I have the following code that adds an observer in the loading of the view.
- (void)viewDidLoad
{
[super viewDidLoad];
[[NSNotificationCenter defaultCenter] addObserverForName:#"com.app.livedata.jsonupdated"
object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *notif) {
NSLog(#"JSONUPDATED");
}];
}
And this fires fine. However when the view is unloaded and I confirm the dealloc is called the Notification is still firing.
There doesn't seem to be a method for deactivating this observer?
Seems the solution is to track the object in the View and then you can reference it in the dealloc methods.
id observer = [[NSNotificationCenter defaultCenter] addObserverForName: /* ... */ ];
And then remove as following:
[[NSNotificationCenter defaultCenter] removeObserver:observer];
observer = nil;
Related
This is my code.
Here create the observer to Notification called Example into ViewController
- (void)addObserverExample
{
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(example:)
name:#"Example"
object:nil];
}
- (void)example:(NSNotification *)notification{
NSLog(#"Example!!!");
}
From viewDidLoad register my observer
- (void)viewDidLoad
{
[self addObserverExample];
}
In my second ViewController. When tapped a button excute this code:
[[NSNotificationCenter defaultCenter] postNotificationName:#"Example" object:self.dictKeys userInfo:nil];
The problem I have is that the notification is never executed.
Any idea.
Have created demo for NSNotificationCenter as per your question and it's working fine for me. Here it is the link of that code: NSNotificationCenter Demo
- (void)viewDidLoad {
[super viewDidLoad];
[self addObserverExample];
}
- (void)addObserverExample
{
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(example:)
name:#"Example"
object:nil];
}
- (void)example:(NSNotification *)notification{
NSLog(#"Example!!!");
NSLog(#"%#",notification.userInfo);
}
- (IBAction)btnFireNotification:(id)sender {
[[NSNotificationCenter defaultCenter] postNotificationName:#"Example" object:nil userInfo:#{#"key" : #"value"}];
}
I believe the problem you're having may be related to the fact that in your second view controller, you're passing self.dictKeys in the object parameter.
If you want to pass data via the NSNotificationCenter, you should use the userInfo parameter instead.
Darshan's example does this the correct way.
Brief : I have a queue that sends notifications. A view controller subscribes to them and when it receives them it displays an image.
Problem : the first time, everything goes well. When I come back later to the view, I see the log that the notification was received but the image is not displayed.
Note : the background thread is a queue : dispatch_queue_create("scan", DISPATCH_QUEUE_SCAN) and from this queue the notification are posted.
#property(strong, nonatomic) id observer;
- (void)viewDidAppear:(BOOL) animated {
[super viewDidAppear:animated];
self.observer = [[NSNotificationCenter defaultCenter] addObserverForName:MY_NOTIF object:nil queue:nil usingBlock:^(NSNotification *note) {
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"notification");
[self.img setImage:[UIImage imageNamed:#"register-ok"]];
[self.img setNeedsDisplay]; // useless
});
}
}
- (void)viewDidDisappear:(BOOL) animated {
[super viewDidDisappear:animated];
[[NSNotificationCenter defaultCenter] removeObserver:observer name:MY_NOTIF object:nil];
}
This is really getting me crazy. Thanks in advance for your help
EDIT : changes done :
NSLog(#"viewDidAppear. createObserver. self: %#");
[[NSNotificationCenter defaultCenter] addObserverForName:MY_NOTIF object:nil queue:nil usingBlock:^(NSNotification *note) {
NSLog(#"will call redraww. self: %#");
[self performSelectorOnMainThread:#selector(redraww:) withObject:nil waitUntilDone:YES];
}
-(void) redraww:(NSObject*)input {
NSLog(#"redraww self : %#", self);
[self.img setImage:[UIImage imageNamed:#"register-ok"]];
}
-(void) onRegistrationFinish {
NSLog(#"remove observer. self %#", self);
[[NSNotificationCenter defaultCenter] removeObserver:self name:(NSString *)NOTIF_SOLE_UUID object:nil];
}
console log
viewDidAppear. createObserver self: <RegisterLeftViewController: 0x15e34550>
will call redraww. self:<RegisterLeftViewController: 0x15e34550>
redrawww self : <RegisterLeftViewController: 0x15e34550>
remove observer. self <RegisterLeftViewController: 0x15e34550>
************************* Second time
viewDidAppear. createObserver self: <RegisterLeftViewController: 0x15e98ba0>
will call redraww. self:<RegisterLeftViewController: 0x15e34550>
redrawww self : <RegisterLeftViewController: 0x15e34550>
remove observer. self <RegisterLeftViewController: 0x15e98ba0>
Seems like you're doing a few things that could cause trouble.
The trouble could be coming from a number of places. Here's some things that may fix the problem:
Move observer registration to viewDidLoad
Move observer de-registration to dealloc
use __block __weak id observer to ensure self is properly captured in your notification handler. See iOS NSNotificationCenter Observer not being removed
Hope that helps
I'm registering three observers in most of my view controllers. Some have more, some less but I want to include part of the registration and unregistration process in a parent class. Is there any problem with calling the unregistering even if there is no observer? And is one call to unregister enough for all three observers?
- (void)registerForKeyboardNotifications
{
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(keyboardWasShown:)
name:UIKeyboardWillShowNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(keyboardWillBeHidden:)
name:UIKeyboardWillHideNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(applicationWillEnterBackground:)
name:UIApplicationWillResignActiveNotification
object:nil];
}
- (void)viewWillDisappear:(BOOL)animated
{
[super viewWillDisappear:animated];
//Has to be unregistered always, otherwise nav controllers down the line will call this method
[[NSNotificationCenter defaultCenter] removeObserver:self];
}
- (void)dealloc
{
[[NSNotificationCenter defaultCenter] removeObserver:self];
}
Yes, that will remove all registrations where the observer is self. It's documented in the NSNotificationCenter Class Reference:
The following example illustrates how to unregister someObserver for all notifications for which it had previously registered:
[[NSNotificationCenter defaultCenter] removeObserver:someObserver];
Note that in theory (but not, as far as I know, in practice as of iOS 7.0), UIViewController could have its own registrations that it doesn't want removed in viewWillDisappear:. It's unlikely to register for any of the notifications in the public API using addObserver:selector:name:object:, because that would preclude you registering for them in your UIViewController subclass, but it could certainly register for non-public notifications now or in a future version.
A safe way to deregister is to send removeObserver:name:object: once for each registration:
- (void)deregisterForKeyboardNotifications {
NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
[center removeObserver:self name:UIKeyboardWillShowNotification object:nil];
[center removeObserver:self name:UIKeyboardWillHideNotification object:nil];
[center removeObserver:self name:UIApplicationWillResignActiveNotification object:nil];
}
- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];
[self deregisterForKeyboardNotifications];
}
- (void)dealloc {
[self deregisterForKeyboardNotifications];
}
Another way is to use addObserverForName:object:queue:usingBlock: to register (instead of addObserver:selector:name:object:). This returns a new observer object reference for each registration. You have to save these away (perhaps in an NSArray instance variable if you don't want to create individual instance variables). Then you pass each one to removeObserver: to deregister its notification. Example:
#implementation MyViewController {
NSMutableArray *observers;
}
- (void)registerForKeyboardNotifications {
NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
NSOperationQueue *queue = [NSOperationQueue mainQueue];
__weak MyViewController *me = self;
observers = [NSMutableArray array];
[observers addObject:[center addObserverForName:UIKeyboardWillShowNotification
object:nil queue:queue usingBlock:^(NSNotification *note) {
[me keyboardWillShow:note];
}]];
[observers addObject:[center addObserverForName:UIKeyboardWillHideNotification
object:nil queue:queue usingBlock:^(NSNotification *note) {
[me keyboardWillHide:note];
}]];
[observers addObject:[center addObserverForName:UIApplicationWillResignActiveNotification
object:nil queue:queue usingBlock:^(NSNotification *note) {
[me applicationWillResignActive:note];
}]];
}
- (void)deregisterForKeyboardNotifications {
for (id observer in observers) {
[[NSNotificationCenter defaultCenter] removeObserver:observer];
}
observers = nil;
}
- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];
[self deregisterForKeyboardNotifications];
}
- (void)dealloc {
[self deregisterForKeyboardNotifications];
}
Since every observer returned by addObserverForName:object:queue:usingBlock: is a new object that has only one registration, each call to removeObserver: is guaranteed to only remove that observer's one registration.
Update for iOS 9 / macOS 10.11 and later
As of iOS 9 and macOS 10.11, NSNotificationCenter automatically deregisters an observer if the observer is deallocated. It is no longer necessary to deregister yourself manually in your dealloc method (or deinit in Swift) if your deployment target is iOS 9 or later or macOS 10.11 or later.
For your first Question unregistering even when there is no observer is OK.
But for the way you're removing the observer, [[NSNotificationCenter defaultCenter] removeObserver:someObserver]; will remove even the super class observers which is highly unrecommended (except in dealloc because the object is unloaded) but in viewWillDisappear you should remove the observers one by one by using [[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillHideNotification object:nil];
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Closed 10 years ago.
Possible Duplicate:
How to tell when controller has resumed from background?
How to refresh View after user enters applicationWillEnterForeground?
I want to complete recall for example HomeViewController.
I have and update function in HomeViewController and I want when user enters to call update function and reload table data.
Any class can register to UIApplicationWillEnterForegroundNotification, and react accordingly. It's not reserved to the app delegate, and helps for better separation of source code.
Create a viewDidLoad method like this for your HomeViewController
- (void)viewDidLoad
{
[super viewDidLoad];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(yourUpdateMethodGoesHere:)
name:UIApplicationWillEnterForegroundNotification
object:nil];
}
// Don't forget to remove the observer in your dealloc method.
// Otherwise it will stay retained by the [NSNotificationCenter defaultCenter]...
- (void) dealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}
If your ViewController is a tableViewController your could also directly call the reload data function:
- (void)viewDidLoad
{
[super viewDidLoad];
[[NSNotificationCenter defaultCenter] addObserver:[self tableView]
selector:#selector(reloadData)
name:UIApplicationWillEnterForegroundNotification
object:nil];
}
- (void) dealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}
Or you could use a block:
[[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationWillEnterForegroundNotification
object:nil
queue:[NSOperationQueue mainQueue]
usingBlock:^(NSNotification *note) {
[[self tableView] reloadData];
}];
You could declare a property in your app delegate class that would point to the HomeViewController object. Then you can call your update function in applicationWillEnterForeground.
Hi I am using NSNotificationCenter defaultCenter to implement the 'like' and 'comment' functions in my app.
//In Answer Table View
#implementation AnswerTableView
- (id)initWithParentController:(UIViewController *)pController andResourcePath:(NSString *)thisResourcePath {
....
// Notification to reload table when a comment is submitted
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(reloadTable)
name:#"Comment Submitted"
object:nil];
// Notification to reload table when an answer is liked
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(reloadTable)
name:#"Answer Liked"
object:nil];
}
- (void)dealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}
//In custom button implementation - THIS BUTTON IS CREATED IN EVERY CELL OF THE TABLEVIEW
#implementation UICustomButton
-(id)initWithButtonType:(NSString *)type {
self = [super init];
if (self) {
//Initialization done here
}
return self;
}
- (void)buttonPressed {
if ([btnType isEqualToString:#"like"]) {
[[NSNotificationCenter defaultCenter] postNotificationName:#"Answer Liked" object:nil];
}
else if ([btnType isEqualToString:#"comment"]) {
[[NSNotificationCenter defaultCenter] postNotificationName:#"Comment Submitted" object:nil];
}
}
However, I realize that after using these functions for a while, the response speed of the table reload gets slower and slower (to a point where it crashes).
Did I miss out anything in the implementation i.e. deallocating etc
You are repeatedly adding observers and the slowdown occurs because the notification code has to cycle over more and more observers to send notifications. You are probably crashing because you are leaking so many of these views.
Put a log statement in your dealloc to see if these instances are ever cleaned up. Also there can be timing issues with removeObserver in a dealloc method. try to remove the observer before dealloc if you can.
Sometimes its good to queue the event with Grand Central Dispatch to make sure its running on the main thread.
dispatch_async(dispatch_get_main_queue()