I want to continue to process image and store it when iOS App in background

Question

I tried to use NSOperation and NSOperationQueue.
But As getting into background, NSOperation didn't work. it stopped.

Calling

- (void)viewDidLoad
{
    findFQueue = [[NSOperationQueue alloc] init];
    FindFeatureOperation *testOperation = [[FindFeatureOperation alloc] init];
    [findFQueue addOperation:testOperation];
}

NSOperation define

#import "FindFeatureOperation.h"

@implementation FindFeatureOperation
- (void)main
{
for (int i = 0; i < 999999 ; i++) {
    NSLog(@"Operation %d", i);
    [NSThread sleepForTimeInterval:1];
}
}
@end

Let me Know Please.

Answer 1

1 way: inside the op

to keep just running the operation:

- (main) {
    bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        // Clean up any unfinished task business by marking where you
        // stopped or ending the task outright.
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    for (int i = 0; i < 999999 ; i++) {
        NSLog(@"Operation %d", i);
        [NSThread sleepForTimeInterval:1];
    }

    [application endBackgroundTask];
}

2 way: using inside of applicationDidEnterBG:

- (void)applicationDidEnterBackground:(UIApplication *)app {
    _bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
         // When Expired the limit time for background NSLog(@"Expired Background Task Limit Time");
             [app endBackgroundTask:_bgTask]; 
            _bgTask = UIBackgroundTaskInvalid;
         }];
        dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
             [findFQueue waitUntilAllOperationsAreFinished]; 
            [app endBackgroundTask];
        }); 
}

Note: you can still be canceled if you use too much cpu / men / energy ... see the docs

Note 2: This can be written anywhere. I placed it in the OP but maybe in the VC is a better place... I don't think so but might be

Note 3: Read the docs! :: https://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html