When call completionHandler with Background Fetch in iOS 7 with NSOperation - objective-c

Question

i want integrate the Background Fetch in my iOS app, i have enabled the Background Fetch in the capabilities of the project, then i have insert this:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
[application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
to enable the background fetch with minimum interval, then in App delegate i insert the delegate method:
-(void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
NSLog(#"Call fetch iCloud");
[[MySingleton sharedManager] startCheckOnCloud];
}
My question is this, i know that i have to call this:
completionHandler(UIBackgroundFetchResultNewData);
somewhere, i have see a lot of example that insert that completionHandler in the performFetchWithCompletionHandler delegate method, but in that method i call a NSOperation in a Singleton, that operation check che iCloud folder and make some changes in a Sqlite DB, if i do this:
-(void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
NSLog(#"Call fetch iCloud");
[[MySingleton sharedManager] startCheckOnCloud];
completionHandler(UIBackgroundFetchResultNewData);
}
the operation start and do anything, maybe because the system make it sleep instantly, instead if i remove the completionHandler give me this warning:
Warning: Application delegate received call to -application:performFetchWithCompletionHandler: but the completion handler was never called.
so my question is, how i can handle the completionHandler with NSOperation?

Answer 1

You must call the completion handler with 30 seconds, but you don't have to block the main thread while you perform the query.
It appears that internally UIKit is detecting that you have not stored a strong reference to the completionHandler and then logging that warning. If you perform something simple like this:
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
// Start asynchronous NSOperation, or some other check
// Ideally the NSOperation would notify when it has completed, but just for
// illustrative purposes, call the completion block after 20 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 20 * NSEC_PER_SEC),
dispatch_get_main_queue(), ^{
// Check result of your operation and call completion block with the result
completionHandler(UIBackgroundFetchResultNewData);
});
}
You will notice that the warning isn't logged even, though the method exited before calling the completion block.

Answer 2

I've had similar situation in my app too.
What I did was to pass the completion handler to my singleton and whenever my code path has ended in operation, either on success or error, I check if I have a reference to a completion block. if so, I simply call the completion handler with appropriate results, if not, that means I'm not in background fetch mode.
Don't forget to set your completion handler's reference to nil in your singleton after wards, because it might fool your instance in the consequent operations.
I did something else too, because of that mandatory 30 second timeout on background fetch, before starting my operation, I check if I'm in background fetch mode, if so, I schedule a timer for 25 seconds (5 second threshold for assurance) and handle the situation myself. if you don't do this, and your app fails to meet 30 second timeout on calling completion handler too many times, you'll end up blocking your app from background fetch on the user's device.
good luck

Answer 3

At some point, your app will know when it's done fetching and processing new data. That can be whenever a new file was stored to disk, new records were inserted in Core Data, or when a web page finished loading.
As soon as that happens, you have to determine whether there was indeed new data, and then call the completion handler with the correct argument. More than likely, that means that you'll have to pass the completion handler to other objects. startCheckOnCloud will become startCheckOnCloudWithCompletionHandler:, and if that method isn't the one to actually do the fetch, you pass the completion handler to a method that gets called in startCheckOnCloudWithCompletionHandler: that does do the fetch.

Answer 4

I also have this problem, I finally solved my mistake is that, due to the presence of the asynchronous operation in the fetch callback, when you start using __block UIBackgroundFetchResult assignment to this:
UIBackgroundFetchResult __block result = UIBackgroundFetchResultNoData;
result = UIBackgroundFetchResultNoData;
post: return; return; because, "did not lead to finally, the method is called," completionHandler (result); "
so it appears a mistake:" the completion handler was never called "," return; "get rid of making the final call methods:" completionHandler (result); "there is no mistake. Under the summary, is must call the method: completionHandler (); can not be wrong.
Thx.