iOS 5 Programming Cookbook

1.29. Sending Notifications with NSNotificationCenter

Problem

You want to broadcast an event in your app and allow any object that is willing to listen to it to take action, depending on the notification that you are broadcasting.

Solution

Use the postNotificationName:object:userInfo: method of the default notification center of type NSNotificationCenter to post a notification that carries an object (usually the object that fires the notification) and a user-info dictionary that can carry extra information about the notification and/or the object that fires the notification.

Discussion

Notification centers are dispatch centrals for notification objects. For instance, when the keyboard pops up anywhere while the user is inside your app, iOS will send a notification to your app. Any object inside your app willing to listen to this notification can add itself to the default notification center as an observer for that particular notification. Once your object’s lifetime comes to an end, it must remove itself from the notification center’s dispatch table. As a result, a notification is a message that gets broadcasted to observers through a notification center. A notification center is an instance of NSNotificationCenter class. We retrieve the default notification center object using the defaultCenter class method of NSNotificationCenter.

Notifications are objects of type NSNotification. A notification object has a name (specified as NSString) and can carry two key pieces of information:

Note

You can specify the name of your notifications yourself. You don’t have to use an API for that. Just make sure that your notification names are unique enough that they won’t clash with a system notification.

Sender Object

This is the instance of the object that fires the notification. The observer can access this object using the object instance method of the NSNotification class.

User-Info Dictionary

This is an optional dictionary that the sender object can create and send alongside a notification object. This dictionary usually contains more information about the notification. For instance, when a keyboard is about to get displayed in iOS for any component inside your app, iOS sends the UIKeyboardWillShowNotification notification to the default notification center. The user-info dictionary of this notification contains values such as the rectangle of the keyboard before and after animation and the animation duration of the keyboard. Using this data, an observer can make a decision as to, for instance, what to do with UI components that potentially will be obstructed once the keyboard gets displayed on the screen.

Warning

Notifications are a great way of implementing decoupled code. By that I mean, using notifications, you can get rid of completion handlers and delegation. However, there is one potential caveat about notifications: they are not delivered immediately. They are dispatched by notification centers, and the implementation of NSNotificationCenter is hidden from application programmers. Delivery might sometimes be delayed by a few milliseconds or in extreme cases (which I have never encountered) a few seconds. As a result, it is up to you to decide where to and where not to use notifications.

In order to construct a notification of type NSNotification, use the notificationWithName:object:userInfo: class method of the NSNotificationClass, as we will soon see.

Note

It is best to suffix your notification names with the word Notification. For instance, it is permitted to give your notification a name similar to ResultOfAppendingTwoStrings. However, it is better to give the name ResultOfAppendingTwoStringsNotification, as that clearly says what this name belongs to.

Let’s have a look at an example. We’ll simply take a first name and a last name, append them to create one string (first name + last name) and then broadcast the result using the default notification center. We will do that in the implementation of our app delegate as soon as the user launches our app:

#import "AppDelegate.h"

@implementation AppDelegate

@synthesize window = _window;

/* The notification name */
const NSString *ResultOfAppendingTwoStringsNotification =
                @"ResultOfAppendingTwoStringsNotification";

/* Keys inside the dictionary that our notification sends */
const NSString
  *ResultOfAppendingTwoStringsFirstStringInfoKey = @"firstString";

const NSString
  *ResultOfAppendingTwoStringsSecondStringInfoKey = @"secondString";

const NSString
  *ResultOfAppendingTwoStringsResultStringInfoKey = @"resultString";

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{

  NSString *firstName = @"Anthony";
  NSString *lastName = @"Robbins";
  NSString *fullName = [firstName stringByAppendingString:lastName];

  NSArray *objects = [[NSArray alloc] initWithObjects:
                      firstName,
                      lastName,
                      fullName,
                      nil];

  NSArray *keys = [[NSArray alloc] initWithObjects:
                   ResultOfAppendingTwoStringsFirstStringInfoKey,
                   ResultOfAppendingTwoStringsSecondStringInfoKey,
                   ResultOfAppendingTwoStringsResultStringInfoKey,
                   nil];

  NSDictionary *userInfo = [[NSDictionary alloc] initWithObjects:objects
                                                         forKeys:keys];

  NSNotification *notificationObject =
  [NSNotification
   notificationWithName:(NSString *)ResultOfAppendingTwoStringsNotification
   object:self
   userInfo:userInfo];

  [[NSNotificationCenter defaultCenter] postNotification:notificationObject];

  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];
  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}

Of course, you don’t have to specify an object or a user-info dictionary for every notification that you wish to broadcast. However, if you are working with a team of developers on the same app or if you are writing a static library, I suggest that you fully document your notifications and clearly mention whether your notifications carry an object and/or an user-info dictionary with them. If they do, you must say what object each notification carries and what keys and values are inside the user-info dictionary. If you are planning on not sending an object or a user-info dictionary, then I suggest you use the postNotificationName:object: instance method of NSBundle. Specify a string that represents the name of your notification as the first parameter, and nil as the second parameter, which is the object that should be carried with the notification. Here is an example:

#import "AppDelegate.h"

@implementation AppDelegate

@synthesize window = _window;

/* The notification name */
const NSString *NetworkConnectivityWasFoundNotification =
              @"NetworkConnectivityWasFoundNotification";

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{

  [[NSNotificationCenter defaultCenter]
   postNotificationName:(NSString *)NetworkConnectivityWasFoundNotification
   object:nil];

  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];
  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}