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NSURLSession后台上传的基本逻辑是：首先创建一个后台模式的NSURLSessionConfiguration，然后通过这个configuration创建一个NSURLSession，接着是创建相关的NSURLSessionTask，最后就是处理相关的代理事件。

1、创建NSURLSession

- (NSURLSession *)backgroundURLSession {static NSURLSession *session = nil;static dispatch_once_t onceToken;dispatch_once(&onceToken, ^{NSURLSessionConfiguration* sessionConfig = nil;NSString *identifier = [NSString stringWithFormat:@"%@.%@", [NSBundle mainBundle].bundleIdentifier, @"HttpUrlManager"];sessionConfig = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:identifier];//请求的缓存策略sessionConfig.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;//数据传输超时，当恢复传输时会清零sessionConfig.timeoutIntervalForRequest = 60;//单条请求超时，决定一条请求的最长生命周期sessionConfig.timeoutIntervalForResource = 60;//请求的服务类型sessionConfig.networkServiceType = NSURLNetworkServiceTypeDefault;//是否允许使用移动网络（电话网络）default is YESsessionConfig.allowsCellularAccess = YES;//后台模式生效，YES允许自适应系统性能调节sessionConfig.discretionary = YES;session = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil];});return session;
}

NSURLSessionConfiguration配置有三种模式： 

//默认模式类似于原来的NSURLConnection，可以使用缓存的Cache，Cookie，鉴权
+ (NSURLSessionConfiguration *)defaultSessionConfiguration;//及时模式不使用缓存的Cache，Cookie，鉴权
+ (NSURLSessionConfiguration *)ephemeralSessionConfiguration;//后台模式在后台完成上传下载，创建Configuration对象的时候需要给一个NSString的ID用于追踪完成工作的Session是哪一个
+ (NSURLSessionConfiguration *)backgroundSessionConfigurationWithIdentifier:(NSString *)identifier

2、后台上传

- (void)upload:(NSString *)urlStr data:(NSData *)data headers:(NSDictionary *)headers parameters:(NSDictionary *)parameters name:(NSString *)name filename:(NSString *)filename mimeType:(NSString *)mimeType success:(void (^)(id responseObject))success failure:(void (^)(int code, NSString *message))failure {NSURL *url = [NSURL URLWithString:urlStr];NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];request.HTTPMethod = @"POST";NSString *string = [NSString stringWithFormat:@"multipart/form-data; charset=utf-8; boundary=%@", kBoundary];[request setValue:string forHTTPHeaderField:@"Content-Type"];if (headers != nil) {for (NSString *key in headers.allKeys) {[request setValue:headers[key] forHTTPHeaderField:key];}}NSData *bodyData = [self bodyFormData:data parameters:parameters name:name filename:filename mimeType:mimeType];NSString *tempPath = NSTemporaryDirectory();NSTimeInterval interval = [NSDate.now timeIntervalSince1970];NSString *tempName = [NSString stringWithFormat:@"temp%.0f_%@", interval, filename];NSString *tempPath = [tempPath stringByAppendingPathComponent:tempName];[bodyData writeToFile:tempPath atomically:YES];NSURLSession *session = self.backgroundURLSession;NSURLSessionUploadTask *uploadTask = [session uploadTaskWithRequest:request fromFile:[NSURL fileURLWithPath:tempPath]];[uploadTask resume];
}- (NSData *)bodyFormData:(NSData *)data parameters:(NSDictionary *)parameters name:(NSString *)name filename:(NSString *)filename mimeType:(NSString *)mimeType {if (data == nil || data.length == 0) {return nil;}NSMutableData *formData = [NSMutableData data];NSData *lineData = [@"\r\n" dataUsingEncoding:NSUTF8StringEncoding];NSData *boundary = [[NSString stringWithFormat:@"--%@", kBoundary] dataUsingEncoding:NSUTF8StringEncoding];if (parameters != nil) {[parameters enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {[formData appendData:boundary];[formData appendData:lineData];NSString *thisFieldString = [NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"\r\n\r\n%@", key, obj];[formData appendData:[thisFieldString dataUsingEncoding:NSUTF8StringEncoding]];[formData appendData:lineData];}];}[formData appendData:boundary];[formData appendData:lineData];NSString *thisFieldString = [NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"; filename=\"%@\"\r\nContent-Type: %@", name, filename, mimeType];[formData appendData:[thisFieldString dataUsingEncoding:NSUTF8StringEncoding]];[formData appendData:lineData];[formData appendData:lineData];[formData appendData:data];[formData appendData:lineData];[formData appendData:[[NSString stringWithFormat:@"--%@--\r\n", kBoundary] dataUsingEncoding:NSUTF8StringEncoding]];return formData;
}

 上传有4种方法：

/* Creates an upload task with the given request.  The body of the request will be created from the file referenced by fileURL */
- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromFile:(NSURL *)fileURL;/* Creates an upload task with the given request.  The body of the request is provided from the bodyData. */
- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromData:(NSData *)bodyData;- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromFile:(NSURL *)fileURL completionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))completionHandler;- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromData:(nullable NSData *)bodyData completionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))completionHandler;

后台模式不支持使用带回调的上传方法，否则会报错：

Completion handler blocks are not supported in background sessions. Use a delegate instead.

后台模式不支持使用NSData的上传方法，否则会报错：

Upload tasks from NSData are not supported in background sessions 

所以如果使用后台模式上传，选择uploadTaskWithRequest:(NSURLRequest *)request fromFile:(NSURL *)fileURL方法。

NSURLSessionDataDelegate上传代理事件

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didSendBodyData:(int64_t)bytesSent totalBytesSent:(int64_t)totalBytesSent totalBytesExpectedToSend:(int64_t)totalBytesExpectedToSend {NSLog(@"URLSession didSendBodyData progress: %f" ,totalBytesSent/(float)totalBytesExpectedToSend);
}- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {NSLog(@"%s", __func__);
}- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {NSMutableData *responseData = self.responsesData[@(dataTask.taskIdentifier)];if (!responseData) {responseData = [NSMutableData dataWithData:data];self.responsesData[@(dataTask.taskIdentifier)] = responseData;} else {[responseData appendData:data];}
}- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {if (error) {NSLog(@"URLSession didCompleteWithError %@ failed: %@", task.originalRequest.URL, error);}NSMutableData *responseData = self.responsesData[@(task.taskIdentifier)];if (responseData) {NSDictionary *response = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:nil];if (response) {NSLog(@"response = %@", response);} else {NSString *errMsg = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];NSLog(@"responseData = %@", errMsg);}[self.responsesData removeObjectForKey:@(task.taskIdentifier)];} else {NSLog(@"responseData is nil");}
}

3、后台请求

- (void)request:(NSString *)urlStr method:(NSString *)method headers:(NSDictionary *)headers parameters:(NSDictionary *)parameters success:(void (^)(id responseObject))success failure:(void (^)(int code, NSString *message))failure {urlStr = [self getFullUrlString:urlStr parameters:parameters];NSURL *url = [NSURL URLWithString:urlStr];NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];request.HTTPMethod = method;if (headers != nil) {for (NSString *key in headers.allKeys) {[request setValue:headers[key] forHTTPHeaderField:key];}}NSURLSession *session = self.backgroundURLSession;NSURLSessionDataTask *task = [session dataTaskWithRequest:request];[task resume];
}- (NSString *)getFullUrlString:(NSString *)urlStr parameters:(NSDictionary *)parameters {NSMutableString *newStr = [NSMutableString stringWithString:urlStr];if (parameters.allKeys.count > 0) {BOOL isFirst = NO;for (NSString *key in parameters) {isFirst = YES;[newStr appendString:isFirst?@"?":@"&"];[newStr appendFormat:@"%@=%@", key, parameters[key]];}}return newStr;
}

4、Session和ApplicationDelegate交互

使用BackgroundSession后台模式，在Task执行的时候，当用户切到后台时，Session会和ApplicationDelegate做交互，在BackgroundSession中的Task还会继续下载/上传。

现在分三个场景分析下Session和Application的关系：

（1）当加入了多个Task，程序没有切换到后台。 

这种情况Task会按照NSURLSessionConfiguration的设置正常下载，不会和ApplicationDelegate有交互。

（2）当加入了多个Task，程序切到后台，所有Task都完成下载

在切到后台之后，Session的Delegate不会再收到，Task相关的消息，直到所有Task全都完成后，系统会调用ApplicationDelegate的application:handleEventsForBackgroundURLSession:completionHandler:回调，之后“汇报”下载工作，对于每一个后台下载的Task调用Session的Delegate中的URLSession:downloadTask:didFinishDownloadingToURL:（成功的话）和URLSession:task:didCompleteWithError:（成功或者失败都会调用）。 

AppDelegate：

@property (copy, nonatomic) void(^backgroundSessionCompletionHandler)();- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {self.backgroundSessionCompletionHandler = completionHandler;
}

Session的Delegate

@interface MyViewController()<NSURLSessionDelegate>
@end@implementation MyViewController- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];if (appDelegate.backgroundSessionCompletionHandler) {void (^completionHandler)() = appDelegate.backgroundSessionCompletionHandler;appDelegate.backgroundSessionCompletionHandler = nil;completionHandler();}NSLog(@"All tasks are finished");
}@end

（3）当加入了多个Task，程序切到后台，下载完成了几个Task，然后用户又切换到前台。（程序没有退出）

切到后台之后，Session的Delegate仍然收不到消息。在下载完成几个Task之后再切换到前台，系统会先汇报已经下载完成的Task的情况，然后继续下载没有下载完成的Task，后面的过程同第一种情况。 

（4）当加入了多个Task，程序切到后台，几个Task已经完成，但还有Task还没有下载完的时候关掉强制退出程序，然后再进入程序的时候。（程序退出了）

由于程序已经退出了，后面没有下完Session就不在了后面的Task肯定是失败了。但是已经下载成功的那些Task，新启动的程序也没有听“汇报”的机会了。经过实验发现，这个时候之前在NSURLSessionConfiguration设置的NSString类型的ID起作用了，当ID相同的时候，一旦生成Session对象并设置Delegate，马上可以收到上一次关闭程序之前没有汇报工作的Task的结束情况（成功或者失败）。