Threads allows you to execute multiple code paths concurrently within your application. The reason you’ll use it is to speed up the performance of your application by running some code on another thread while other operations are happening on the main thread. This is helpful for when you need to pre-load data. I’ve used this before in a magazine app where I needed to pre-load the next couple of pages while the user is reading the current page, this made the application much smoother when swiping to the next page.

There are a couple of disadvantages to threads, one problem with creating threads yourself is that they add uncertainty to your code. Threads are a relatively low-level and complicated way to support concurrency in your application. If you do not fully understand the implications of your design choices, you could easily encounter synchronization or timing issues, the severity of which can range from subtle behavioral changes to the crashing of your application and the corruption of the user’s data. Another problem area with Cocoa and multithreading comes from user interface updates. All UI updates in Cocoa must be done on the main thread, or instability can result. If you have a background thread performing a calculation, make sure to wrap whatever method updates the UI in a -performSelectorOnMainThread:withObject:waitUntilDone: method call. The single biggest source of crashes with multithreaded Cocoa applications is simultaneous access to a shared resource.

Grand Central Dispatch (GCD)

Grand Central Dispatch is one of the most common ways to add threads. With GCD, you define the task you want to perform and add it to a work queue, which handles the scheduling of your task on an appropriate thread. Work queues take into account the number of available cores and the current load to execute your tasks more efficiently than you could do yourself using threads.

Adding a Task to a Queue

dispatch_async(myQueue, ^{
    // Insert code to be executed on another thread here
    [self methodName];
});
dispatch_release(myQueue);

To execute a task, you must dispatch it to an appropriate dispatch queue. You can dispatch tasks synchronously or asynchronously. Once in a queue, the queue becomes responsible for executing your tasks as soon as possible, given its constraints and the existing tasks already in the queue. Do not call the dispatch_sync function from a task that is executing on the same queue that you pass to your function call. Doing so will deadlock the queue. If you need to dispatch to the current queue, do so asynchronously using the dispatch_async function.

Performing Tasks on the Main Thread

You can use to execute tasks on your application’s main thread while you’re running code on another thread. You can get the dispatch queue for your application’s main thread by calling the dispatch_get_main_queue function. Tasks added to this queue are performed serially on the main thread itself. Therefore, you can use this queue as a synchronization point for work being done in other parts of your application.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
    // Insert code to be executed on another thread here
    [self methodName];
    dispatch_async(dispatch_get_main_queue(), ^{
        // Insert code to be executed on the main thread here
        [self methodName];
    });
});
dispatch_release(queue);

Suspending and Resuming Queues

It’s not possible to stop a queue after it has been called because it would cause your application to be unstable but you can pause and resume the queue. You suspend a dispatch queue using the dispatch_suspend function and resume it using the dispatch_resume function.

-(void) startRunning {
    // queue will need to be defined in your header since it's use globally in the class
    queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        // Insert code to be executed on another thread here
        [self methodName];
    });
}

-(void) userCancels {
    if (queue) {
       dispatch_suspend(queue);
    }
}

-(void) userResumes {
    if (queue) {
       dispatch_resume(queue);
    }
}

-(void) dealloc {
    if (queue) dispatch_release(queue);

    [super dealloc[;
}

I know Push Notifications is all the rage lately, but let’s not forget about Local Notifications. They are much easier to implement and can sometimes be just as effective in getting Users to come back to your app. The short tutorial below will describe the steps needed to setup Local Notifications in your app.

Setup your Local Notification

-(void) setLocalNotification {
    
    NSDateComponents *comps = [[NSDateComponents alloc] init];
    [comps setYear:2013];
    [comps setMonth:05];
    [comps setDay:01];
    [comps setHour:12];
    [comps setMinute:0];
    [comps setTimeZone:[NSTimeZone systemTimeZone]];
    NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDate *setTime = [cal dateFromComponents:comps];
    
    UILocalNotification *localNotif = [[UILocalNotification alloc] init];
    if (localNotif == nil)
        return;
    localNotif.fireDate = setTime;
    localNotif.timeZone = [NSTimeZone defaultTimeZone];
	
    // Notification details
    localNotif.alertBody = @"Don't forget to come back to the app name today for your bonus!";
    // Set the action button
    localNotif.alertAction = @"View";
	
    localNotif.soundName = UILocalNotificationDefaultSoundName;
    localNotif.applicationIconBadgeNumber = 1;
	
    // Schedule the notification
    [[UIApplication sharedApplication] scheduleLocalNotification:localNotif];
}

To customize the code above change the NSDate entry to the date and time you would like your notification to fire. This method could easily be modified to take an NSDate parameter to allow you to call this method based on a dynamic date and time. Next you’ll want to change the alertBody to the message you want the user to see. It should contain a call to action so the user will be compelled to tap it to return to your app. SoundName can be changed to to accept an mp3 or wav if you wish. The example above uses the default sound setup on the users device. The application icon red badge bubble is then set to one to notify the user that their is a notification for you app that has not yet been tapped by them.

Cancelling All Notifications

    // Cancel all Notifications
    UIApplication* app = [UIApplication sharedApplication];
    NSArray *notifications = [app scheduledLocalNotifications];
    if ([notifications count] > 0)
       [app presentLocalNotificationNow:notifications[0]];

Receiving the Notification

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    
    self.mainVC = [[MainViewController alloc] initWithNibName:@"MainViewController" bundle:nil];
    self.window.rootViewController = self.mainVC;
    [self.window makeKeyAndVisible];
    
    // Reset the Icon Alert Number back to Zero
    application.applicationIconBadgeNumber = 0;
    
    // Detect the Notification after a user taps it
    UILocalNotification *localNotif =
    [launchOptions objectForKey:UIApplicationLaunchOptionsLocalNotificationKey];
    if (localNotif) 
    {
	NSLog(@"Recieved Notification %@",localNotif);
    }
    
    return YES;
}

- (void)application:(UIApplication *)app didReceiveLocalNotification:(UILocalNotification *)notif {
	// Handle the notification when the app is running
	NSLog(@"Recieved Notification %@",notif);
}

If you would need to programmatically set an event in the users Calendar you’ll need to leverage the EventKit framework API. The EventKit framework provides classes for accessing and manipulating calendar events and reminders.

First, access your target settings, click on “Summary”, scroll down to the “Linked Frameworks and Libraries” group and click the + button to add a new framework to your project. Search for ‘EventKit.Framework’.

Then you will be able to add a method like the one below to your class.

First add the EventKit framework to the top of your *.m file.

#import 

Next use the method below to set the iCal event.

-(void) setiCalEvent {

   // Set the Date and Time for the Event
    NSDateComponents *comps = [[NSDateComponents alloc] init];
    [comps setYear:2013];
    [comps setMonth:3];
    [comps setDay:5];
    [comps setHour:9];
    [comps setMinute:0];
    [comps setTimeZone:[NSTimeZone systemTimeZone]];
    NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDate *eventDateAndTime = [cal dateFromComponents:comps];

    // Set iCal Event
    EKEventStore *eventStore = [[EKEventStore alloc] init];

    EKEvent *event = [EKEvent eventWithEventStore:eventStore];
    event.title = @"EVENT TITLE";

    event.startDate = eventDateAndTime;
    event.endDate = [[NSDate alloc] initWithTimeInterval:600 sinceDate:event.startDate];

    // Check if App has Permission to Post to the Calendar
    [eventStore requestAccessToEntityType:EKEntityTypeEvent completion:^(BOOL granted, NSError *error) {
        if (granted){
            //---- code here when user allows your app to access their calendar.
            [event setCalendar:[eventStore defaultCalendarForNewEvents]];
            NSError *err;
            [eventStore saveEvent:event span:EKSpanThisEvent error:&err];
        }else
        {
            //----- code here when user does NOT allow your app to access their calendar.
            [event setCalendar:[eventStore defaultCalendarForNewEvents]];
            NSError *err;
            [eventStore saveEvent:event span:EKSpanThisEvent error:&err];
        }
    }];
}

To customize it for your app, set the date and time of the iCal event, then change the event.title to what you want the user to see in their calendar, and finally the length of the event in seconds (i.e. 600 secs = 1 hour).

    [eventStore requestAccessToEntityType:EKEntityTypeEvent completion:^(BOOL granted, NSError *error) {
        if (granted){
            //---- code here when user allows your app to access their calendar.

        }else
        {
            //----- code here when user does NOT allow your app to access their calendar.

        }
    }];

On iOS6, Apple introduced a new privacy control. The user can control permissions for the contact and calender by each app. So in the code side, you need to add some way to request the permission. In iOS5 or before, you can always call it without the user having to give permission.