我在项目中工作，需要在后台下载大型JSON响应（即使应用程序崩溃）。
我还想了解后台传输服务的内部原理，即iOS在内部进行的操作以及操作方式。因此，我已将调用堆栈保存在UserDefaults中。
有2种情况。
1）没有应用崩溃。
(
Download task created with taskidentifier : 1,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 1,
didCompleteWithError->success for taskIdentifier 1,
)
2）当我在下载开始后使用空指针取消引用手动使应用程序崩溃时，结果很奇怪。
(
Download task created with taskidentifier : 1,
Download task created with taskidentifier : 2,
handleEventsForBackgroundURLSession,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 1,
didCompleteWithError->success for taskIdentifier 1,
URLSessionDidFinishEventsForBackgroundURLSession,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 2,
didCompleteWithError->success for taskIdentifier 2
)
我只创建了1个任务。 iOS是否会自动创建第二个任务？ 这里发生了什么。
这是我的代码
#import "KiviSyncLogin.h"
#import "AppDelegate.h"
@implementation KiviSyncLogin
-(instancetype)init{
self = [super init];
return self;
}
-(void)startSync{
if(self.session != nil){
NSLog(@"session is not nil");
return;
}
self.session = [self setUpSession]; // init session with once token
NSLog(@"This is session : %@",self.session);
[self pullDataFromServerWithSession:self.session];
}
-(NSURLSession *)setUpSession{
static dispatch_once_t onceToken;
static NSURLSession *session = nil;
dispatch_once(&onceToken, ^{
NSURLSessionConfiguration *config = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"com.kivi.login.sync"];
config.HTTPMaximumConnectionsPerHost = 1;
session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];
});
return session;
}
-(void)pullDataFromServerWithSession:(NSURLSession *)session{
NSString *serverUrlString = @"apiurl";
NSMutableDictionary *mydic = [[NSMutableDictionary alloc] init];
[mydic setObject:@"email" forKey:@"email"];
[mydic setObject:@"password" forKey:@"password"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:serverUrlString]];
request.HTTPMethod = @"POST";
request.HTTPBody = [[self encodedString:mydic] dataUsingEncoding:NSUTF8StringEncoding];
self.downloadTask = [self.session downloadTaskWithRequest:request];
NSLog(@"Download task %@ created with identifier : %ld", self.downloadTask,self.downloadTask.taskIdentifier);
[self.downloadTask resume];
}
/* common delegate Start */
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location{
NSLog(@"file downloaded");
NSFileManager *fileManager = [NSFileManager defaultManager];
NSArray *URLs = [fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask];
NSURL *documentsDirectory = [URLs objectAtIndex:0];
NSURL *originalURL = [[downloadTask originalRequest] URL];
NSURL *destinationURL = [documentsDirectory URLByAppendingPathComponent:[originalURL lastPathComponent]];
NSError *errorCopy;
[fileManager removeItemAtURL:destinationURL error:NULL];
BOOL success = [fileManager copyItemAtURL:location toURL:destinationURL error:&errorCopy];
if (success){
/* Store data in database */
}
else{
NSLog(@"Error during the copy: %@", [errorCopy localizedDescription]);
}
}
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
if(task.taskIdentifier == self.downloadTask.taskIdentifier){
if(error){
NSLog(@"Download task completed with error : %@",error);
/* Show error that data is not downloaded */
[self windUpSession];
}else{
NSLog(@"Download task completed successfully");
}
}
}
-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session
{
NSLog(@"Session %@ URL Session Did Finish Events For Background URL Session\n", session);
dispatch_async(dispatch_get_main_queue(), ^{
AppDelegate *appDelegate = (AppDelegate *)[UIApplication sharedApplication].delegate;
[session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
if ([downloadTasks count] == 0 && [uploadTasks count] == 0) {
if (appDelegate.backgroundTransferCompletionHandler != nil) {
NSLog(@"I have completion handler");
void(^completionHandler)(void) = appDelegate.backgroundTransferCompletionHandler;
appDelegate.backgroundTransferCompletionHandler = nil;
[self windUpSession];
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
completionHandler();
}];
}
}
}];
});
}
- (void)windUpSession{
//[self.session invalidateAndCancel];
self.session = nil;
self.downloadTask = nil;
}
- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error{
if(error == nil){
NSLog(@"Session is invalidated successfully");
}else{
NSLog(@"Error while invalidating session");
}
}
/* Common Delegates End */
/* Helper methods */
-(NSString *)encodedString:(NSDictionary *)mydic{
NSString *params = @"";//[NSString stringWithFormat:@"email=%@&password=%@",email,token];
int i = 0;
for (NSString *key in [mydic allKeys]) {
NSString *stringToBeAppended;
if(i++ == 0)
stringToBeAppended = [NSString stringWithFormat:@"%@=%@",key,mydic[key]];
else
stringToBeAppended = [NSString stringWithFormat:@"&%@=%@",key,mydic[key]];
params = [params stringByAppendingString:stringToBeAppended];
}
return params;
}
@end
如果我在创建下载任务时添加了[self.session finishTasksAndInvalidate];，它将创建第二个任务，但由于会话无效而无法执行。
在这种情况下，调用堆栈为（如果我手动操纵应用程序。）
(
Download task created with taskidentifier : 1,
Download task created with taskidentifier : 2,
handleEventsForBackgroundURLSession,
didfinishdownloadingtourl,
File downloaded for taskidentifier : 1,
didCompleteWithError->success for taskIdentifier 1,
URLSessionDidFinishEventsForBackgroundURLSession
)
编辑1
我还注意到，当我的应用程序崩溃时，我正在崩溃的应用程序以及从其初始化类并开始获取的viewController的viewDidLoad方法被调用。