How to keep downloading new images in background even if user force quits the app in iOS objective C?

Question

I want to download many images around 300, and display it in the UI.

I have two questions:

1. If the app is in foreground, and suppose user sends it to background mode(by clicking on home button) then how do I assure the download to continue?

2. If the user force quits the app(doubleclick on home button and swipe the app from app switcher), then how do I ensure the app downloads the images?

I've been reading about background stuffs a lot. Few also say that in 2. case the download cannot continue.

Below are the links I referred:

http://www.appcoda.com/ios7-background-fetch-programming/ http://www.appcoda.com/background-transfer-service-ios7/ iOS Background downloads when the app is not active

I'm not getting the right approach to download the images in foreground, background/suspended, user force quits the app. I'm using AFNetworking for webservice calls.

Below is my code:

I've image details like the URL in .json file uploaded to amazon. To download the file I do,

[NSData dataWithContentsOfURL:[NSURL URLWithString:@"https://someurl/imageFile.json"]];

From this file I read the urls of images and download them along with image details. This is a big process. How Do I handle it? Please help..

Answer 1

Solution for your 1 question is below :

Background sessions let you perform uploads and downloads of content in the background while your app is not running. You can create a background session configuration by calling the backgroundSessionConfiguration: method on the NSURLSessionConfiguration class.

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
NSURLSessionConfiguration *sessionConfig;
float timeout = 5 * 60.0f;

BOOL iOS8OrNew = [[[UIDevice currentDevice] systemVersion] floatValue] >= 8.0;
if (iOS8OrNew) {
    sessionConfig = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:identifier];
request.timeoutInterval = timeout;
}
else {
    sessionConfig = [NSURLSessionConfiguration backgroundSessionConfiguration:identifier];
sessionConfig.timeoutIntervalForRequest = timeout;
}

sessionConfig.HTTPMaximumConnectionsPerHost = 10;
AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:sessionConfig];

 NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request];


[manager setDidFinishEventsForBackgroundURLSessionBlock:^(NSURLSession * _Nonnull session) {
    AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    if (appDelegate.backgroundSessionCompletionHandler) {
        void (^completionHandler)() = appDelegate.backgroundSessionCompletionHandler;
        appDelegate.backgroundSessionCompletionHandler = nil;
        completionHandler();
    }
    NSLog(@"All tasks are finished");
}];

Add below code in your AppDelegate:

 - (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier
  completionHandler:(void (^)())completionHandler {
   self.backgroundSessionCompletionHandler = completionHandler;

   //add notification
   [self presentNotification];
}

-(void)presentNotification{
    UILocalNotification* localNotification = [[UILocalNotification alloc] init];
    localNotification.alertBody = @"Download Complete!";
    localNotification.alertAction = @"Background Transfer Download!";

    //On sound
    localNotification.soundName = UILocalNotificationDefaultSoundName;

    //increase the badge number of application plus 1
    localNotification.applicationIconBadgeNumber = [[UIApplication sharedApplication] applicationIconBadgeNumber] + 1;

    [[UIApplication sharedApplication] presentLocalNotificationNow:localNotification];
}

For your 2 solution

If the system kills your app and your background session has active downloads, your downloads will continue and the system will launch your app when the downloads complete. However, if a user force quits your app, all tasks get cancelled.