合代表我們已設(shè)置的配置屬性寇蚊，
     */
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self.mutableObservedChangedKeyPaths containsObject:keyPath]) {
            [mutableRequest setValue:[self valueForKeyPath:keyPath] forKey:keyPath];
        }
    }

    /*
     將傳入的parameters進(jìn)行編碼笔时，添加到request中
     */
    mutableRequest = [[self requestBySerializingRequest:mutableRequest withParameters:parameters error:error] mutableCopy];
    
    return mutableRequest;
}

開始閱讀該方法≌贪叮可以看到允耿，首先是對(duì)幾個(gè)參數(shù)的斷言。然后以URLString生成url扒怖，再以url生成mutableRequest较锡，并為其該HTTP請(qǐng)求設(shè)置配置屬性。然后它是將最核心的模塊封裝在了一個(gè)本類的方法里盗痒。我們跳入該方法蚂蕴，準(zhǔn)備繼續(xù)閱讀，此時(shí)發(fā)現(xiàn)不光本類中有這個(gè)方法俯邓，它的幾個(gè)子類中也實(shí)現(xiàn)了該方法骡楼。原來這個(gè)方法便是定義在AFURLRequestSerialization協(xié)議中的方法。在不同的子類中有不同的實(shí)現(xiàn)稽鞭，用以實(shí)現(xiàn)不同的功能鸟整。下面是本類中該方法的實(shí)現(xiàn)：

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

    NSMutableURLRequest *mutableRequest = [request mutableCopy];

    /*
     1.為request設(shè)置請(qǐng)求頭
     */
    [self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
        if (![request valueForHTTPHeaderField:field]) {
            [mutableRequest setValue:value forHTTPHeaderField:field];
        }
    }];
    
    
    /*
     2.參數(shù)序列化處理（將參數(shù)字典解析為url query）
     */
    NSString *query = nil;
    if (parameters) {
        // 若自定義了queryStringSerialization，那么就使用自定義的queryStringSerialization構(gòu)建方式
        if (self.queryStringSerialization)
        {
            NSError *serializationError;
            query = self.queryStringSerialization(request, parameters, &serializationError); // 通過queryStringSerialization構(gòu)建query字符串

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

                return nil;
            }
        }
        else
        {
            switch (self.queryStringSerializationStyle) {
                case AFHTTPRequestQueryStringDefaultStyle:
                    query = AFQueryStringFromParameters(parameters); // 由參數(shù)生成url的query部分
                    break;
            }
        }
    }

    // HTTPMethodsEncodingParametersInURI是個(gè)NSSet川慌，裝了"GET"吃嘿、"HEAD"祠乃、"DELETE"請(qǐng)求方式(在該類的初始化方法里初始化了)梦重，因?yàn)檫@幾個(gè)請(qǐng)求方式的query是直接拼接在url后面的。
    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 // 若請(qǐng)求方式為"POST"亮瓷、"PUT"等琴拧，則需要將query設(shè)置到http body上
    {
        // #2864: an empty string is a valid x-www-form-urlencoded payload
        if (!query) {
            query = @"";
        }
        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {
            [mutableRequest setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
        }
        [mutableRequest setHTTPBody:[query dataUsingEncoding:self.stringEncoding]]; // 將query string編碼
    }

    return mutableRequest;
}

該方法里的脈絡(luò)很清晰，分為兩個(gè)步驟嘱支。第一步便是為request設(shè)置請(qǐng)求頭屬性蚓胸；第二步是參數(shù)序列化處理：先看是否實(shí)現(xiàn)自定義的序列化處理block，若有除师，則調(diào)用自定義的block沛膳，讓使用者自己實(shí)現(xiàn)參數(shù)序列化；若無汛聚，則調(diào)用AFQueryStringFromParameters()方法锹安，由參數(shù)字典轉(zhuǎn)換為query string。關(guān)于這個(gè)函數(shù)如何將字典解析為query string的釋義，這篇文章已經(jīng)解釋得很好：AFNetworking源碼閱讀（二）叹哭∪趟危總之，現(xiàn)在有了query string了风罩，但還是要根據(jù)HTTP method的不同糠排，將其放在正確的位置上。方法里接下來的代碼就是完成這部分的超升。

值得注意的是在HTTP method為POST等其他時(shí)入宦，它設(shè)置Content-Type為application/x-www-form-urlencoded，這個(gè)代表什么意思呢室琢？我趕緊百度了下云石。收集了以下干貨資料：
四種常見的 POST 提交數(shù)據(jù)方式
HTTP協(xié)議之multipart/form-data請(qǐng)求分析
關(guān)于 Content-Type:application/x-www-form-urlencoded 和 Content-Type:multipart/related

結(jié)尾

剛說到了Content-Type了，但時(shí)間不早了研乒，明天把這部分補(bǔ)充完整汹忠。