Update data after entering the foreground

Question

After changing the default settings, I would like to update myViewController data when I enter the foreground in AppDelegate. What i do is

AppDelegate.m

- (void)applicationDidFinishLaunching:(UIApplication *)application { [window addSubview:[navigationController view]]; NSLog(@"APPLICATION DID FINISH LAUNCHING"); // listen for changes to our preferences when the Settings app does so, // when we are resumed from the backround, this will give us a chance to update our UI // [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(defaultsChanged:) name:NSUserDefaultsDidChangeNotification object:nil]; } - (void)applicationWillEnterForeground:(UIApplication *)application { /* Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background. */ NSLog(@"APPLICATION WILL ENTER BACKGROUND"); [myViewController viewDidLoad]; } 

myViewController.m

 - (void)viewDidLoad { NSLog(@"VIEW DID LOAD IN MY VIEW CONTROLLER"); // watch when the app has finished launching so we can update our preference settings and apply them to the UI [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateSettings:) name:UIApplicationDidFinishLaunchingNotification object:nil]; } - (void)updateSettings:(NSNotification *)notif { myLabel.text = @"data has just been modified"; } 

However, nothing has changed.

Answer 1

You have 2 problems in the code. At first, the sequence of startup functions (the so-called execution states) seems incomprehensible to you, and secondly, you added a listener to the updateSettings function, but you never called it in the code above, and that’s why nothing changed when your application started.

Let me explain the initial sequence first. When the application is loaded from a turned off device, these states are triggered:

 application:didFinishLaunchingWithOptions: applicationDidBecomeActive: 

After that, if you click the "Home" button, these states will be launched:

 applicationWillResignActive: applicationDidEnterBackground: 

Then, if you re-enter the application, the following will happen:

 applicationWillEnterForeground: applicationDidBecomeActive: 

Please note that the download status occurs only once at the first boot (but not after returning from a home press). Now, for each view, the viewDidLoad function will only call one time that was first called by that view. If you call this view again (after loading it), the viewWillAppear function will be called viewWillAppear . Therefore, usually updating occurs in viewWillAppear .

The important thing that I noticed in your code is inappropriate is to use the basic functions of a delegate. In applicationWillEnterForeground you manually call viewDidLoad until you have to do this, as this function will be called automatically, as I explained above. I also see that you are adding Notification centers that are not needed.

Now consider the second problem in your code. You add the Notification Center for the updateSettings function to viewDidLoad . Well, loading this view will happen after the UIApplicationDidFinishLaunchingNotification event, so in practice you never called the updateSettings function. Moreover, since this function is a member of your class, you do not need a notification center to call it. We usually use the Notification Center when we need to call a function from another class. Just what you need to do is call this function directly from viewDidLoad as follows:

 [self updateSettings] 

And if you need to update after pressing the home button, call the function from viewWillAppear .

Hope this quick explanation helps you.

EDIT: answer your comment below

If you have only one view (no navigation controllers ...), after its appearance for the first time, it will remain in memory and it will not appear anymore (so this function is not called). Here you should catch the UIApplicationDidBecomeActiveNotification event to do the following:

In viewDidLoad add a notification:

 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateSettings) name:UIApplicationDidBecomeActiveNotification object:[UIApplication sharedApplication]]; 

This will allow you to call the updateSettings function every time your application wakes up. Also remember to remove this listener at the end:

 [[NSNotificationCenter defaultCenter] removeObserver:self]; 

Answer 2

You have a lot of problems with your code.

confusion between Foreground and Background

 - (void)applicationWillEnterForeground:(UIApplication *)application { NSLog(@"APPLICATION WILL ENTER BACKGROUND"); ... 

Well, no, he will enter FOREGROUND and leave the BACKGROUND.

missing supercall in viewDidLoad

 - (void)viewDidLoad { ... 

You must add [super viewDidLoad];

direct call viewDidLoad

 - (void)applicationWillEnterForeground:(UIApplication *)application { [myViewController viewDidLoad]; ... 

Well, no, do not call viewDidLoad yourself, as it should only be called once by the system. Superclasses or a subclass may not be compatible with multiple calls.

unbalanced observers

 - (void)viewDidLoad { [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateSettings:) name:UIApplicationDidFinishLaunchingNotification object:nil]; ... 

By calling viewDidLoad several times, you actually logged multiple observers for the same event. You must have symmetry in your code with the number of calls to removeObserver than addObserver (note that you can also remove multiple observers at the same time).

missing implementation?

 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(defaultsChanged:) name:NSUserDefaultsDidChangeNotification object:nil]; 

Well, we don’t see your implementation of defaultsChanged: so it’s not clear what you were trying to achieve. Should this set BOOL to YES and subsequently check this value to determine if preferences have been changed? Something like that?

 - (void)defaultsChanged:(id)notif { self.refreshData = YES; } - (void)applicationWillEnterForeground:(UIApplication *)application { if (self.refreshData) { self.refreshData = NO; // we refresh data now ... } }