我想在应用关闭时保存未完成下载的数据。试过了，但是resumeData总是空的:[[NSNotificationCenterdefaultCenter]addObserver:selfselector:@selector(appWillTerminate)name:UIApplicationWillTerminateNotificationobject:nil];-(void)appWillTerminate{[self.downloadTaskcancelByProducingResumeData:^(NSData*resumeData){if(resumeData)[selfsav