How to correctly setup NSNotification and post?

Question

I want to make a singleton Reachability observer for each connection delegate to use, however I couldn't build it correctly, here is some snippet of code.

MYReachability.m

static MYReachability *sharedInstance;

+ (MYReachability *)sharedInstance
{
    if (sharedInstance == NULL) {
        sharedInstance = [[MYReachability alloc] init];
    }

    return sharedInstance;
}

- (void)addReachabilityObserver
{
    [[NSNotificationCenter defaultCenter] addObserver: self
                                             selector: @selector(reachabilityChanged:)
                                                 name: kReachabilityChangedNotification
                                               object: nil];

    Reachability *reach = [Reachability reachabilityWithHostname: @"www.apple.com"];
    [reach startNotifier];
}

- (void) reachabilityChanged: (NSNotification *)notification {
    NSLog(@"notification: %@", notification);
    Reachability *reach = [notification object];

    if( [reach isKindOfClass: [Reachability class]]) {
        NetworkStatus status = [reach currentReachabilityStatus];
        NSLog(@"reachability status: %u", status);
        if (status == NotReachable) {
            // Insert your code here
            UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error"
                                                            message:@"Cannot connect to server..."
                                                           delegate:nil
                                                  cancelButtonTitle:@"OK"
                                                  otherButtonTitles:nil];
            [alert show];
        }
    }
}

in MYConnectionDelegate.m

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    if (error.code) {
        // sending notification to viewController
        [[MYReachability sharedInstance] addReachabilityObserver];
        MYTestClass *myTestClass = [MYTestClass new];
        [myTestClass notificationTrigger];
    }
}

in MYTestClass.m

- (void)notificationTrigger
{
    // All instances of TestClass will be notified
    [[NSNotificationCenter defaultCenter]
     postNotificationName:kReachabilityChangedNotification
     object:self];
}

Rick, Thanks, that works, but here comes another problem, each time it calls will generates one more notifications into stack... I made a dealloc in MYReachability.m

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

but previous notification still exists.

Answer 1

You're calling the method notificationTrigger but your test class only has the method notificationTrigger: with a colon, ie one parameter.

Change [myTestClass notificationTrigger]; to [myTestClass notificationTrigger:self]; and it should work.

If you only want the notification to show once, remove yourself as an observer once the alert view has been shown, like this:

- (void) reachabilityChanged: (NSNotification *)notification {
NSLog(@"notification: %@", notification);
Reachability *reach = [notification object];

if( [reach isKindOfClass: [Reachability class]]) {
    NetworkStatus status = [reach currentReachabilityStatus];
    NSLog(@"reachability status: %u", status);
    if (status == NotReachable) {
        // Insert your code here
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Cannot connect to server..." delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
        [alert show];
        [[NSNotificationCenter defaultCenter] removeObserver:self];
        }
    }
}

Answer 2

In one class:

[[NSNotificationCenter defaultCenter] postNotificationName:@"notifyMe" object:self];

In another class:

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(doSomething)
                                             name:@"notifyMe"
                                           object:nil];