[Objective-C] バックグラウンドで継続して処理を実行する ref: http://qiita.com/edo_m18/items/52fb7190d4285aabc1c9

file0.m

#import "ViewController.h"

@interface ViewController ()

@property (nonatomic, strong) NSTimer *timer;
@property (nonatomic, assign) UIBackgroundTaskIdentifier bgid;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    [NSNotificationCenter.defaultCenter addObserver:self
                                           selector:@selector(willResignActive:)
                                               name:UIApplicationWillResignActiveNotification
                                             object:nil];
    [NSNotificationCenter.defaultCenter addObserver:self
                                           selector:@selector(didBecomeActive:)
                                               name:UIApplicationDidBecomeActiveNotification
                                             object:nil];

    [self update];
}

- (void)update
{
    if (self.timer) {
        [self.timer invalidate];
        self.timer = nil;
    }
    self.timer = [NSTimer scheduledTimerWithTimeInterval:3
                                                  target:self
                                                selector:@selector(update)
                                                userInfo:nil
                                                 repeats:YES];

    NSLog(@"timer");
}

- (void)willResignActive:(NSNotification *)notification
{
    NSLog(@"resign");

    UIApplication *app = UIApplication.sharedApplication;
    self.bgid = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:self.bgid];
        self.bgid = UIBackgroundTaskInvalid;
    }];
}

- (void)didBecomeActive:(NSNotification *)notification
{
    NSLog(@"become");

    [UIApplication.sharedApplication endBackgroundTask:self.bgid];
}

@end