1 概述

AFURLRequestSerialization主要实现了根据不同情况和参数初始化NSURLRequest对象的功能。只有AFHTTPSessionManager有requestSerialization，默认是AFHTTPRequestSerializer对象。尤其是我们使用MultipartForm请求的时候，可以使用它帮我们完成繁杂的请求头拼接过程，这个是最值得推荐的。

在阅读源码之前，一定要对multipart/form-data非常熟悉，不然会有很多地方看不懂。具体可以看AFNetWorking源码之AFHTTPSessionManager关于它的那部分。

2 AFURLRequestSerialization的api分析

AFURLRequestSerialization包含了四个部分：

• 全局方法:AFPercentEscapedStringFromString和AFQueryStringFromParameters。
• 协议AFURLRequestSerialization提供了一个序列化parameters参数的方法。我们可以把参数转换为查询字符串、HTTP请求体、设置恰当的请求头等。
• AFHTTPRequestSerializer继承自AFURLRequestSerialization协议。提供了查询字符串/URL格式的参数序列化、默认请求头处理。同时以提供HTTP状态码和返回数据的验证等工作。
  _ AFMultipartFormData协议。主要用于添加multipart/form-data请求的Content-Disposition: file; filename=#{generated filename}; name=#{name}" 和 Content-Type: #{generated mimeType}的请求体域。
• 类型AFJSONRequestSerializer和AFPropertyListRequestSerializer。主要针对JSON和Plist类型的序列化优化。

AFPercentEscapedStringFromString返回一个字符串的百分号编码格式的字符串。因为url只有普通英文字符和数字，特殊字符$-_.+!*'()还有保留字符。所以很多字符都需要编码,非ASCII编码的字符串先转换为ASCII编码，然后再转换为百分号编码。

/**
AFPercentEscapedStringFromString方法的作用就是把一个普通字符串转换为百分号编码的字符串
 http://blog.csdn.net/qq_32010299/article/details/51790407
 @param string 一个字符串
 @return 百分号编码的字符串
 */
NSString * AFPercentEscapedStringFromString(NSString *string) {
    //可能需要做百分号编码处理的字符串
    static NSString * const kAFCharactersGeneralDelimitersToEncode = @":#[]@"; 
    static NSString * const kAFCharactersSubDelimitersToEncode = @"!$&'()*+,;=";
    //不需要做百分号编码的字符串集合
    NSMutableCharacterSet * allowedCharacterSet = [[NSCharacterSet URLQueryAllowedCharacterSet] mutableCopy];
    //获取目前系统中最终需要做百分号编码转换的字符集合
    [allowedCharacterSet removeCharactersInString:[kAFCharactersGeneralDelimitersToEncode stringByAppendingString:kAFCharactersSubDelimitersToEncode]];

    static NSUInteger const batchSize = 50;
    NSUInteger index = 0;
    NSMutableString *escaped = @"".mutableCopy;
    //迭代字符串做百分号编码
    while (index < string.length) {
        NSUInteger length = MIN(string.length - index, batchSize);
        NSRange range = NSMakeRange(index, length);
        //移除字符串中的一些非法字符。比如👴🏻👮🏽
        range = [string rangeOfComposedCharacterSequencesForRange:range];
        NSString *substring = [string substringWithRange:range];
        //指定范围内的字符做百分号编码
        NSString *encoded = [substring stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacterSet];
        [escaped appendString:encoded];
        index += range.length;
    }
    //返回处理以后的字符串
    return escaped;
}

私有类AFQueryStringPair的主要功能就是把一个key和vaue的键值对转换为百分号编码格式的键值对并且用=链接起来

@interface AFQueryStringPair : NSObject
@property (readwrite, nonatomic, strong) id field;
@property (readwrite, nonatomic, strong) id value;
- (instancetype)initWithField:(id)field value:(id)value;
- (NSString *)URLEncodedStringValue;
@end
@implementation AFQueryStringPair
- (instancetype)initWithField:(id)field value:(id)value {
    self = [super init];
    if (!self) {
        return nil;
    }
    self.field = field;
    self.value = value;
    return self;
}
/**
 把key、value键值对转换为百分号编码，并且链接起来
 @return 转换后的字符串
 */
- (NSString *)URLEncodedStringValue {
    if (!self.value || [self.value isEqual:[NSNull null]]) {
        return AFPercentEscapedStringFromString([self.field description]);
    } else {
        //先用百分号编码处理，然后再拼接
        return [NSString stringWithFormat:@"%@=%@", AFPercentEscapedStringFromString([self.field description]), AFPercentEscapedStringFromString([self.value description])];
    }
}
@end

方法AFQueryStringPairsFromDictionary和AFQueryStringPairsFromKeyAndValue分别把一个字典或者key、value键值对转换为url的query参数。

/**
 把一个字典转换为百分号编码的query参数
 @param parameters 要转换的字典
 @return query参数
 */
NSString * AFQueryStringFromParameters(NSDictionary *parameters) {
    NSMutableArray *mutablePairs = [NSMutableArray array];
    for (AFQueryStringPair *pair in AFQueryStringPairsFromDictionary(parameters)) {
        //调用`AFQueryStringPair`序列化
        [mutablePairs addObject:[pair URLEncodedStringValue]];
    }
    return [mutablePairs componentsJoinedByString:@"&"];
}
NSArray * AFQueryStringPairsFromDictionary(NSDictionary *dictionary) {
    return AFQueryStringPairsFromKeyAndValue(nil, dictionary);
}
/**
 分别把一个字典、数组、集合转换为一个AFQueryStringPair对象的的数组。
 @param key key
 @param value value
 @return AFQueryStringPair类型数组
 */
NSArray * AFQueryStringPairsFromKeyAndValue(NSString *key, id value) {
    NSMutableArray *mutableQueryStringComponents = [NSMutableArray array];
    //使用`description`排序
    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"description" ascending:YES selector:@selector(compare:)];
    if ([value isKindOfClass:[NSDictionary class]]) {
        NSDictionary *dictionary = value;
        for (id nestedKey in [dictionary.allKeys sortedArrayUsingDescriptors:@[ sortDescriptor ]]) {
            id nestedValue = dictionary[nestedKey];
            if (nestedValue) {
                //如果是字典，就取出每一对key、value处理
                [mutableQueryStringComponents addObjectsFromArray:AFQueryStringPairsFromKeyAndValue((key ? [NSString stringWithFormat:@"%@[%@]", key, nestedKey] : nestedKey), nestedValue)];
            }
        }
    } else if ([value isKindOfClass:[NSArray class]]) {
        NSArray *array = value;
        for (id nestedValue in array) {
            //如果是数组，则取出元素，添加一个额外的key处理
            [mutableQueryStringComponents addObjectsFromArray:AFQueryStringPairsFromKeyAndValue([NSString stringWithFormat:@"%@[]", key], nestedValue)];
        }
    } else if ([value isKindOfClass:[NSSet class]]) {
        NSSet *set = value;
        for (id obj in [set sortedArrayUsingDescriptors:@[ sortDescriptor ]]) {
            //如果是集合，就是用默认key和集合元素处理
            [mutableQueryStringComponents addObjectsFromArray:AFQueryStringPairsFromKeyAndValue(key, obj)];
        }
    } else {
        //添加处理后的key和value
        [mutableQueryStringComponents addObject:[[AFQueryStringPair alloc] initWithField:key value:value]];
    }
    //返回`AFQueryStringPair`对象数组
    return mutableQueryStringComponents;
}

AFHTTPRequestSerializerObservedKeyPaths全局方法指定了request请求序列化要观察的属性列表、是一个数组，里面有对蜂窝数据、缓存策略、cookie、管道、网络状态、超时这几个元素。

static NSArray * AFHTTPRequestSerializerObservedKeyPaths() {
    static NSArray *_AFHTTPRequestSerializerObservedKeyPaths = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _AFHTTPRequestSerializerObservedKeyPaths = @[NSStringFromSelector(@selector(allowsCellularAccess)), NSStringFromSelector(@selector(cachePolicy)), NSStringFromSelector(@selector(HTTPShouldHandleCookies)), NSStringFromSelector(@selector(HTTPShouldUsePipelining)), NSStringFromSelector(@selector(networkServiceType)), NSStringFromSelector(@selector(timeoutInterval))];
    });

    return _AFHTTPRequestSerializerObservedKeyPaths;
}

2.1 AFHTTPRequestSerializer的解析

AFHTTPRequestSerializer主要实现了大部分request拼接转化功能。比如通用请求头的添加如userAgent、request属性的KVO观察、手动指定请求头序列化的Block、负责具体的request对象的初始化等。

1 AFHTTPRequestSerializer的属性和初始化

//属性列表
@interface AFHTTPRequestSerializer ()
//某个request需要观察的属性集合
@property (readwrite, nonatomic, strong) NSMutableSet *mutableObservedChangedKeyPaths;
//存储request的请求头域
@property (readwrite, nonatomic, strong) NSMutableDictionary *mutableHTTPRequestHeaders;
//用于修改或者设置请求体域的dispatch_queue_t。
@property (readwrite, nonatomic, strong) dispatch_queue_t requestHeaderModificationQueue;
@property (readwrite, nonatomic, assign) AFHTTPRequestQueryStringSerializationStyle queryStringSerializationStyle;
//手动指定parameters参数序列化的Block
@property (readwrite, nonatomic, copy) AFQueryStringSerializationBlock queryStringSerialization;
@end
//初始化方法
- (instancetype)init {
    self = [super init];
    if (!self) {
        return nil;
    }
    //指定序列化编码格式
    self.stringEncoding = NSUTF8StringEncoding;
    //请求头保存在一个字典中，方便后面构建request的时候拼装。
    self.mutableHTTPRequestHeaders = [NSMutableDictionary dictionary];
    //初始化一个操作request的header域的dispatch_queue_t
    self.requestHeaderModificationQueue = dispatch_queue_create("requestHeaderModificationQueue", DISPATCH_QUEUE_CONCURRENT);

    NSMutableArray *acceptLanguagesComponents = [NSMutableArray array];
    /*
     *枚举系统的language列表。然后设置`Accept-Language`请求头域。优先级逐级降低，最多五个。
     */
    [[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;
    }];
    //数组元素使用`, `分割
    [self setValue:[acceptLanguagesComponents componentsJoinedByString:@", "] forHTTPHeaderField:@"Accept-Language"];
    /*
     *设置User-Agent请求头域的值。
     */
    NSString *userAgent = nil;
    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]];
    if (userAgent) {
        /*
         *如果userAgent里面包含非ASCII码的字符，比如中文，则需要转换。这里是转换为对应的拉丁字母。
         AFNetWorking3.X源码阅读/1.0 (iPhone; iOS 10.2; Scale/2.00)
         AFNetWorking3.X yuan ma yue du/1.0 (iPhone; iOS 10.2; Scale/2.00)
         */
        if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
            NSMutableString *mutableUserAgent = [userAgent mutableCopy];
            //转换为拉丁字母
            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
    //需要把parameters转换为query参数的方法集合。
    self.HTTPMethodsEncodingParametersInURI = [NSSet setWithObjects:@"GET", @"HEAD", @"DELETE", nil];

    self.mutableObservedChangedKeyPaths = [NSMutableSet set];
    /*
     添加对蜂窝数据、缓存策略、cookie、管道、网络状态、超时这几个属性的观察。
     */
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
            [self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];
        }
    }
    return self;
}

2 AFHTTPRequestSerializer的各种setter方法

首先通过automaticallyNotifiesObserversForKey方法来阻止一些属性的KVO机制的触发，然后我们通过重写蜂窝数据、缓存策略、cookie、管道、网络状态、超时的观察。可以用于测试这些属性变化是否崩溃等。

/**
 如果kvo的触发机制是默认出发。则返回true，否则返回false。在这里，只要是`AFHTTPRequestSerializerObservedKeyPaths`里面的属性，我们都取消自动出发kvo机制，使用手动触发。

 @param key kvo的key
 @return bool值
 */
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
    if ([AFHTTPRequestSerializerObservedKeyPaths() containsObject:key]) {
        return NO;
    }
    return [super automaticallyNotifiesObserversForKey:key];
}
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(__unused id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    //是否是选择要观察的属性
    if (context == AFHTTPRequestSerializerObserverContext) {
        //如果属性值为null，则表示么有这个属性，移除对其的观察
        if ([change[NSKeyValueChangeNewKey] isEqual:[NSNull null]]) {
            [self.mutableObservedChangedKeyPaths removeObject:keyPath];
        } else {
            //添加到要观察的属性的集合
            [self.mutableObservedChangedKeyPaths addObject:keyPath];
        }
    }
}

通过重写属性的setter方法来手动触发kvo

#pragma mark - 手动触发蜂窝数据、缓存策略、cookie、管道、网络状态、超时的观察。可以用于测试这些属性变化是否崩溃等。
- (void)setAllowsCellularAccess:(BOOL)allowsCellularAccess {
    [self willChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];
    _allowsCellularAccess = allowsCellularAccess;
    [self didChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];
}

- (void)setCachePolicy:(NSURLRequestCachePolicy)cachePolicy {
    [self willChangeValueForKey:NSStringFromSelector(@selector(cachePolicy))];
    _cachePolicy = cachePolicy;
    [self didChangeValueForKey:NSStringFromSelector(@selector(cachePolicy))];
}

- (void)setHTTPShouldHandleCookies:(BOOL)HTTPShouldHandleCookies {
    [self willChangeValueForKey:NSStringFromSelector(@selector(HTTPShouldHandleCookies))];
    _HTTPShouldHandleCookies = HTTPShouldHandleCookies;
    [self didChangeValueForKey:NSStringFromSelector(@selector(HTTPShouldHandleCookies))];
}

- (void)setHTTPShouldUsePipelining:(BOOL)HTTPShouldUsePipelining {
    [self willChangeValueForKey:NSStringFromSelector(@selector(HTTPShouldUsePipelining))];
    _HTTPShouldUsePipelining = HTTPShouldUsePipelining;
    [self didChangeValueForKey:NSStringFromSelector(@selector(HTTPShouldUsePipelining))];
}

- (void)setNetworkServiceType:(NSURLRequestNetworkServiceType)networkServiceType {
    [self willChangeValueForKey:NSStringFromSelector(@selector(networkServiceType))];
    _networkServiceType = networkServiceType;
    [self didChangeValueForKey:NSStringFromSelector(@selector(networkServiceType))];
}

- (void)setTimeoutInterval:(NSTimeInterval)timeoutInterval {
    [self willChangeValueForKey:NSStringFromSelector(@selector(timeoutInterval))];
    _timeoutInterval = timeoutInterval;
    [self didChangeValueForKey:NSStringFromSelector(@selector(timeoutInterval))];
}

3 AFHTTPRequestSerializer的各种请求头域处理方法

/**
 返回请求头域key和vaue

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

/**
 设置一个请求头域

 @param value vaue
 @param field 域名
 */
- (void)setValue:(NSString *)value
forHTTPHeaderField:(NSString *)field
{
    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
        [self.mutableHTTPRequestHeaders setValue:value forKey:field];
    });
}
/**
 返回指定请求头域的值

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

/**
 设置Basic Authorization的用户名和密码。记住需要是base64编码格式的。
 @param username 用户
 @param password 密码
 */
- (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"];
}
/**
 移除Basic Authorization的请求头
 */
- (void)clearAuthorizationHeader {
    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
        [self.mutableHTTPRequestHeaders removeObjectForKey:@"Authorization"];
    });
}

4 AFHTTPRequestSerializer的各种创建NSMutableURLRequest的方法

通过下面这三种方法处理不同类型的request对象的初始化和参数序列化。

/**
 根据给定的url、方法名、参数构建一个request。

 @param method 方法名
 @param URLString url地址
 @param parameters 参数，根据不同的请求方法构建出不同的模式
 @param error 构建出错
 @return 返回一个非multipartForm请求
 */
- (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;
    /*
     *mutableObservedChangedKeyPaths集合里面的属性都通过`setValue: forKey`手动设置一下。估计目的是触发这几个属性的kvo。
     */
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self.mutableObservedChangedKeyPaths containsObject:keyPath]) {
            [mutableRequest setValue:[self valueForKeyPath:keyPath] forKey:keyPath];
        }
    }
    /*
     根据parameters和HTTPRequestHeaders构建一个request
     */
    mutableRequest = [[self requestBySerializingRequest:mutableRequest withParameters:parameters error:error] mutableCopy];
    return mutableRequest;
}

/**
 构建一个multipartForm的request。并且通过`AFMultipartFormData`类型的formData来构建请求体

 @param method 方法名，一般都是POST
 @param URLString 请求地址
 @param parameters 请求头参数
 @param block 用于构建请求体的Block
 @param error 构建请求体出错
 @return 返回一个构建好的request
 */
- (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"]);
    /*
     先构建一个普通的request对象，然后在构建出multipartFrom的request
     * 在这一步将会把parameters加入请求头或者请求体。然后把`AFURLRequestSerialization`指定的headers加入request的请求头中。这个request就只差构建multipartFrom部分了
     */
    NSMutableURLRequest *mutableRequest = [self requestWithMethod:method URLString:URLString parameters:nil error:error];
    /*
     *初始化一个`AFStreamingMultipartFormData`对象。用于封装multipartFrom的body部分
     */
    __block AFStreamingMultipartFormData *formData = [[AFStreamingMultipartFormData alloc] initWithURLRequest:mutableRequest stringEncoding:NSUTF8StringEncoding];
    if (parameters) {
        /*
         把parameters拼接成`AFQueryStringPair`对象。然后根据取出的key和value处理。
         */
        for (AFQueryStringPair *pair in AFQueryStringPairsFromDictionary(parameters)) {
            NSData *data = nil;
            //把value处理为NSData类型
            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);
    }
    //body具体序列化操作
    return [formData requestByFinalizingMultipartFormData];
}

/**
 通过一个Multipart-Form的request创建一个request。新request的httpBody是`fileURL`指定的文件。
 并且是通过`HTTPBodyStream`这个属性添加，`HTTPBodyStream`属性的数据会自动添加为httpBody。

 @param request 原request
 @param fileURL 文件的url
 @param handler 错误处理
 @return 处理完成的request
 */
- (NSMutableURLRequest *)requestWithMultipartFormRequest:(NSURLRequest *)request
                             writingStreamContentsToFile:(NSURL *)fileURL
                                       completionHandler:(void (^)(NSError *error))handler
{
    NSParameterAssert(request.HTTPBodyStream);
    NSParameterAssert([fileURL isFileURL]);
    //获取`HTTPBodyStream`属性
    NSInputStream *inputStream = request.HTTPBodyStream;
    //获取文件的数据流
    NSOutputStream *outputStream = [[NSOutputStream alloc] initWithURL:fileURL append:NO];
    __block NSError *error = nil;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //把读和写的操作加入当前线程的runloop
        [inputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
        [outputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
        //打开读和写数据流
        [inputStream open];
        [outputStream open];
        //循环做读和写操作
        while ([inputStream hasBytesAvailable] && [outputStream hasSpaceAvailable]) {
            uint8_t buffer[1024];

            NSInteger bytesRead = [inputStream read:buffer maxLength:1024];
            if (inputStream.streamError || bytesRead < 0) {
                error = inputStream.streamError;
                break;
            }

            NSInteger bytesWritten = [outputStream write:buffer maxLength:(NSUInteger)bytesRead];
            if (outputStream.streamError || bytesWritten < 0) {
                error = outputStream.streamError;
                break;
            }

            if (bytesRead == 0 && bytesWritten == 0) {
                break;
            }
        }
        //读和写完成。关闭读和写数据流
        [outputStream close];
        [inputStream close];
        //如果有handler，调用handler这个Block
        if (handler) {
            dispatch_async(dispatch_get_main_queue(), ^{
                handler(error);
            });
        }
    });
    //获取一个新的request，新的request的httpBody已经通过`HTTPBodyStream`转换成功
    NSMutableURLRequest *mutableRequest = [request mutableCopy];
    mutableRequest.HTTPBodyStream = nil;
    //返回一个request对象
    return mutableRequest;
}

3 AFStreamingMultipartFormData私有类的解析

首先，我们看几个全局方法。下面几个方法用于拼接multipart/form-data的分隔符和文件的MIMEType。

/*
 生成multipartForm的request的boundary
 */
static NSString * AFCreateMultipartFormBoundary() {
    return [NSString stringWithFormat:@"Boundary+%08X%08X", arc4random(), arc4random()];
}
//回车换行符
static NSString * const kAFMultipartFormCRLF = @"\r\n";
//生成一个request的请求体中的参数的开始符号，第一个
static inline NSString * AFMultipartFormInitialBoundary(NSString *boundary) {
    return [NSString stringWithFormat:@"--%@%@", boundary, kAFMultipartFormCRLF];
}
//生成一个request的请求体中的参数的开始符号，菲第一个。
static inline NSString * AFMultipartFormEncapsulationBoundary(NSString *boundary) {
    return [NSString stringWithFormat:@"%@--%@%@", kAFMultipartFormCRLF, boundary, kAFMultipartFormCRLF];
}
//生成一个request的请求体中的参数的结束符号
static inline NSString * AFMultipartFormFinalBoundary(NSString *boundary) {
    return [NSString stringWithFormat:@"%@--%@--%@", kAFMultipartFormCRLF, boundary, kAFMultipartFormCRLF];
}
/*
根据文件的扩展名获取文件的`MIMEType`
 */
static inline NSString * AFContentTypeForPathExtension(NSString *extension) {
    NSString *UTI = (__bridge_transfer NSString *)UTTypeCreatePreferredIdentifierForTag(kUTTagClassFilenameExtension, (__bridge CFStringRef)extension, NULL);
    NSString *contentType = (__bridge_transfer NSString *)UTTypeCopyPreferredTagWithClass((__bridge CFStringRef)UTI, kUTTagClassMIMEType);
    if (!contentType) {
        return @"application/octet-stream";
    } else {
        return contentType;
    }
}

AFStreamingMultipartFormData负责multipart/form-data的Body的具体构建。比如boundary的指定、请求体数据的拼接等。

- (instancetype)initWithURLRequest:(NSMutableURLRequest *)urlRequest
                    stringEncoding:(NSStringEncoding)encoding
{
    self = [super init];
    if (!self) {
        return nil;
    }
    //需要添加httpbody的request
    self.request = urlRequest;
    //字符编码
    self.stringEncoding = encoding;
    //指定boundary
    self.boundary = AFCreateMultipartFormBoundary();
    //这个属性用于存储httpbody数据
    self.bodyStream = [[AFMultipartBodyStream alloc] initWithStringEncoding:encoding];
    return self;
}
/*
 根据文件的url添加一个`multipart/form-data`请求的请求体域
 */
- (BOOL)appendPartWithFileURL:(NSURL *)fileURL
                         name:(NSString *)name
                        error:(NSError * __autoreleasing *)error
{
    NSParameterAssert(fileURL);
    NSParameterAssert(name);
    //文件扩展名
    NSString *fileName = [fileURL lastPathComponent];
    //获取文件的mimetype的类型
    NSString *mimeType = AFContentTypeForPathExtension([fileURL pathExtension]);

    return [self appendPartWithFileURL:fileURL name:name fileName:fileName mimeType:mimeType error:error];
}

/**
 根据指定类型的fileurl，把数据添加进入bodyStream中。以提供给后面构建request的body。

 @param fileURL 文件的url
 @param name 参数名称
 @param fileName 文件名称
 @param mimeType 文件类型
 @param error 错误
 @return 是否成功
 */
- (BOOL)appendPartWithFileURL:(NSURL *)fileURL
                         name:(NSString *)name
                     fileName:(NSString *)fileName
                     mimeType:(NSString *)mimeType
                        error:(NSError * __autoreleasing *)error
{
    NSParameterAssert(fileURL);
    NSParameterAssert(name);
    NSParameterAssert(fileName);
    NSParameterAssert(mimeType);
    /*
     各种错误情况判断
     */
    if (![fileURL isFileURL]) {
        NSDictionary *userInfo = @{NSLocalizedFailureReasonErrorKey: NSLocalizedStringFromTable(@"Expected URL to be a file URL", @"AFNetworking", nil)};
        if (error) {
            *error = [[NSError alloc] initWithDomain:AFURLRequestSerializationErrorDomain code:NSURLErrorBadURL userInfo:userInfo];
        }
        return NO;
    } else if ([fileURL checkResourceIsReachableAndReturnError:error] == NO) {
        NSDictionary *userInfo = @{NSLocalizedFailureReasonErrorKey: NSLocalizedStringFromTable(@"File URL not reachable.", @"AFNetworking", nil)};
        if (error) {
            *error = [[NSError alloc] initWithDomain:AFURLRequestSerializationErrorDomain code:NSURLErrorBadURL userInfo:userInfo];
        }
        return NO;
    }
    //获取指定路径文件的属性
    NSDictionary *fileAttributes = [[NSFileManager defaultManager] attributesOfItemAtPath:[fileURL path] error:error];
    if (!fileAttributes) {
        return NO;
    }
    //添加`Content-Disposition`和`Content-Type`这两个请求体域
    NSMutableDictionary *mutableHeaders = [NSMutableDictionary dictionary];
    [mutableHeaders setValue:[NSString stringWithFormat:@"form-data; name=\"%@\"; filename=\"%@\"", name, fileName] forKey:@"Content-Disposition"];
    [mutableHeaders setValue:mimeType forKey:@"Content-Type"];
    //把一个完整的请求体域封装进一个`AFHTTPBodyPart`对象中。
    AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init];
    bodyPart.stringEncoding = self.stringEncoding;
    bodyPart.headers = mutableHeaders;
    bodyPart.boundary = self.boundary;
    bodyPart.body = fileURL;
    bodyPart.bodyContentLength = [fileAttributes[NSFileSize] unsignedLongLongValue];
    [self.bodyStream appendHTTPBodyPart:bodyPart];

    return YES;
}
/**
 根据指定类型的数据流，把数据添加进入bodyStream中。以提供给后面构建request的body。
 
 @param inputStream 输入的数据流
 @param name 参数名称
 @param fileName 文件名称
 @param mimeType 文件类型
 */
- (void)appendPartWithInputStream:(NSInputStream *)inputStream
                             name:(NSString *)name
                         fileName:(NSString *)fileName
                           length:(int64_t)length
                         mimeType:(NSString *)mimeType
{
    NSParameterAssert(name);
    NSParameterAssert(fileName);
    NSParameterAssert(mimeType);
    //添加`Content-Disposition`和`Content-Type`这两个请求体域
    NSMutableDictionary *mutableHeaders = [NSMutableDictionary dictionary];
    [mutableHeaders setValue:[NSString stringWithFormat:@"form-data; name=\"%@\"; filename=\"%@\"", name, fileName] forKey:@"Content-Disposition"];
    [mutableHeaders setValue:mimeType forKey:@"Content-Type"];
    //把一个完整的请求体域封装进一个`AFHTTPBodyPart`对象中
    AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init];
    bodyPart.stringEncoding = self.stringEncoding;
    bodyPart.headers = mutableHeaders;
    bodyPart.boundary = self.boundary;
    bodyPart.body = inputStream;
    bodyPart.bodyContentLength = (unsigned long long)length;
    [self.bodyStream appendHTTPBodyPart:bodyPart];
}

/**
 根据指定的data添加到请求体域中

 @param data 数据
 @param name 名称
 @param fileName 文件名称
 @param mimeType mimeType
 */
- (void)appendPartWithFileData:(NSData *)data
                          name:(NSString *)name
                      fileName:(NSString *)fileName
                      mimeType:(NSString *)mimeType
{
    NSParameterAssert(name);
    NSParameterAssert(fileName);
    NSParameterAssert(mimeType);

    NSMutableDictionary *mutableHeaders = [NSMutableDictionary dictionary];
    [mutableHeaders setValue:[NSString stringWithFormat:@"form-data; name=\"%@\"; filename=\"%@\"", name, fileName] forKey:@"Content-Disposition"];
    [mutableHeaders setValue:mimeType forKey:@"Content-Type"];
    
    [self appendPartWithHeaders:mutableHeaders body:data];
}

/**
 根据指定的key和value拼接到`Content-Disposition`属性中

 @param data 参数值
 @param name 参数名
 */
- (void)appendPartWithFormData:(NSData *)data
                          name:(NSString *)name
{
    NSParameterAssert(name);

    NSMutableDictionary *mutableHeaders = [NSMutableDictionary dictionary];
    [mutableHeaders setValue:[NSString stringWithFormat:@"form-data; name=\"%@\"", name] forKey:@"Content-Disposition"];
    //把处理好的数据加入对应的request的请求体中`Content-Disposition`部分
    [self appendPartWithHeaders:mutableHeaders body:data];
}

/**
 给一个multipartForm的`Content-Disposition`添加boundary

 @param headers 请求头域
 @param body 值
 */
- (void)appendPartWithHeaders:(NSDictionary *)headers
                         body:(NSData *)body
{
    NSParameterAssert(body);

    AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init];
    bodyPart.stringEncoding = self.stringEncoding;
    bodyPart.headers = headers;
    bodyPart.boundary = self.boundary;
    bodyPart.bodyContentLength = [body length];
    bodyPart.body = body;

    [self.bodyStream appendHTTPBodyPart:bodyPart];
}

- (void)throttleBandwidthWithPacketSize:(NSUInteger)numberOfBytes
                                  delay:(NSTimeInterval)delay
{
    self.bodyStream.numberOfBytesInPacket = numberOfBytes;
    self.bodyStream.delay = delay;
}

/**
 根据一个request对应的`AFStreamingMultipartFormData`对象获取封装好的request对象

 @return multipart/form的request对象
 */
- (NSMutableURLRequest *)requestByFinalizingMultipartFormData {
    if ([self.bodyStream isEmpty]) {
        return self.request;
    }
    // Reset the initial and final boundaries to ensure correct Content-Length
    //重置boundary，从而确保`Content-Length`正确
    [self.bodyStream setInitialAndFinalBoundaries];
    //把拼接好的bodyStream添加进入request中
    [self.request setHTTPBodyStream:self.bodyStream];
    //给requst的请求头添加Content-Type属性指定为`multipart/form-data`类型的request。同时设置请求体的长度Content-Length。
    [self.request setValue:[NSString stringWithFormat:@"multipart/form-data; boundary=%@", self.boundary] forHTTPHeaderField:@"Content-Type"];
    [self.request setValue:[NSString stringWithFormat:@"%llu", [self.bodyStream contentLength]] forHTTPHeaderField:@"Content-Length"];
    return self.request;
}

4 AFJSONRequestSerializer和AFPropertyListRequestSerializer

这两个类继承自AFHTTPRequestSerializer。他们的基本实现都是继承自父类。但是也根据自身不同情况，做了处理。

对于AFJSONRequestSerializer。需要把Content-Type指定为"application/json。同时HTTPBody
需要使用JSON序列化：

- (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];
    //把`HTTPRequestHeaders`中的值添加进入请求头中。
    [self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
        if (![request valueForHTTPHeaderField:field]) {
            [mutableRequest setValue:value forHTTPHeaderField:field];
        }
    }];
    if (parameters) {
        //设置请求头的`Content-Type`类型
        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;
        }
        //JSON序列化的数据设置为httpbody
        [mutableRequest setHTTPBody:jsonData];
    }
    return mutableRequest;
}

对于AFPropertyListRequestSerializer也是同样的道理：

- (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];
    //把`HTTPRequestHeaders`中的值添加进入请求头中。
    [self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
        if (![request valueForHTTPHeaderField:field]) {
            [mutableRequest setValue:value forHTTPHeaderField:field];
        }
    }];
    if (parameters) {
        //设置请求头的`Content-Type`类型
        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {
            [mutableRequest setValue:@"application/x-plist" forHTTPHeaderField:@"Content-Type"];
        }
        //把parameters转换为Plist序列化的data
        NSData *plistData = [NSPropertyListSerialization dataWithPropertyList:parameters format:self.format options:self.writeOptions error:error];
        if (!plistData) {
            return nil;
        }
        //Plist序列化的数据设置为httpbody
        [mutableRequest setHTTPBody:plistData];
    }
    return mutableRequest;
}

5 总结

这个类主要实现了对于不同情况的请求的request对象的封装。尤其是对于multipart/form-data类型的request的封装，简化了我们自己封装过程的痛苦。如果我们要使用multipart/form-data类型的请求。强烈推荐使用AFHTTPSessionManager对象的AFHTTPRequestSerialization来处理参数的序列化过程。下面就是使用AFHTTPRequestSerailization序列化和自己拼装的不同：

- (IBAction)updatePic:(id)sender {
    //请求头参数
    NSDictionary *dic = @{
                          @"businessType":@"CC_USER_CENTER",
                          @"fileType":@"image",
                          @"file":@"img.jpeg"
                          };
    //请求体图片数据
    NSData *imageData = UIImagePNGRepresentation([UIImage imageNamed:@"1.png"]);
    //创建request
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:[NSURL URLWithString:url]];
    //post方法
    [request setHTTPMethod:@"POST"];
    AFHTTPSessionManager *manager = [[AFHTTPSessionManager alloc]initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
    NSURLSessionDataTask *task = [manager POST:url parameters:dic constructingBodyWithBlock:^(id<AFMultipartFormData>  _Nonnull formData) {
        //请求体里面的参数
        NSDictionary *bodyDic = @{
                                  @"Content-Disposition":@"form-data;name=\"file\";filename=\"img.jpeg\"",
                                  @"Content-Type":@"image/png",
                                  };
        [formData appendPartWithHeaders:bodyDic body:imageData];
    } progress:^(NSProgress * _Nonnull uploadProgress) {
        NSLog(@"下载进度");
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        NSLog(@"下载成功:%@",responseObject);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        NSLog(@"下载失败%@",error);
    }];
    [task resume];
}
- (IBAction)multipartformPost3:(id)sender {
    //参数
    NSDictionary *dic = @{
                          @"businessType":@"CC_USER_CENTER",
                          @"fileType":@"image",
                          @"file":@"img.jpeg"
                          };
    NSString *boundaryString = @"xxxxx";
    NSMutableString *str = [NSMutableString string];
    [dic enumerateKeysAndObjectsUsingBlock:^(id  _Nonnull key, id  _Nonnull obj, BOOL * _Nonnull stop) {
        [str appendFormat:@"--%@\r\n",boundaryString];
        [str appendFormat:@"%@name=\"%@\"\r\n\r\n",@"Content-Disposition: form-data;",key];
        [str appendFormat:@"%@\r\n",obj];
    }];
    
    NSMutableData *requestMutableData=[NSMutableData data];
    
    [str appendFormat:@"--%@\r\n",boundaryString];
    [str appendFormat:@"%@:%@",@"Content-Disposition",@"form-data;"];
    [str appendFormat:@"%@=\"%@\";",@"name",@"file"];
    [str appendFormat:@"%@=\"%@\"\r\n",@"filename",@"img1.jpeg"];
    [str appendFormat:@"%@:%@\r\n\r\n",@"Content-Type",@"image/png"];
    //转换成为二进制数据
    [requestMutableData appendData:[str dataUsingEncoding:NSUTF8StringEncoding]];
    NSData *imageData = UIImagePNGRepresentation([UIImage imageNamed:@"1.png"]);
    //文件数据部分
    [requestMutableData appendData:imageData];
    //添加结尾boundary
    [requestMutableData appendData:[[NSString stringWithFormat:@"\r\n--%@--\r\n",boundaryString] dataUsingEncoding:NSUTF8StringEncoding]];

    
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:[NSURL URLWithString:url]];
    //post方法
    [request setHTTPMethod:@"POST"];
    // 设置请求头格式为Content-Type:multipart/form-data; boundary=xxxxx
    [request setValue:[NSString stringWithFormat:@"multipart/form-data; boundary=%@",boundaryString] forHTTPHeaderField:@"Content-Type"];
    request.HTTPBody = requestMutableData;
    
    NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
    NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSString *result = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];
        NSLog(@"%@",result);
    }];
    
    [task resume];

}

最后原文地址,demo地址。