I bult an iOS app and in -(void)viewDidLoad I parse data from web, and display it on load. But often when I open my app it displays old data(app loads fast) and I need to kill it and open it again, after that it parses data and shows new. Why is that happening?
It doesn't work that way because viewDidLoad is only called once, when the view is created. After backgrounding and returning, your view still exists.
If you want to reload your data whenever the app returns from the background, you need to either override applicationDidBecomeActive: in your UIApplicationDelegate implementation, or you need to listen for the appropriate notification:
- (void)viewDidLoad
{
[super viewDidLoad];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(applicationDidBecomeActive:)
name:UIApplicationDidBecomeActiveNotification
object:nil];
}
- (void)applicationDidBecomeActive:(NSNotification *)notification
{
// reload your data here.
}
Don't forget to remove yourself as an observer when you no longer need the notification.
Related
I have an app that is connected to a device by Bluetooth.
I want the app to send a command that indicates that the app is going to close in the app delegate method : (void)applicationWillTerminate:(UIApplication *)application {
One word: NSNotificationCenter
I'm not sure what you need to set the data to because you can't pass the data seamlessly via NSNotificationCenter; however you were going to figure that out in your UIApplicationDelegate anyway, so why can't you do it in the view controller directly.
In your case there is no need to do anything in your application delegate, because this notification allows your view controller to act as a mini app delegate (in that you can get termination status and so on).
Therefore...
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(TXdata:) name:UIApplicationWillTerminateNotification object:nil];
}
- (void)TXdata:(NSString *) data {
NSString *newData = data;
if (newData == nil) {
newData = ... // Figure out what your data should be here.
}
//do whatever with your data here.
}
I quote:
UIApplicationWillTerminateNotification
Posted when the app is about to terminate.
This notification is associated with the delegate applicationWillTerminate: method. This notification does not contain a userInfo dictionary.
You should create a class separate from the view controller and app delegate to handle the BLE communication. That way, the view controller and the app delegate can both have access and provide a better "separation of concerns" for your app. This new class might work well as a singleton.
Normally I add my observers in viewWillAppear and remove them in viewWillDisappear. In this case I need one of the observers to continue even after the view is gone so that it can finish some work. In order to make sure that the observer is only added once with this view, I do the following:
-(void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:YES];
[[NSNotificationCenter defaultCenter]removeObserver:self
name:#"imageSaved"
object:nil];
[[NSNotificationCenter defaultCenter]addObserver:self
selector:#selector(postMessageWithImage:)
name:#"imageSaved"
object:nil];
}
I have performed a search through the rest of the application to ensure that this observer is NOT registered anywhere else. Unfortunately sometimes, but not all times and there is no consistent factor, the notification is fired twice. I have also ensured with breakpoints and NSLog that the postNotifcationName is NOT called more than once. I have not been able to reproduce on the iPhone as the problem seems confined to the iPad.
In further troubleshooting I have checked that the method is being called from the same thread (no reason it wouldn't be but just to check). This problem DOES go away if I put the removeObserverin viewWillDisappear, however, again that is not how I need this to work.
Clearly this is a case where the observer for this is being registered twice but I cannot find a reason why that would be. As you can see from the code, any time this observer is registered it is first removed. My only other thought is whether self could get "corrupted" such that the removeObserverwouldn't function properly?
Add your observer when the view will show, and remove it when will disappear.
ADD:
- (void)viewDidLoad
{
[[NSNotificationCenter defaultCenter]addObserver:self
selector:#selector(postMessageWithImage:)
name:#"imageSaved"
object:nil];
}
REMOVE:
- (void)postMessageWithImage:(NSNotification*)aNotification
{
[[NSNotificationCenter defaultCenter]removeObserver:self
name:#"imageSaved"
object:nil];
// here do your job
}
This is perfectly valid and efficient.
Instead of adding the observer in viewWillAppear:, try adding the observer you wish to persist when the view disappears in viewDidLoad. Then you can call your removeObserver:name:object: in your dealloc method
If you just want something to be executed once, put it in the predicate of a dispatch_once() call, like
static dispatch_once_t lock;
dispatch_once(&lock, ^{
// put your addObserver call here
});
Within an App I make use of several viewcontrollers. On one viewcontroller an observer is initialized as follows:
[[NSNotificationCenter defaultCenter] removeObserver:self name:#"MyNotification" object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(myMethod:) name:#"MyNotification" object:nil];
Even when removing the NSNotification before initializing the number of executions of myMethod: is being summed up by the amount of repeated views on the respective viewcontroller.
Why does this happen and how can I avoid myMethod: being called more then once.
Note: I made sure by using breakpoints that I did not made mistakes on calling postNotification multiple times.
Edit: This is how my postNotification looks like
NSArray * objects = [NSArray arrayWithObjects:[NSNumber numberWithInt:number],someText, nil];
NSArray * keys = [NSArray arrayWithObjects:#"Number",#"Text", nil];
NSDictionary * userInfo = [NSDictionary dictionaryWithObjects:objects forKeys:keys];
[[NSNotificationCenter defaultCenter] postNotificationName:#"myNotification" object:self userInfo:userInfo];
edit: even after moving my subscribing to viewwillappear: I get the same result. myMethod: is called multiple times. (number of times i reload the viewcontroller).
-(void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
[[NSNotificationCenter defaultCenter] removeObserver:self name:#"MyNotification" object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(myMethod:) name:#"MyNotification" object:nil];
}
edit: something seems wrong with my lifecycle. ViewDidUnload and dealloc are not getting called, however viewdiddisappear is getting called.
The way I push my Viewcontroller to the stack is as follows where parent is a tableview subclass (on clicking the row this viewcontroller is initiated:
detailScreen * screen = [[detailScreen alloc] initWithContentID:ID andFullContentArray:fullContentIndex andParent:parent];
[self.navigationController pushViewController:screen animated:YES];
Solution:
Moving removal of nsnotification to viewdiddisappear did the trick. Thanks for guidance!
Based on this description, a likely cause is that your viewcontrollers are over-retained and not released when you think they are. This is quite common even with ARC if things are over-retained. So, you think that you have only one instance of a given viewcontroller active, whereas you actually have several live instances, and they all listen to the notifications.
If I was in this situation, I would put a breakpoint in the viewcontroller’s dealloc method and make sure it is deallocated correctly, if that’s the intended design of your app.
In which methods did you register the observers?
Apple recommends that observers should be registered in viewWillAppear: and unregistered in viewWillDissapear:
Are you sure that you don't register the observer twice?
Ran into this issue in an application running swift. The application got the notification once when first launched. the notification increases the number of times you go into the background and come back. i.e
app launches one - add observer gets gets called once in view will appear or view did load - notification is called once
app goes into background and comes back, add observer gets called again in view will appear or view did load. notification gets called twice.
the number increases the number of times you go into background and come back.
code in view will disappear will make no difference as the view is still in the window stack and has not been removed from it.
solution:
observe application will resign active in your view controller:
NSNotificationCenter.defaultCenter().addObserver(self, selector: "applicationWillResign:", name: UIApplicationWillResignActiveNotification, object: nil)
func applicationWillResign(notification : NSNotification) {
NSNotificationCenter.defaultCenter().removeObserver(self)
}
this will make sure that your view controller will remove the observer for the notification when the view goes into background.
it is quite possible you are subscribing to the notifications
[[NSNotificationCenter defaultCenter] postNotificationName:#"myNotification" object:self userInfo:userInfo];
before self gets initialized. And trying to unsubscribe 'self' which isn't really subscribed to, and you will get all global myNotification notifications.
If your view was hooked up in IB, use -awakeFromNib: as the starting point to register for notifications
It is possible that the class with the observer is, quite appropriately, instantiated multiple times. When you are debugging it will kinda look like the notification is being posted multiple times. But if you inspect self you might see that each time is for a different instance.
This could easily be the case if your app uses a tab bar and the observer is in a base class of which your view controllers are subclasses.
App delegate:
- (void)applicationDidBecomeActive:(UIApplication *)application {
[[NSNotificationCenter defaultCenter] postNotificationName:APP_REFRESH_NOTIFICATION object:nil];
}
In my view controller:
- (void)viewDidLoad {
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(doStuff) postNotificationName:APP_REFRESH_NOTIFICATION object:self];
}
- (void)doStuff never gets called. Why?
I assume that you've typed your question incorrectly and you'd meant to write addObserver:selector:name:object:, instead of addObserver:selector: postNotificationName:object: (such method doesn't exist).
In the documentation of - (void)addObserver:(id)notificationObserver selector:(SEL)notificationSelector name:(NSString *)notificationName object:(id)notificationSender
we can read:
notificationSender
The object whose notifications the observer wants
to receive; that is, only notifications sent by this sender are
delivered to the observer. If you pass nil, the notification center
doesn’t use a notification’s sender to decide whether to deliver it to
the observer.
So in your case, as you're passing object:nil in postNotificationName:object:, you also have to set object:nil in addObserver:selector:name:object:.
According to the documentation you also should replace the method doStuff with:
- (void)doStuff:(NSNotification *)notification
and use #selector(doStuff:) in addObserver:selector:name:object:.
You're passing self as the object parameter to addObserver:selector:name:object:, but doStuff doesn't accept any parameters, so the method call fails (silently). Your viewDidLoad should look like this:
- (void)viewDidLoad {
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(doStuff)
name:APP_REFRESH_NOTIFICATION
object:nil];
}
You're app delegate is posting a notification when the app becomes active, but your view controller isn't subscribing to that until its view gets loaded. If your app delegate is creating your view controller and loading it (which is probable) then your controller doesn't even exist at the time the notification is posted, which is why it isn't receiving it. If you use a storyboard, and that controller is the entry point in the storyboard, AND you use the info.plist for your app to set that storyboard as the main interface, then it will have already instantiated the controller and loaded its view by the time -applicationDidBecomeActive: is called, solving your problem.
I'm getting data off the server via JSON and displaying it on Labels.
I've added that method in viewDidLoad.
I want to refresh the data when the user opens the app again. Currently, even if I kill the app in the simulator and start the app again, it doesn't refresh.
I tried the viewDidAppear method, but it isn't being executed for some reason.
-(void)viewDidAppear{
NSLog(#"Called viewDidAppear");
}
This is never called. I tried to minimize the app but it didn't work.
You can listen for notifications and respond appropriately. Try using these and decide what works for your intended workflow.
UIApplicationDidBecomeActiveNotification
UIApplicationWillEnterForegroundNotification
You can use respond to the notification like this.
[[NSNotificationCenter defaultCenter] addObserverForName: UIApplicationDidBecomeActiveNotification object: nil queue: [NSOperationQueue mainQueue] usingBlock:^(NSNotification *note) {
// LOAD JSON
}];
I followed this tutorial - http://leejon.es/notifying-a-viewcontroller-with-uiapplicationdidbecomeactivenotification/
First, attach to the notification in the viewWillAppear method of the target view controller:
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
[[NSNotificationCenter defaultCenter] addObserver: self
selector: #selector( appActivated: )
name: UIApplicationDidBecomeActiveNotification
object: nil];
}
- (void)viewWillDisappear:(BOOL)animated
{
[super viewWillDisappear:animated];
[[NSNotificationCenter defaultCenter] removeObserver:self ];
}
- (void)appActivated:(NSNotification *)note
{
[self update];
}
The viewDidAppear: method takes a bool parameter wether the view was displayed with an animation which you are missing. Also you have to call the implementation of the superclass:
- (void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear: animated];
NSLog(#"Called viewDidAppear");
}
In your app delegate implementation, there is a method called:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions;
This method is called each time the app is launched, so I think it fits your needs. If you place your code here, it should work.
Also, be aware you should not perform a synchronous call here, because you will delay the app launch.
EDIT:
This method will be only called when the app launches. You could place your code inside a method, and call it from application didFinishLaunchingWithOptions, and then also call it from the method:
- (void)applicationWillEnterForeground:(UIApplication *)application;
This method will be called when the application enters the foreground, but not after the first launch, so beware.
I also think you should check the UIApplicationDelegate methods from apple developer page: http://developer.apple.com/library/ios/#documentation/uikit/reference/UIApplicationDelegate_Protocol/Reference/Reference.html
Also, check out the application state changes:
http://developer.apple.com/library/ios/#documentation/iphone/conceptual/iphoneosprogrammingguide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html