本文回详细的介绍一下Multipart协议在AFN中的封装
Multipart协议
Multipart协议是基于post方法的组合实现，和post协议的主要区别在于请求头和请求体的不同
multipart/form-data的请求头必须包含一个特殊的头信息：Content-Type，且其值也必须规定为multipart/form-data，同时还需要规定一个内容分割符用于分割请求体中的多个post的内容，如文件内容和文本内容自然需要分割开来，不然接收方就无法正常解析和还原这个文件了
multipart/form-data的请求体也是一个字符串，不过和post的请求体不同的是它的构造方式，post是简单的name=value值连接，而multipart/form-data则是添加了分隔符等内容的构造体
下面我们列举一个具有代表性的例子
--${fengefu}
Content-Disposition: form-data; name="Filename"
Test.txt
--${fengefu}
Content-Disposition: form-data; name="file000"; filename="Test测试.txt"
Content-Type: application/octet-stream
%PDF-1.5
file content
%%EOF
--${fengefu}
Content-Disposition: form-data; name="Upload"
Submit Query
--${fengefu}--
现在我们想对上面的例子使用AFNetworking进行调用：
AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
[manager POST:@"postURLString" parameters:@{@"Filename":@"Test.txt"} constructingBodyWithBlock:^(id<AFMultipartFormData> _Nonnull formData) {
[formData appendPartWithFileData:[txt文件具体内容(NSData *)]
name:@"file000"
fileName:@"Test测试.txt"
mimeType:@"application/octet-stream"];
[formData appendPartWithFormData:[@"Submit Query" dataUsingEncoding:NSUTF8StringEncoding]
name:@"Upload"];
} progress:nil success:nil failure:nil];
此处带constructingBodyWithBlock的POST方法与- [AFHTTPSessionManager POST:parameters:progress:success:failure:]明显的区别在于构建request的时候，使用的是multipartFormRequestWithMethod:以及构建NSURLSessionDataTask的时候使用的是uploadTaskWithStreamedRequest:。
multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:error:除了需要使用普通的request构造函数requestWithMethod:URLString:parameters:error:来构造request，还需要根据multipart独有的属性来修饰这个request，其中最关键的就是要构造http body(请求体)部分。
因为multipart是基于POST请求的，所以应该首先排除GET和HEAD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);
NSParameterAssert(![method isEqualToString:@"GET"] && ![method isEqualToString:@"HEAD"]);
NSMutableURLRequest *mutableRequest = [self requestWithMethod:method URLString:URLString parameters:nil error:error];
__block AFStreamingMultipartFormData *formData = [[AFStreamingMultipartFormData alloc] initWithURLRequest:mutableRequest stringEncoding:NSUTF8StringEncoding];
if (parameters) {
for (AFQueryStringPair *pair in AFQueryStringPairsFromDictionary(parameters)) {
NSData *data = nil;
if ([pair.value isKindOfClass:[NSData class]]) {
data = pair.value;
} else if ([pair.value isEqual:[NSNull null]]) {
data = [NSData data];
} else {
data = [[pair.value description] dataUsingEncoding:self.stringEncoding];
}
if (data) {
[formData appendPartWithFormData:data name:[pair.field description]];
}
}
}
if (block) {
block(formData);
}
return [formData requestByFinalizingMultipartFormData];
}
下面我们通过详细分析请求的三个部分来看一下AFNetworking在multipart中是如何实现的。
每一个request都分为三个部分：请求行、请求头和请求体
multipart请求头
- (NSMutableURLRequest *)requestByFinalizingMultipartFormData {
if ([self.bodyStream isEmpty]) {
return self.request;
}
[self.bodyStream setInitialAndFinalBoundaries];
[self.request setHTTPBodyStream:self.bodyStream];
[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;
}
其中主要的信息内容有self.bodyStream和self.boundary
self.bodyStream的构造
self.bodyStream是区分普通post请求方法和multipart请求的关键要素
所以我们首先介绍bodyStream
事实上对于bodyStream的构建就是对AFStreamingMultipartFormData对象的处理，比如函数- [AFHTTPRequestSerializer multipartFormRequestWithMethod:URLString:parameters:constructingBodyWithBlock:error:]的那个formData就是一个AFStreamingMultipartFormData对象
AFStreamingMultipartFormData类中的appendPart函数最终目的就是给bodyStream中HTTPBodyParts添加一个AFHTTPBodyPart对象
如何添加一个AFHTTPBodyPart呢？
(BOOL) - appendPartWithFileURL:name:error: 根据文件位置构造数据源，使用文件类型名作为mimeType
(BOOL) - appendPartWithFileURL:name:fileName:mimeType:error: 根据文件位置构造数据源，需要提供mimeType
(void) - appendPartWithInputStream:name:fileName:length:mimeType: 直接使用NSInputStream作为数据源
(void) - appendPartWithFileData:name:fileName:mimeType: 使用NSData作为数据源
(void) - appendPartWithFormData:name: 使用NSData作为数据源，NSData并不是一个文件，可能只是一个字符串
新建一个AFHTTPBodyPart对象bodyPart，然后给bodyPart设置各种参数，其中比较重要的参数是headers和body这两个。最后使用appendHTTPBodyPart:方法，将bodyPart添加到bodyStream的HTTPBodyParts上。
如：
- (void)appendPartWithInputStream:(NSInputStream *)inputStream
name:(NSString *)name
fileName:(NSString *)fileName
length:(int64_t)length
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"];
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];
}
appendPartWithFileURL:函数会首先检查fileURL是否可用，使用[fileURL isFileURL]检查文件位置格式是否正确。使用[fileURL checkResourceIsReachableAndReturnError:error]来检查该文件是否存在，是否能获取到。最后使用NSFileManager获取到文件attributes，并判断attributes是否存在。另外注意到此处直接使用的是fileURL作为AFHTTPBodyPart对象的body属性。
appendPartWithFileData:和appendPartWithFormData:两个函数实现中，最后使用的是appendPartWithHeaders:构建AFHTTPBodyPart对象
self.boundary的构造
boundary是用来分割不同数据内容的，其实就是上面举的那个例子中的${fengefu}。我们注意到boundary需要处理以下几个情况：
创建boundary字符串
此处AFNetworking自定义了个函数创建boundary字符串。
static NSString * AFCreateMultipartFormBoundary() {
return [NSString stringWithFormat:@"Boundary+%08X%08X", arc4random(), arc4random()];
}
如果是开头分隔符的，那么只需在分隔符结尾加一个换行符
static inline NSString * AFMultipartFormInitialBoundary(NSString *boundary) {
return [NSString stringWithFormat:@"--%@%@", boundary, kAFMultipartFormCRLF];
}
如果是中间部分分隔符，那么需要分隔符前面和结尾都加换行符
static inline NSString * AFMultipartFormEncapsulationBoundary(NSString *boundary) {
return [NSString stringWithFormat:@"%@--%@%@", kAFMultipartFormCRLF, boundary, kAFMultipartFormCRLF];
}
如果是末尾，还得使用–分隔符–作为请求体的结束标志
static inline NSString * AFMultipartFormFinalBoundary(NSString *boundary) {
return [NSString stringWithFormat:@"%@--%@--%@", kAFMultipartFormCRLF, boundary, kAFMultipartFormCRLF];
boundary的用处
除了设置Content-Type外，在设置Content-Length时使用的[self.bodyStream contentLength]中会使用到boundary的这些相关函数:
// AFMultipartBodyStream函数
- (unsigned long long)contentLength {
unsigned long long length = 0;
for (AFHTTPBodyPart *bodyPart in self.HTTPBodyParts) {
length += [bodyPart contentLength];
}
return length;
}
- (unsigned long long)contentLength {
unsigned long long length = 0;
NSData *encapsulationBoundaryData = [([self hasInitialBoundary] ? AFMultipartFormInitialBoundary(self.boundary) : AFMultipartFormEncapsulationBoundary(self.boundary)) dataUsingEncoding:self.stringEncoding];
length += [encapsulationBoundaryData length];
NSData *headersData = [[self stringForHeaders] dataUsingEncoding:self.stringEncoding];
length += [headersData length];
length += _bodyContentLength;
NSData *closingBoundaryData = ([self hasFinalBoundary] ? [AFMultipartFormFinalBoundary(self.boundary) dataUsingEncoding:self.stringEncoding] : [NSData data]);
length += [closingBoundaryData length];
return length;
}
第二种multipart的request构建方法
- (NSMutableURLRequest *)requestWithMultipartFormRequest:(NSURLRequest *)request
writingStreamContentsToFile:(NSURL *)fileURL
completionHandler:(void (^)(NSError *error))handler
{
NSParameterAssert(request.HTTPBodyStream);
NSParameterAssert([fileURL isFileURL]);
NSInputStream *inputStream = request.HTTPBodyStream;
NSOutputStream *outputStream = [[NSOutputStream alloc] initWithURL:fileURL append:NO];
__block NSError *error = nil;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[inputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
[outputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
[inputStream open];
[outputStream open];
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;
}
}
[outputStream close];
[inputStream close];
if (handler) {
dispatch_async(dispatch_get_main_queue(), ^{
handler(error);
});
}
});
NSMutableURLRequest *mutableRequest = [request mutableCopy];
mutableRequest.HTTPBodyStream = nil;
return mutableRequest;
}
读取字节的方法
- (NSInteger)read:(uint8_t *)buffer
maxLength:(NSUInteger)length
{
if ([self streamStatus] == NSStreamStatusClosed) {
return 0;
}
NSInteger totalNumberOfBytesRead = 0;
while ((NSUInteger)totalNumberOfBytesRead < MIN(length, self.numberOfBytesInPacket)) {
if (!self.currentHTTPBodyPart || ![self.currentHTTPBodyPart hasBytesAvailable]) {
if (!(self.currentHTTPBodyPart = [self.HTTPBodyPartEnumerator nextObject])) {
break;
}
} else {
NSUInteger maxLength = MIN(length, self.numberOfBytesInPacket) - (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];
}
}
}
}
return totalNumberOfBytesRead;
}
单个bodyPart的读取
- (NSInteger)read:(uint8_t *)buffer
maxLength:(NSUInteger)length
{
NSInteger totalNumberOfBytesRead = 0;
if (_phase == AFEncapsulationBoundaryPhase) {
NSData *encapsulationBoundaryData = [([self hasInitialBoundary] ? AFMultipartFormInitialBoundary(self.boundary) : AFMultipartFormEncapsulationBoundary(self.boundary)) dataUsingEncoding:self.stringEncoding];
totalNumberOfBytesRead += [self readData:encapsulationBoundaryData intoBuffer:&buffer[totalNumberOfBytesRead] maxLength:(length - (NSUInteger)totalNumberOfBytesRead)];
}
if (_phase == AFHeaderPhase) {
NSData *headersData = [[self stringForHeaders] dataUsingEncoding:self.stringEncoding];
totalNumberOfBytesRead += [self readData:headersData intoBuffer:&buffer[totalNumberOfBytesRead] maxLength:(length - (NSUInteger)totalNumberOfBytesRead)];
}
if (_phase == AFBodyPhase) {
NSInteger numberOfBytesRead = 0;
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];
}
}
}
if (_phase == AFFinalBoundaryPhase) {
NSData *closingBoundaryData = ([self hasFinalBoundary] ? [AFMultipartFormFinalBoundary(self.boundary) dataUsingEncoding:self.stringEncoding] : [NSData data]);
totalNumberOfBytesRead += [self readData:closingBoundaryData intoBuffer:&buffer[totalNumberOfBytesRead] maxLength:(length - (NSUInteger)totalNumberOfBytesRead)];
}
return totalNumberOfBytesRead;
}
Phase的切换
根据对应的阶段切换到下一个阶段，其中主要是inputStream的开关
- (BOOL)transitionToNextPhase {
if (![[NSThread currentThread] isMainThread]) {
dispatch_sync(dispatch_get_main_queue(), ^{
[self transitionToNextPhase];
});
return YES;
}
switch (_phase) {
case AFEncapsulationBoundaryPhase:
_phase = AFHeaderPhase;
break;
case AFHeaderPhase:
[self.inputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
[self.inputStream open];
_phase = AFBodyPhase;
break;
case AFBodyPhase:
[self.inputStream close];
_phase = AFFinalBoundaryPhase;
break;
case AFFinalBoundaryPhase:
default:
_phase = AFEncapsulationBoundaryPhase;
break;
}
_phaseReadOffset = 0;
return YES;
}
总结：上面就是所有关于multipart协议的分析和介绍，其实现方法可以说是一个对POST请求的在此封装。传输多种参数，多种资料型态混合的信息时会使用到multipart协议。
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