Send local notification when download completes through NSURLSession / NSURLSessionDownloadTask - ios

Question

I am using NSURLSessionDownloadTask objects on an NSURLSession to allow users to download documents while the app is in the background / device locked. I also want to inform the user that individual downloads have finished through a local notification.
To that end, I am triggering a local notification in the -URLSession:downloadTask:didFinishDownloadingToURL: download task delegate method, however I am wondering if there might be a better place to add the code triggering a notification, since the way Apple explains it, the download task will be passed to the system, and from that I am deriving that those delegates will not be called anymore on the download task's delegate once (or shortly after) the app is backgrounded.
My question: What is the best place to add the code for triggering the local notifications? Has anybody had any previous experience in adding this sort of a functionality to their application?

Answer 1

Answer on your question can be found in Apple documentation URL Loading System Programming Guide:
In iOS, when a background transfer completes or requires credentials,
if your app is no longer running, iOS automatically relaunches your
app in the background and calls the
application:handleEventsForBackgroundURLSession:completionHandler:
method on your app’s UIApplicationDelegate object. This call provides
the identifier of the session that caused your app to be launched.
Your app should store that completion handler, create a background
configuration object with the same identifier, and create a session
with that configuration object. The new session is automatically
reassociated with ongoing background activity. Later, when the session
finishes the last background download task, it sends the session
delegate a URLSessionDidFinishEventsForBackgroundURLSession: message.
Your session delegate should then call the stored completion handler.
If any task completed while your app was suspended, the delegate’s
URLSession:downloadTask:didFinishDownloadingToURL: method is then
called with the task and the URL for the newly downloaded file
associated with it.
As you see it's much more complicated then just set delegate object. By delegate methods you will be notified only if app in foreground mode. In other cases (app in background mode, app is terminated) you need handle AppDelegate methods that are described in above quote.
Also Apple provides example project, that shows how to work with background download/upload tasks. This example will help you to find place where to put "Local Notification" code.

Answer 2

As Visput explained above, this method will be called once the download completes.application:handleEventsForBackgroundURLSession:completionHandler:
This will happen if you use the NSURLSessionConfiguration class with the backgroundSessionConfiguraton. You might be missing that piece.
NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfiguration:#"com.BGTransfer"];
sessionConfiguration.HTTPMaximumConnectionsPerHost = 5; // To set the max concurrent connections
It is explained in detail here.

Answer 3

As Suggested by #Gautam Jain u have to use backgroundSessionConfiguration to achieve ur objective.Below i have attached a example ,hope it helps you
DownloadModel.h
#import "AppDelegate.h"
#interface DownloadModel : NSObject<NSURLSessionDelegate,NSURLSessionTaskDelegate,NSURLSessionDownloadDelegate>{
NSString *resp;
}
+(instancetype)shared;
-(NSURLSessionDownloadTask *) downloadTaskWithURL:(NSURL*)url ;
#end
DownloadModel.m
#import "DownloadModel.h"
#interface DownloadModel ()
#property (strong,nonatomic) NSURLSession *downloadSession;
#end
#implementation DownloadModel
+(instancetype)shared{
static dispatch_once_t onceToken;
static DownloadModel *downloader=nil;
dispatch_once(&onceToken, ^{
downloader=[DownloadModel new];
});
return downloader;
}
-(id)init{
self=[super init];
if(self){
NSURLSessionConfiguration *downloadConfig=[NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:#"DownloadDemo"];
// downloadConfig.timeoutIntervalForRequest = 30;
// downloadConfig.timeoutIntervalForResource = 30;
// downloadConfig.HTTPMaximumConnectionsPerHost = 1;
// downloadConfig.sessionSendsLaunchEvents=YES;
downloadConfig.allowsCellularAccess = YES;
downloadConfig.networkServiceType = NSURLNetworkServiceTypeBackground;
// downloadConfig.discretionary = YES;
self.downloadSession=[NSURLSession sessionWithConfiguration:downloadConfig delegate:self delegateQueue:nil];
self.downloadSession.sessionDescription=#"Video Downloader";
}
return self;
}
-(NSURLSessionDownloadTask *) downloadTaskWithURL:(NSURL*)url{
return [self.downloadSession downloadTaskWithURL:url];
}
#pragma mark download delegate
use notification OR Local Notification in this method
- (void)URLSession:(NSURLSession *)session downloadTask(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL(NSURL *)location{
[[NSNotificationCenter defaultCenter] postNotificationName:#"DownloadFinish" object:downloadTask userInfo:nil];
}
For Progress of Download
- (void)URLSession:(NSURLSession *)session downloadTask(NSURLSessionDownloadTask *)downloadTask didWriteData(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite{
CGFloat progress=(CGFloat)totalBytesWritten/totalBytesExpectedToWrite;
NSDictionary *userInfo=#{#"progress":#(progress)};
[[NSNotificationCenter defaultCenter] postNotificationName:#"DownloadProgress" object:downloadTask userInfo:userInfo];
}
#pragma mark delegate
-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
AppDelegate *appdelegate=[[UIApplication sharedApplication] delegate];
if(appdelegate.backgroundSessionCompletionHandler){
appdelegate.backgroundSessionCompletionHandler();
appdelegate.backgroundSessionCompletionHandler=nil;
}
}
#end
AppDelegate.h
#interface AppDelegate : UIResponder <UIApplicationDelegate>
#property (strong, nonatomic) UIWindow *window;
#property (copy ,nonatomic) void(^backgroundSessionCompletionHandler)();
#end
AppDelegate.m
-(void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler{
self.backgroundSessionCompletionHandler=completionHandler;
[DownloadModel shared];
}
ViewController.m Call this Method -(NSURLSessionDownloadTask *) downloadTaskWithURL:(NSURL*)url
- (void)viewDidLoad {
//Add Notification observers to track download progress and call the above method
[DownloadModel shared] downloadTaskWithURL:url];
}
Don't Forget to enable Background Fetch