iOS源码解析—AFNetworking(RequestSerializer)

概述

​ AFNetworking作为著名的网络请求框架，被开发者广泛运用。自从AF3.x开始，废弃了之前NSURLConnection的版本，使用NSURLSession，但是之前的AF2.x版本仍然有许多值得学习的地方。以2.6.2版本为例，本系列将对该框架进行全面的学习和分析。AFNetworking框架主要包含网络通信、序列化/反序列化、网络性能监听、网络通信安全四个模块，本文主要讲解AFURLRequestSerialization相关类。

初始化

AFHTTPRequestSerializer是用于构建NSURLRequest的类，通过-init:方法进行初始化，指定编码方式为NSUTF-8。HTTP请求分为请求头和请求体，请求头主要包含以下字段：

User-Agent、Pragma、Content-Type、Content-Length、Accept-Language、Accept、Accept-Encoding、Cookie

init方法，指定了请求头的Accept-Language，根据版本号、操作系统版本、设备屏幕scale等信息生成User-Agent，然后设置一些参数，如HTTPMethodsEncodingParametersInURI包含了指定的HTTP method，mutableObservedChangedKeyPaths存放监听到的属性，同时开始kvo监听。部分代码注释如下：

- (instancetype)init {    self.stringEncoding = NSUTF8StringEncoding; //UTF-8    self.mutableHTTPRequestHeaders = [NSMutableDictionary dictionary]; //请求头配置    ...    [self setValue:[acceptLanguagesComponents componentsJoinedByString:@", "] forHTTPHeaderField:@"Accept-Language"]; //设置Accept-Language字段    ...    [self setValue:userAgent forHTTPHeaderField:@"User-Agent"];//设置User-Agent字段    ...    self.HTTPMethodsEncodingParametersInURI = [NSSet setWithObjects:@"GET", @"HEAD", @"DELETE", nil]; //包含指定的http method    self.mutableObservedChangedKeyPaths = [NSMutableSet set]; //kvo属性集合    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {        if ([self respondsToSelector:NSSelectorFromString(keyPath)]) { //kvo监听部分属性            [self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];        }    }    return self;}

AFHTTPRequestSerializer重写了属性的set方法，手动实现kvo通知，这样当调用这些set方法时，会监听到这些属性值的改变，存入mutableObservedChangedKeyPaths中，例如：

- (void)setAllowsCellularAccess:(BOOL)allowsCellularAccess {    [self willChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];    _allowsCellularAccess = allowsCellularAccess;    [self didChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];}

下面开始构建NSURLRequest，构建的类型主要有两种，普通类型和multipart类型。首先看一下构建普通类型的方法。

普通类型

通过-requestWithMethod:URLString:parameters:error方法构建NSURLRequest，下面是代码注释：

- (NSMutableURLRequest *)requestWithMethod:(NSString *)method                                 URLString:(NSString *)URLString                                parameters:(id)parameters                                     error:(NSError *__autoreleasing *)error{    NSParameterAssert(method);    NSParameterAssert(URLString);    NSURL *url = [NSURL URLWithString:URLString]; //构建URL    NSParameterAssert(url);    //根据url创建URLRequest对象    NSMutableURLRequest *mutableRequest = [[NSMutableURLRequest alloc] initWithURL:url];    mutableRequest.HTTPMethod = method; //http method    //监听的发生变化的属性，设置为http的请求头    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {        if ([self.mutableObservedChangedKeyPaths containsObject:keyPath]) {            [mutableRequest setValue:[self valueForKeyPath:keyPath] forKey:keyPath];        }    }    //进一步处理NSURLRequest    mutableRequest = [[self requestBySerializingRequest:mutableRequest withParameters:parameters error:error] mutableCopy];    return mutableRequest;}

首先创建NSURLRequest对象，然后调用-requestBySerializingRequest:withParameters:error:方法根据参数进一步设置NSURLRequest对象，下面是代码注释：

- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request                               withParameters:(id)parameters                                        error:(NSError *__autoreleasing *)error{    ...    //构建请求头参数    [self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {        if (![request valueForHTTPHeaderField:field]) {            [mutableRequest setValue:value forHTTPHeaderField:field];        }    }];    NSString *query = nil;    if (parameters) {        if (self.queryStringSerialization) {            NSError *serializationError;            //自定义方法，构建query            query = self.queryStringSerialization(request, parameters, &serializationError);            ...        } else {            switch (self.queryStringSerializationStyle) {                case AFHTTPRequestQueryStringDefaultStyle:                    //按照一定规则构建query                    query = AFQueryStringFromParameters(parameters);                    break;            }        }    }    if ([self.HTTPMethodsEncodingParametersInURI containsObject:[[request HTTPMethod] uppercaseString]]) {        if (query) { //将query拼在url后面            mutableRequest.URL = [NSURL URLWithString:[[mutableRequest.URL absoluteString] stringByAppendingFormat:mutableRequest.URL.query ? @"&%@" : @"?%@", query]];        }    } else {        ...        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {            [mutableRequest setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];        }        //query设置为httpBody        [mutableRequest setHTTPBody:[query dataUsingEncoding:self.stringEncoding]];    }    return mutableRequest;}

该方法首先设置http请求的head参数，然后将parameters转化为url的query，通过AFQueryStringFromParameters方法返回一个字符串，格式是：”key1=value1&key2=value2”，代码注释如下：

static NSString * AFQueryStringFromParameters(NSDictionary *parameters) {    NSMutableArray *mutablePairs = [NSMutableArray array];    for (AFQueryStringPair *pair in AFQueryStringPairsFromDictionary(parameters)) {        [mutablePairs addObject:[pair URLEncodedStringValue]]; //url encode，返回key=value格式的字符串    }    return [mutablePairs componentsJoinedByString:@"&"];}

思路是首先根据AFQueryStringPairsFromDictionary方法的会根据parameters创建一个数组，无论parameters是否嵌套，创建后的数组中的元素在同一层级。数组中的元素是AFQueryStringPair对象，包装了key/value，然后调用URLEncodedStringValue方法将pair对象转化为key=value格式的字符串，且key和value都做了encode。最后将返回的字符传用”&”符号连接，生成query字符串。

接着判断当前http请求的method是否包含在HTTPMethodsEncodingParametersInURI中，如果是GET请求，则包含在内，会将生成的将query拼在url后面，因为GET请求不设置http的request body。如果是POST请求，则将query字符串序列化成nsdata后，设置为http的request body。

另外，有两个子类继承了AFHTTPRequestSerializer类，即AFJSONRequestSerializer和AFPropertyListRequestSerializer，都重写了父类的方法，下面看一下AFJSONRequestSerializer的方法：

- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request                               withParameters:(id)parameters                                        error:(NSError *__autoreleasing *)error{    NSParameterAssert(request);    //如果是"GET、HEAD、DELETE"方法，和父类一样    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]) {            //设置http请求头参数            [mutableRequest setValue:value forHTTPHeaderField:field];        }    }];    //parameters，设置http请求体    if (parameters) {        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {            [mutableRequest setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];        }        [mutableRequest setHTTPBody:[NSJSONSerialization dataWithJSONObject:parameters options:self.writingOptions error:error]];    }    return mutableRequest;}

分析该方法，当http的method是POST类型时，AFJSONRequestSerializer和父类的处理不同，该方法将parameters序列化为JSON数据，设置为http的请求体，且请求头的”Content-Type”参数设置为”application/json”。AFPropertyListRequestSerializer的方法如下：

- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request                               withParameters:(id)parameters                                        error:(NSError *__autoreleasing *)error{    ...    if (parameters) {        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {            [mutableRequest setValue:@"application/x-plist" forHTTPHeaderField:@"Content-Type"];        }        [mutableRequest setHTTPBody:[NSPropertyListSerialization dataWithPropertyList:parameters format:self.format options:self.writeOptions error:error]];    }    return mutableRequest;}

该方法逻辑类似，当http的method是POST类型时，调用NSPropertyListSerialization的方法将parameters序列化成NSData，赋值给http的请求体。

multipart类型

Multipart是HTTP协议为web表单新增的上传文件的协议，基于POST方法，数据放在http请求体，不同于普通post请求的key/value格式或者，json格式，multipart类型的请求体较长，且遵循一定的格式，下面是格式：

--BoundaryAaB03x content-disposition: form-data; name="name"//空行abcdef... --BoundaryAaB03x content-disposition: form-data; name=”pic”; filename=“content.txt” Content-Type: text/plain//空行 ...contents of abc.txt...--BoundaryAaB03x content-disposition: form-data; name=”pic”; filename=“content.txt” Content-Type: text/plain//空行 ...contents of abc.txt...--BoundaryAaB03x--

可以看出请求的数据可以分成若干个段落，每个段落由分隔符隔开，且顶部和中间的分隔符前面有”–”，底部的分隔符前后都有”–”，每个段落的格式固定如下：

头部字段//空行数据实体

下面来分析一下multipart类型的请求方法：

- (NSMutableURLRequest *)multipartFormRequestWithMethod:(NSString *)method                                              URLString:(NSString *)URLString                                             parameters:(NSDictionary *)parameters                              constructingBodyWithBlock:(void (^)(id <AFMultipartFormData> formData))block                                                  error:(NSError *__autoreleasing *)error{    NSParameterAssert(method);    //不支持GET、HEAD类型    NSParameterAssert(![method isEqualToString:@"GET"] && ![method isEqualToString:@"HEAD"]);    //创建NSURLRequest对象    NSMutableURLRequest *mutableRequest = [self requestWithMethod:method URLString:URLString parameters:nil error:error];    //创建AFStreamingMultipartFormData对象    __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对象追加参数                [formData appendPartWithFormData:data name:[pair.field description]];            }        }    }    //formData对象追加参数    if (block) {        block(formData);    }    //设置request header、request body    return [formData requestByFinalizingMultipartFormData];}

首先排除了GET和HEAD类型的请求，然后根据url创建NSURLRequest对象，然后创建创建AFStreamingMultipartFormData对象，该对象负责为NSURLRequest对象拼接请求体数据，下面主要分析AFStreamingMultipartFormData对象的结构和方法：

@interface AFStreamingMultipartFormData ()@property (readwrite, nonatomic, copy) NSMutableURLRequest *request; //URLRequest@property (readwrite, nonatomic, assign) NSStringEncoding stringEncoding; //字符串编码@property (readwrite, nonatomic, copy) NSString *boundary; //边界标识@property (readwrite, nonatomic, strong) AFMultipartBodyStream *bodyStream;//body数据@end//初始化- (id)initWithURLRequest:(NSMutableURLRequest *)urlRequest          stringEncoding:(NSStringEncoding)encoding{    self = [super init];    if (!self) {        return nil;    }    self.request = urlRequest;    self.stringEncoding = encoding;    self.boundary = AFCreateMultipartFormBoundary(); //生成边界字符串    self.bodyStream = [[AFMultipartBodyStream alloc] initWithStringEncoding:encoding]; //创建请求体数据    return self;}

AFStreamingMultipartFormData内部维护了一个NSMutableURLRequest对象，编码方式（字符串转成NAData的编码方式），边界字符串和AFMultipartBodyStream对象，其中AFMultipartBodyStream对象负责维护请求体的各个数据段落。下面是该对象的定义注释：

@interface AFMultipartBodyStream () <NSCopying>@property (readwrite, nonatomic, assign) NSStringEncoding stringEncoding;@property (readwrite, nonatomic, strong) NSMutableArray *HTTPBodyParts;@property (readwrite, nonatomic, strong) NSEnumerator *HTTPBodyPartEnumerator;@property (readwrite, nonatomic, strong) AFHTTPBodyPart *currentHTTPBodyPart;@property (readwrite, nonatomic, strong) NSOutputStream *outputStream;@property (readwrite, nonatomic, strong) NSMutableData *buffer;@end

其中HTTPBodyParts是一个数组，数组中每个元素是一个AFHTTPBodyPart对象，AFHTTPBodyPart对象封装了请求体数据中每个段落的信息，代码注释如下：

@interface AFHTTPBodyPart : NSObject@property (nonatomic, assign) NSStringEncoding stringEncoding; //编码方式@property (nonatomic, strong) NSDictionary *headers; //段落头部信息@property (nonatomic, copy) NSString *boundary; //边界字符串@property (nonatomic, strong) id body; //段落体数据@property (nonatomic, assign) unsigned long long bodyContentLength; //段落体数据长度@property (nonatomic, strong) NSInputStream *inputStream; //数据流，用于读取body内如进buffer@property (nonatomic, assign) BOOL hasInitialBoundary; //是否是开始边界@property (nonatomic, assign) BOOL hasFinalBoundary; //是否是结束边界...@end

-multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:方法接下来将parameters参数转化成AFQueryStringPair对象数组，将每个pair对象的key和value添加到formData对象维护的段落数组中。formData支持三种格式的数据，即NData、FileURL和NSInputStream，下面依次分析一下：

1. NSData格式：

   例如-appendPartWithFormData:name:方法和-appendPartWithFileData:name:fileName:mimeType:方法，首先拼装段落头部信息，设置Content-Disposition字段和Content-Type字段，然后将data作为body构建AFHTTPBodyPart对象，添加进bodyStream中的段落数组HTTPBodyParts中。下面是代码注释：

   - (void)appendPartWithFileData:(NSData *)data                         name:(NSString *)name                     fileName:(NSString *)fileName                     mimeType:(NSString *)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]; //mutableHeaders和data包装成AFHTTPBodyPart对象，加入HTTPBodyParts数组中}

2. FileURL格式：

   例如-appendPartWithFileURL:name:fileName:mimeType:error:方法，首先构建段落头部信息，设置Content-Disposition字段和Content-Type字段，然后构建AFHTTPBodyPart对象，body属性是fileURL，即本地文件路径，bodyContentLength属性是文件的大小，最后添加进HTTPBodyParts中。下面是代码注释：

   - (BOOL)appendPartWithFileURL:(NSURL *)fileURL                        name:(NSString *)name                    fileName:(NSString *)fileName                    mimeType:(NSString *)mimeType                       error:(NSError * __autoreleasing *)error{   ...   NSMutableDictionary *mutableHeaders = [NSMutableDictionary dictionary];   [mutableHeaders setValue:[NSString stringWithFormat:@"form-data; name=\"%@\"; filename=\"%@\"", name, fileName] forKey:@"Content-Disposition"];   [mutableHeaders setValue:mimeType forKey:@"Content-Type"]; //段落头部headers   AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init]; //创建AFHTTPBodyPart对象   bodyPart.stringEncoding = self.stringEncoding;   bodyPart.headers = mutableHeaders;   bodyPart.boundary = self.boundary;   bodyPart.body = fileURL; //body属性是文件路径   bodyPart.bodyContentLength = [fileAttributes[NSFileSize] unsignedLongLongValue];   [self.bodyStream appendHTTPBodyPart:bodyPart]; //添加进HTTPBodyParts中   return YES;}

3. NSInputStream格式：

   例如-appendPartWithInputStream:name:fileName:length:mimeType:方法，首先构建段落信息，设置Content-Disposition字段和Content-Type字段，然后构建AFHTTPBodyPart对象，body属性是NSInputStream对象，添加进HTTPBodyParts中。下面是代码注释：

   - (void)appendPartWithInputStream:(NSInputStream *)inputStream                            name:(NSString *)name                        fileName:(NSString *)fileName                          length:(int64_t)length                        mimeType:(NSString *)mimeType{   ...   NSMutableDictionary *mutableHeaders = [NSMutableDictionary dictionary];   [mutableHeaders setValue:[NSString stringWithFormat:@"form-data; name=\"%@\"; filename=\"%@\"", name, fileName] forKey:@"Content-Disposition"];   [mutableHeaders setValue:mimeType forKey:@"Content-Type"];//段落头部headers   AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init]; //创建AFHTTPBodyPart对象   bodyPart.stringEncoding = self.stringEncoding;   bodyPart.headers = mutableHeaders;   bodyPart.boundary = self.boundary;   bodyPart.body = inputStream; //body属性是NSInputStream   bodyPart.bodyContentLength = (unsigned long long)length;   [self.bodyStream appendHTTPBodyPart:bodyPart]; //添加进HTTPBodyParts中}

以上3种格式的数据添加进HTTPBodyParts中时，在接下来的过程中，都会转化二进制数据拼装在http的请求体报文中。

-multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:方法接下来调用block(formData)供外界调用层继续添加数据，方式可以使以上3种中的任意一种。然后调用-requestByFinalizingMultipartFormData方法为NSURLRequest对象添加请求报文的头，因为采用multipart类型的报文格式，所以设置Content-Type为”multipart/form-data”，下面是代码注释：

- (NSMutableURLRequest *)requestByFinalizingMultipartFormData {    if ([self.bodyStream isEmpty]) {        return self.request;    }    //设置出初始和结尾的边界    [self.bodyStream setInitialAndFinalBoundaries];    //将bodyStream作为请求报文体    [self.request setHTTPBodyStream:self.bodyStream];    //设置请求头的Content-Type和Content-Length字段    [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;}

其中self.bodyStream属性是AFMultipartBodyStream类型，继承于NSInputStream，将该属性设置为http的HTTPBodyStream属性，这样在发送http请求时，会按照文件流的方式发送请求报文，且上传数据是分片的，不会一次性将报文体数据读入内存中后发送，关于这种方式的比较，可以参考JSPatch大神bang的文章。

接下来通过NSURLSession发送请求时，调用-uploadTaskWithStreamedRequest:方法构建NSURLSessionUploadTask对象uploadTask，调用resume开始发送请求。由于已经设置bodyStream为NSInputStream对象，这种方式发送请求时会读取bodyStream的数据，调用NSInputStream对象的-read:maxLength:方法，而AFMultipartBodyStream对象重写了-read:maxLength:方法，代码注释如下：

- (NSInteger)read:(uint8_t *)buffer        maxLength:(NSUInteger)length{    ...    while ((NSUInteger)totalNumberOfBytesRead < MIN(length, self.numberOfBytesInPacket)) {        if (!self.currentHTTPBodyPart || ![self.currentHTTPBodyPart hasBytesAvailable]) {            if (!(self.currentHTTPBodyPart = [self.HTTPBodyPartEnumerator nextObject])) {                break;            }        } else {            NSUInteger maxLength = length - (NSUInteger)totalNumberOfBytesRead;            NSInteger numberOfBytesRead = [self.currentHTTPBodyPart read:&buffer[totalNumberOfBytesRead] maxLength:maxLength];            if (numberOfBytesRead == -1) {                self.streamError = self.currentHTTPBodyPart.inputStream.streamError;                break;            } else {                totalNumberOfBytesRead += numberOfBytesRead;                if (self.delay > 0.0f) {                    [NSThread sleepForTimeInterval:self.delay];                }            }        }    }#pragma clang diagnostic pop    return totalNumberOfBytesRead;}

该方法遍历HTTPBodyParts数组中的各个元素，其中HTTPBodyPartEnumerator是枚举器，依附于HTTPBodyParts数组，调用nextObject方法取出数组中的对象作为当前对象currentHTTPBodyPart。然后调用AFHTTPBodyPart的-read:maxLength:方法将AFHTTPBodyPart对象中的各个部分读入buffer中。下面是代码注释：

- (NSInteger)read:(uint8_t *)buffer        maxLength:(NSUInteger)length{    NSInteger totalNumberOfBytesRead = 0;    if (_phase == AFEncapsulationBoundaryPhase) { //边界部分        ...    }    if (_phase == AFHeaderPhase) { //header部分        ...    }    if (_phase == AFBodyPhase) { //body部分        ...    }    if (_phase == AFFinalBoundaryPhase) { //尾部边界        ...    }    return totalNumberOfBytesRead;}

通过一组枚举类型AFHTTPBodyPartReadPhase表示段落中的不同部分，依次将它们生成报文数据，然后读入buffer中，生成数据的时候需要按照rfc1867规定，在相应地方添加\r\n换行符。当前阶段读完数据后调用-transitionToNextPhase方法进入下一个部分。需要注意的是，_phase==AFBodyPhase阶段时，通过self.inputStream对象读取数据进buffer中，self.inputStream是内部持有的一个属性，body中的数据由inputStream负责加载，通过懒加载的方式初始化：

- (NSInputStream *)inputStream {    if (!_inputStream) {        if ([self.body isKindOfClass:[NSData class]]) {            _inputStream = [NSInputStream inputStreamWithData:self.body];        } else if ([self.body isKindOfClass:[NSURL class]]) {            _inputStream = [NSInputStream inputStreamWithURL:self.body];        } else if ([self.body isKindOfClass:[NSInputStream class]]) {            _inputStream = self.body;        } else {            _inputStream = [NSInputStream inputStreamWithData:[NSData data]];        }    }    return _inputStream;}

inputStream会根据之前存入AFHTTPBodyPart对象中的body数据类型来初始化inputStream，说明之前3种格式的数据最终都会转化成NSInputStream对象inputStream，然后读取inputStream就可以。关于读取body数据进buffer的代码注释如下：

if (_phase == AFBodyPhase) {    NSInteger numberOfBytesRead = 0;    //从inputStream中读取数据    numberOfBytesRead = [self.inputStream read:&buffer[totalNumberOfBytesRead] maxLength:(length - (NSUInteger)totalNumberOfBytesRead)];    if (numberOfBytesRead == -1) {        return -1;    } else {        totalNumberOfBytesRead += numberOfBytesRead;        if ([self.inputStream streamStatus] >= NSStreamStatusAtEnd) {            [self transitionToNextPhase];        }    }}

totalNumberOfBytesRead表示总共读取的字节数。AFMultipartBodyStream对象通过重写父类的-read:maxLength:方法实现了拼接请求报文的body数据。