I want to remove a notification observer and I am using the method:
[[NSNotificationCenter defaultCenter] removeObserver: name:#"myNotification" object:nil];
for this. Now there are many observers who are listening to this notification and I want to remove all of them in one shot from a centralised place. Can I pass 'nil' in first parameter and it will remove all observers who are listening to myNotification?
You can remove an object from the notification center all together which means no notifications will get triggered. For example, when I have a view controller that has registered for notifications, I include this line in my dealloc.
[[NSNotificationCenter defaultCenter] removeObserver:self];
This is at the object level...so it will unregister for many notifications. It won't unregister for one notification in many objects.
Hope I understood your question correctly.
In case of Swift, you doing it like this:
NSNotificationCenter.defaultCenter().removeObserver(self)
And in Swift 3:
NotificationCenter.default.removeObserver(self)
Unfortunately, there is no way to remove all observers of a specific notification in one place. While there are certainly cases where this would be nice, it would be a dangerous thing to do as generally, the object doing the observing should be responsible for adding and removing itself as an observer of a particular notification. This ensures no unpredictable behavior b/c observers can come and go so they configure and clean up after themselves.
If an object that generates notifications goes away, it won't matter to the observer as the observer doesn't know about that object anyway. It just means that object won't be generating any more notifications.
[EDIT: RESPONSE TO YOUR COMMENT RE CLASS B STOPPING CLASS A FROM OBSERVING]
I just saw your comment. There are different ways to accomplish this, particularly if class B knows about class A. As you reference classes it sounds like you want to affect all instances of a class vs a particular instance. If you have some condition you can check when handling the notification, that's how I would approach this. In the notification handler something like:
if ([self shouldRespondToNotificationNamed:notification.name]) {
[self performNotificationAction];
}
If you don't have a condition you can check, then create one either in the class in question as an iVar or in a place where you can access it globally for all class instances. I generally use a singleton to store global app state that doesn't persist. If it persists, then use whatever method you're using for other state.
Related
I'm trying to think through the ramifications of passing an NSNotification object as a var to a method.
It seems like it's difficult to trace what happens to a the flow of logic with a notification anyway (without some great commenting and method/object naming) so passing the notification to another method/class/lib/project would just make it even less fun to debug or respond intelligently to an error.
But it just feels like it's the sort of thing that has a lot of hidden gotchas potentially.
And I know there are still a lot of people out there who love notifications.
So does anyone have some pros/cons/better-practices about passing the actual NSNotification object as a var between methods?
Just for clarity's sake here's some pseudo code:
[[NSNotificationCenter defaultCenter] postNotificationName:kSuperImportantNotification object:self userInfo:soMetaDict];
... Later in the code:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(importantThingHappened:) name:kSuperImportantNotification object:nil];
... Later in the code:
- (void) importantThingHappened:(NSNotification *)notification {
[respondingInterestedClass executeSomethingWith:notification];
}
... In "respondingInterestedClass":
- (void)executeServicesDiscoveredBlock:(NSNotification *)notification {
// ... do something with the NSNotificaiton object.
}
Thanks
It depends in what you are doing, usually this is when to use what:
One-to-One relationship and loose coupling: Delegate
One-to-One relationship and tight coupling: Direct method calls
One-to-Many relationship and loose coupling: Notifications
One-to-Many relationship and tight coupling: KVO
So you should not use Notifications for all kind of communication, lot of developers use them heavily even when its not needed.
Saying this, it is totally fine to pass the notification object to the method handler, but don't pass it anywhere else.
Putting aside how and when notifications should be used and focusing purely on the ephemeral set of uses that are totally the right choice....
Once received, the notification object is a state container that contains some set of state in a format that is convenient to the system. You can shoehorn whatever you want into it by shoving key/value pairs into a dictionary and passing it as the userInfo, if you want, but it is still the system state.
In a well designed body of code, there are hard fought barriers through which the format of the data and the encapsulated messaging are well defined. In a Model-View-Controller system, for example, a Model class won't be a subclass of UIView or handle user events directly.
And Notifications should be treated the same. When the notification is received, unwrap it into whatever internal-and-specific-to-your-architecture representation you want and pass that around.
Or, consider:
- (void) somethingChangedNotification:(NSNotification *)changed
{
[myModelGoop applyChange:...];
}
#implementation MyModelLayer
- (void)applyChange:(....)change
{
}
In the above, you could certainly pass the notification object into the model layer. But that's just pushing a system specific implementation detail into your model.
What happens the moment you have changes that aren't triggered via an NSNotification? Your either left to encapsulate your change into a fake NSNotification instance (because posting a notification may have side effects depending on who is observing) or you have to refactor the above to no longer take a notification in the model layer.
Better to get rid of the dependencies on system encapsulation early.
To that end, the NSNotification instance should never be passed out of the method that handles the notification.
I have a shared singleton that contains all relevant information on the current user and its session, through the object [IWSession session] and [IWSession session].currentUser.
The current user (which actually refers to the one logged in the application) might have some of its properties updated frequently through webservice calls (triggered by iBeacon, triggered by a change in its location, etc).
This implies to update the GUI accordingly at different places in the app, let's say 5 or 6 class instances.
What's the proper way to update information displayed in the app as soon as any property is updated ?
I thought about
1) Adding a KVO on the [IWSession session].currentUser on himself and for all properties regarding the following link
Key Value Observing - how to observe all the properties of an object?
2) The KVO would then trigger a
[[NSNotificationCenter defaultCenter] postNotificationName:#"userUpdated" object:nil];
and all classes which need their layout to be updated would listen to that notification.
Is it a good approach ?
Any other suggestion ?
If you use Notifications then memory will not be released for all the objects in which notification is posted. Because if you use notifications then reference to the object is stored in the heap. So I don't this it would be a good idea to use Notifications.
I recently have been using a weak-observer list using hashtables
mObservers = [NSHashTable weakObjectsHashTable];
with semi-delegation messages, for example:
#protocol UserSessionObserver <NSObject>
- (void) userSession:(id)session didUpdateUser:(id)userProfile;
#end
So, any object that would be interested in changes to userProfile or userSession can simply add itself as an ad-hoc observer to the shared userSession. Because it is a weak entry, the object will be removed automatically from the observer table on dealloc.
Trick is to create the correct addObserver message:
- (void) addObserver:(__weak id<UserSessionObserver>)observer;
While this requires you to write your own observer logic, it also means:
1. No memory management issues with strong retain cycles,
2. Simplified and to the point observer messaging (the system notifications and KVO implementation is so generic, it is bordering on incomprehensible imo, but also adds several layers of logic that merely slows down processing).
An example implementation: Weak Observer example
I use the following line to add an observer:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(itemDidFinishPlaying) name:AVPlayerItemDidPlayToEndTimeNotification object:self.playerItem];
My observer (self) will never be deallocated.
But when the user starts playing a different item, the old self.playerItem will be deallocated---but will always be replaced with a new one, which I want to continue observing.
When that happens, what happens in regards to my observer's status as an observer? Do I need to do something to stop observing the deallocated object, as is required with KVO? Or will I continue observing the new object at self.playerItem? Or will my observer automatically be "unregistered?"
If I need to remove the observer, I wonder why there's no corresponding removeObserver method that enables one to specify a selector; it seems I can remove an observer only wholesale via removeObserver:(id)notificationObserver.
According to the NSNotificationCenter class reference:
Be sure to invoke removeObserver: or removeObserver:name:object:
before notificationObserver or any object specified in
addObserver:selector:name:object: is deallocated.
So: you should unregister your observer before self.playerItem deallocated.
But when the user starts playing a different item, the old
self.playerItem will be deallocated---but will always be replaced with
a new one, which I want to continue observing.
You may pass nil as the last parameter of addObserver:selector:name:object: method:
Adds an entry to the receiver’s dispatch table with an observer, a notification selector and optional criteria: notification name and sender.
If you don't specify
If you pass nil, the notification center doesn’t use a notification’s sender to decide whether to deliver it to the observer.
So you will receive notification AVPlayerItemDidPlayToEndTimeNotification from any object that posts it.
Since iOS 9 it is no longer needed to remove an observer from an object:
In OS X 10.11 and iOS 9.0 NSNotificationCenter and NSDistributedNotificationCenter will no longer send notifications to registered observers that may be deallocated.
However, block based observers need to be un-registered as before:
Block based observers via the -[NSNotificationCenter addObserverForName:object:queue:usingBlock] method still need to be un-registered when no longer in use since the system still holds a strong reference to these observers.
More information can be found here:
https://developer.apple.com/library/content/releasenotes/Foundation/RN-FoundationOlderNotes/index.html#10_11NotificationCenter
I'd like to retrieve a list of observers (objects and selectors) for a given notification name. I know there's no official API for that. I also know I could subclass NSNotificationCenter to accomplish this. Sometimes however this is not a viable option because NSNotificationCenter usage is spread all over the code or even binary frameworks.
So I'm looking for an unofficial/private way to do this. (Since it's about debugging only, that's fine.)
Finally, Apple added a way to print all notification center observers:
po [NSNotificationCenter defaultCenter]
It prints a comma separated list with Name, Object, Observer, and Options:
<NSNotificationCenter:0x7f997b307500>
Name, Object, Observer, Options
WebPreferencesRemovedNotification, 0x11165b680, 0x116c87ff8, 1400
UIApplicationWillEnterForegroundNotification, 0x11165b680, 0x7f997a838000, 1400
...
If you don't want to subclass NSNotificationCenter you can rename original addObserver:selector:name:object method and create your own with such name and add observers in there to some array then call original renamed method.
Take a look at following methods: class_addMethod, class_replaceMethod, class_getMethodImplementation.
Also look at this SO question: Method Swizzling
I am not sure why you want observers but you might find this class useful, which removes observers automatically for you which I think might be what you want. SFObservers
i have an NSNotificationCenter selector,
where to put it ? in the delegate (if yes then where?) in the controller?
where to put the method as well.
do i need to dealloc the NSNotificationCenter ?
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(deviceNotificationReceived:) name:UIApplicationDidBecomeActiveNotification object:nil];
- (void)deviceNotificationReceived:(NSNotification *)notification
{
[self.soundMgr endInterruption];
}
The deviceNotificationReceived: method must be an instance method of the argument to addObserver:. It is self in this instance, so your method should go in the same class.
You should not release the NotificationCenter, as you did not create or retain it.
Your question was a little hard to understand, is this what you were asking?
Hi, i have an NSNotificationCenter selector,
okay, you mean you have a selector for a method in NSNotificationCenter.
In Objective-C, “selector” has two
meanings. It can be used to refer
simply to the name of a method when
it’s used in a source-code message to
an object. It also, though, refers to
the unique identifier that replaces
the name when the source code is
compiled.
http://developer.apple.com/mac/library/documentation/cocoa/....../ocSelectors.html
So you have created a selector that refer to a method.
where to put it ?
It's a variable, you can store it where ever you feel it fits in your design.
in the delegate
See above.
(if yes then where?)
It's a variable, it depends on your usage.
in the controller?
Do you have controller? Depends on your design.
where to put the method as well.
Which method?
do i need to dealloc the NSNotificationCenter ?
No, [NSNotificationCenter defaultCenter] returns a reference to the notification center, you don't dealloc it.
Since you are subscribing to the UIApplicationDidBecomeActiveNotification notification, the most logical place to put the notification is in the applicationdDidFinishLaunching method of your app delegate.
That's the first point your code gets called, so you cannot set it earlier.
where to put it ?
It depend on when you need to register for notification. One way is to add observer in 'init' method of the class and remove notification in 'dealloc'method of the class.