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
objectivec
- (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 YES
sessionConfig.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：
objectivec
//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 *)identifier2、后台上传
objectivec
- (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：
objectivec
/* 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
objectivec
- (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
objectivec
- (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：
objectivec
@property (copy, nonatomic) void(^backgroundSessionCompletionHandler)();
- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier
completionHandler:(void (^)())completionHandler {
self.backgroundSessionCompletionHandler = completionHandler;
}Session的Delegate
objectivec
@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）。