iOS原始碼解析—AFNetworking(RequestSerializer)

weixin_34337265發表於2017-03-14
iOS原始碼

概述

​ AFNetworking作為著名的網路請求框架，被開發者廣泛運用。自從AF3.x開始，廢棄了之前NSURLConnection的版本，使用NSURLSession，但是之前的AF2.x版本仍然有許多值得學習的地方。以2.6.2版本為例，本系列將對該框架進行全面的學習和分析。AFNetworking框架主要包含網路通訊、序列化/反序列化、網路效能監聽、網路通訊安全四個模組，本文主要講解AFURLRequestSerialization相關類。

初始化

AFHTTPRequestSerializer是用於構建NSURLRequest的類，通過-init:方法進行初始化，指定編碼方式為NSUTF-8。HTTP請求分為請求頭和請求體，請求頭主要包含以下欄位：

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

init方法，指定了請求頭的Accept-Language，根據版本號、作業系統版本、裝置螢幕scale等資訊生成User-Agent，然後設定一些引數，如HTTPMethodsEncodingParametersInURI包含了指定的HTTP method，mutableObservedChangedKeyPaths存放監聽到的屬性，同時開始kvo監聽。部分程式碼註釋如下：

- (instancetype)init {
    self.stringEncoding = NSUTF8StringEncoding; //UTF-8
    self.mutableHTTPRequestHeaders = [NSMutableDictionary dictionary]; //請求頭配置
    ...
    [self setValue:[acceptLanguagesComponents componentsJoinedByString:@", "] forHTTPHeaderField:@"Accept-Language"]; //設定Accept-Language欄位
    ...
    [self setValue:userAgent forHTTPHeaderField:@"User-Agent"];//設定User-Agent欄位
    ...
    self.HTTPMethodsEncodingParametersInURI = [NSSet setWithObjects:@"GET", @"HEAD", @"DELETE", nil]; //包含指定的http method

    self.mutableObservedChangedKeyPaths = [NSMutableSet set]; //kvo屬性集合
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self respondsToSelector:NSSelectorFromString(keyPath)]) { //kvo監聽部分屬性
            [self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];
        }
    }
    return self;
}

AFHTTPRequestSerializer重寫了屬性的set方法，手動實現kvo通知，這樣當呼叫這些set方法時，會監聽到這些屬性值的改變，存入mutableObservedChangedKeyPaths中，例如：

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

下面開始構建NSURLRequest，構建的型別主要有兩種，普通型別和multipart型別。首先看一下構建普通型別的方法。

普通型別

通過-requestWithMethod:URLString:parameters:error方法構建NSURLRequest，下面是程式碼註釋：

- (NSMutableURLRequest *)requestWithMethod:(NSString *)method
                                 URLString:(NSString *)URLString
                                parameters:(id)parameters
                                     error:(NSError *__autoreleasing *)error
{
    NSParameterAssert(method);
    NSParameterAssert(URLString);
    NSURL *url = [NSURL URLWithString:URLString]; //構建URL
    NSParameterAssert(url);
    //根據url建立URLRequest物件
    NSMutableURLRequest *mutableRequest = [[NSMutableURLRequest alloc] initWithURL:url];
    mutableRequest.HTTPMethod = method; //http method

    //監聽的發生變化的屬性，設定為http的請求頭
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self.mutableObservedChangedKeyPaths containsObject:keyPath]) {
            [mutableRequest setValue:[self valueForKeyPath:keyPath] forKey:keyPath];
        }
    }
    //進一步處理NSURLRequest
    mutableRequest = [[self requestBySerializingRequest:mutableRequest withParameters:parameters error:error] mutableCopy];
    return mutableRequest;
}

首先建立NSURLRequest物件，然後呼叫-requestBySerializingRequest:withParameters:error:方法根據引數進一步設定NSURLRequest物件，下面是程式碼註釋：

- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
                               withParameters:(id)parameters
                                        error:(NSError *__autoreleasing *)error
{
    ...
    //構建請求頭引數
    [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;
            //自定義方法，構建query
            query = self.queryStringSerialization(request, parameters, &serializationError);
            ...
        } else {
            switch (self.queryStringSerializationStyle) {
                case AFHTTPRequestQueryStringDefaultStyle:
                    //按照一定規則構建query
                    query = AFQueryStringFromParameters(parameters);
                    break;
            }
        }
    }
    if ([self.HTTPMethodsEncodingParametersInURI containsObject:[[request HTTPMethod] uppercaseString]]) {
        if (query) { //將query拼在url後面
            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設定為httpBody
        [mutableRequest setHTTPBody:[query dataUsingEncoding:self.stringEncoding]];
    }
    return mutableRequest;
}

該方法首先設定http請求的head引數，然後將parameters轉化為url的query，通過AFQueryStringFromParameters方法返回一個字串，格式是："key1=value1&key2=value2"，程式碼註釋如下：

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格式的字串
    }
    return [mutablePairs componentsJoinedByString:@"&"];
}

思路是首先根據AFQueryStringPairsFromDictionary方法的會根據parameters建立一個陣列，無論parameters是否巢狀，建立後的陣列中的元素在同一層級。陣列中的元素是AFQueryStringPair物件，包裝了key/value，然後呼叫URLEncodedStringValue方法將pair物件轉化為key=value格式的字串，且key和value都做了encode。最後將返回的字元傳用"&"符號連線，生成query字串。

接著判斷當前http請求的method是否包含在HTTPMethodsEncodingParametersInURI中，如果是GET請求，則包含在內，會將生成的將query拼在url後面，因為GET請求不設定http的request body。如果是POST請求，則將query字串序列化成nsdata後，設定為http的request body。

另外，有兩個子類繼承了AFHTTPRequestSerializer類，即AFJSONRequestSerializer和AFPropertyListRequestSerializer，都重寫了父類的方法，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，和父類一樣
    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]) {
            //設定http請求頭引數
            [mutableRequest setValue:value forHTTPHeaderField:field];
        }
    }];
    //parameters，設定http請求體
    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;
}

分析該方法，當http的method是POST型別時，AFJSONRequestSerializer和父類的處理不同，該方法將parameters序列化為JSON資料，設定為http的請求體，且請求頭的"Content-Type"引數設定為"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;
}

該方法邏輯類似，當http的method是POST型別時，呼叫NSPropertyListSerialization的方法將parameters序列化成NSData，賦值給http的請求體。

multipart型別

Multipart是HTTP協議為web表單新增的上傳檔案的協議，基於POST方法，資料放在http請求體，不同於普通post請求的key/value格式或者，json格式，multipart型別的請求體較長，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--

可以看出請求的資料可以分成若干個段落，每個段落由分隔符隔開，且頂部和中間的分隔符前面有"--"，底部的分隔符前後都有"--"，每個段落的格式固定如下：

頭部欄位
//空行
資料實體

下面來分析一下multipart型別的請求方法：

- (NSMutableURLRequest *)multipartFormRequestWithMethod:(NSString *)method
                                              URLString:(NSString *)URLString
                                             parameters:(NSDictionary *)parameters
                              constructingBodyWithBlock:(void (^)(id <AFMultipartFormData> formData))block
                                                  error:(NSError *__autoreleasing *)error
{
    NSParameterAssert(method);
    //不支援GET、HEAD型別
    NSParameterAssert(![method isEqualToString:@"GET"] && ![method isEqualToString:@"HEAD"]);
    //建立NSURLRequest物件
    NSMutableURLRequest *mutableRequest = [self requestWithMethod:method URLString:URLString parameters:nil error:error];
    //建立AFStreamingMultipartFormData物件
    __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物件追加引數
                [formData appendPartWithFormData:data name:[pair.field description]];
            }
        }
    }
    //formData物件追加引數
    if (block) {
        block(formData);
    }
    //設定request header、request body
    return [formData requestByFinalizingMultipartFormData];
}

首先排除了GET和HEAD型別的請求，然後根據url建立NSURLRequest物件，然後建立建立AFStreamingMultipartFormData物件，該物件負責為NSURLRequest物件拼接請求體資料，下面主要分析AFStreamingMultipartFormData物件的結構和方法：

@interface AFStreamingMultipartFormData ()
@property (readwrite, nonatomic, copy) NSMutableURLRequest *request; //URLRequest
@property (readwrite, nonatomic, assign) NSStringEncoding stringEncoding; //字串編碼
@property (readwrite, nonatomic, copy) NSString *boundary; //邊界標識
@property (readwrite, nonatomic, strong) AFMultipartBodyStream *bodyStream;//body資料
@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(); //生成邊界字串
    self.bodyStream = [[AFMultipartBodyStream alloc] initWithStringEncoding:encoding]; //建立請求體資料
    return self;
}

AFStreamingMultipartFormData內部維護了一個NSMutableURLRequest物件，編碼方式（字串轉成NAData的編碼方式），邊界字串和AFMultipartBodyStream物件，其中AFMultipartBodyStream物件負責維護請求體的各個資料段落。下面是該物件的定義註釋：

@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是一個陣列，陣列中每個元素是一個AFHTTPBodyPart物件，AFHTTPBodyPart物件封裝了請求體資料中每個段落的資訊，程式碼註釋如下：

@interface AFHTTPBodyPart : NSObject
@property (nonatomic, assign) NSStringEncoding stringEncoding; //編碼方式
@property (nonatomic, strong) NSDictionary *headers; //段落頭部資訊
@property (nonatomic, copy) NSString *boundary; //邊界字串
@property (nonatomic, strong) id body; //段落體資料
@property (nonatomic, assign) unsigned long long bodyContentLength; //段落體資料長度
@property (nonatomic, strong) NSInputStream *inputStream; //資料流，用於讀取body內如進buffer
@property (nonatomic, assign) BOOL hasInitialBoundary; //是否是開始邊界
@property (nonatomic, assign) BOOL hasFinalBoundary; //是否是結束邊界
...
@end

-multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:方法接下來將parameters引數轉化成AFQueryStringPair物件陣列，將每個pair物件的key和value新增到formData物件維護的段落陣列中。formData支援三種格式的資料，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，首先拼裝段落頭部資訊，設定Content-Disposition欄位和Content-Type欄位，然後將data作為body構建AFHTTPBodyPart物件，新增進bodyStream中的段落陣列HTTPBodyParts中。下面是程式碼註釋：

    - (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包裝成AFHTTPBodyPart物件，加入HTTPBodyParts陣列中
}

  2. FileURL格式：

    例如-appendPartWithFileURL:name:fileName:mimeType:error:方法，首先構建段落頭部資訊，設定Content-Disposition欄位和Content-Type欄位，然後構建AFHTTPBodyPart物件，body屬性是fileURL，即本地檔案路徑，bodyContentLength屬性是檔案的大小，最後新增進HTTPBodyParts中。下面是程式碼註釋：

    - (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"]; //段落頭部headers
    AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init]; //建立AFHTTPBodyPart物件
    bodyPart.stringEncoding = self.stringEncoding;
    bodyPart.headers = mutableHeaders;
    bodyPart.boundary = self.boundary;
    bodyPart.body = fileURL; //body屬性是檔案路徑
    bodyPart.bodyContentLength = [fileAttributes[NSFileSize] unsignedLongLongValue];
    [self.bodyStream appendHTTPBodyPart:bodyPart]; //新增進HTTPBodyParts中
    return YES;
}

  3. NSInputStream格式：

    例如-appendPartWithInputStream:name:fileName:length:mimeType:方法，首先構建段落資訊，設定Content-Disposition欄位和Content-Type欄位，然後構建AFHTTPBodyPart物件，body屬性是NSInputStream物件，新增進HTTPBodyParts中。下面是程式碼註釋：

    - (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"];//段落頭部headers
    AFHTTPBodyPart *bodyPart = [[AFHTTPBodyPart alloc] init]; //建立AFHTTPBodyPart物件
    bodyPart.stringEncoding = self.stringEncoding;
    bodyPart.headers = mutableHeaders;
    bodyPart.boundary = self.boundary;
    bodyPart.body = inputStream; //body屬性是NSInputStream
    bodyPart.bodyContentLength = (unsigned long long)length;
    [self.bodyStream appendHTTPBodyPart:bodyPart]; //新增進HTTPBodyParts中
}

以上3種格式的資料新增進HTTPBodyParts中時，在接下來的過程中，都會轉化二進位制資料拼裝在http的請求體報文中。

-multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:方法接下來呼叫block(formData)供外界呼叫層繼續新增資料，方式可以使以上3種中的任意一種。然後呼叫-requestByFinalizingMultipartFormData方法為NSURLRequest物件新增請求報文的頭，因為採用multipart型別的報文格式，所以設定Content-Type為"multipart/form-data"，下面是程式碼註釋：

- (NSMutableURLRequest *)requestByFinalizingMultipartFormData {
    if ([self.bodyStream isEmpty]) {
        return self.request;
    }
    //設定出初始和結尾的邊界
    [self.bodyStream setInitialAndFinalBoundaries];
    //將bodyStream作為請求報文體
    [self.request setHTTPBodyStream:self.bodyStream];
    //設定請求頭的Content-Type和Content-Length欄位
    [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屬性是AFMultipartBodyStream型別，繼承於NSInputStream，將該屬性設定為http的HTTPBodyStream屬性，這樣在傳送http請求時，會按照檔案流的方式傳送請求報文，且上傳資料是分片的，不會一次性將報文體資料讀入記憶體中後傳送，關於這種方式的比較，可以參考JSPatch大神bang的文章。

接下來通過NSURLSession傳送請求時，呼叫-uploadTaskWithStreamedRequest:方法構建NSURLSessionUploadTask物件uploadTask，呼叫resume開始傳送請求。由於已經設定bodyStream為NSInputStream物件，這種方式傳送請求時會讀取bodyStream的資料，呼叫NSInputStream物件的-read:maxLength:方法，而AFMultipartBodyStream物件重寫了-read:maxLength:方法，程式碼註釋如下：

- (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;
}

該方法遍歷HTTPBodyParts陣列中的各個元素，其中HTTPBodyPartEnumerator是列舉器，依附於HTTPBodyParts陣列，呼叫nextObject方法取出陣列中的物件作為當前物件currentHTTPBodyPart。然後呼叫AFHTTPBodyPart的-read:maxLength:方法將AFHTTPBodyPart物件中的各個部分讀入buffer中。下面是程式碼註釋：

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

通過一組列舉型別AFHTTPBodyPartReadPhase表示段落中的不同部分，依次將它們生成報文資料，然後讀入buffer中，生成資料的時候需要按照rfc1867規定，在相應地方新增\r\n換行符。當前階段讀完資料後呼叫-transitionToNextPhase方法進入下一個部分。需要注意的是，_phase==AFBodyPhase階段時，通過self.inputStream物件讀取資料進buffer中，self.inputStream是內部持有的一個屬性，body中的資料由inputStream負責載入，通過懶載入的方式初始化：

- (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會根據之前存入AFHTTPBodyPart物件中的body資料型別來初始化inputStream，說明之前3種格式的資料最終都會轉化成NSInputStream物件inputStream，然後讀取inputStream就可以。關於讀取body資料進buffer的程式碼註釋如下：

if (_phase == AFBodyPhase) {
    NSInteger numberOfBytesRead = 0;
    //從inputStream中讀取資料
    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表示總共讀取的位元組數。AFMultipartBodyStream物件通過重寫父類的-read:maxLength:方法實現了拼接請求報文的body資料。

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