iOS 7 Programming Cookbook

17.1. Sending Notifications

Problem

You want to decouple parts of your app and send a notification where it can be picked up by another component in your app.

Solution

Compose an instance of NSNotification and broadcast it to your app using the class’s postNotification: method. You can get an instance of the notification center using its defaultCenter class method, like so:

#import "AppDelegate.h"

NSString *const kNotificationName = @"NotificationNameGoesHere";

@implementation AppDelegate

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
    
    NSNotification *notification = [NSNotification
                                    notificationWithName:kNotificationName
                                    object:self
                                    userInfo:@{@"Key 1" : @"Value 1",
                                               @"Key 2" : @2}];
    
    [[NSNotificationCenter defaultCenter] postNotification:notification];
    
    self.window = [[UIWindow alloc]
                   initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    return YES;
}

Discussion

A notification object is encapsulated in an instance of the NSNotification class. A notification object on its own is really nothing until it has been posted to the app using a notification center. A notification object has three important properties:

Name

This is a string. When a listener starts listening for notifications, it has to specify the name of the notification, as we will see later in this chapter. If you are posting a notification in a class of yours, ensure that the name of that notification ...