I have a View Controller in which my value is 0 (label) and when I open that View Controller from another ViewController
I have set viewDidAppear
to set value 20 on label. It works fine but when I close my app and than again I open my app but the value doesn't change because viewDidLoad
, viewDidAppear
and viewWillAppear
nothing get called. How can I call when I open my app. Do I have to do anything from applicationDidBecomeActive
?
This question is related to
ios
objective-c
iphone
xcode
swift
Register with the NotificationCenter in viewDidLoad
to be notified when the app returns from background
NotificationCenter.default.addObserver(self, selector: #selector(doSomething), name: UIApplication.willEnterForegroundNotification, object: nil)
Implement the method that should be called.
@objc private func doSomething() {
// Do whatever you want, for example update your view.
}
You can remove the observer once the ViewController
is destroyed. This is only required below iOS9 and macOS 10.11
deinit {
NotificationCenter.default.removeObserver(self)
}
As per Apple's documentation:
(void)beginAppearanceTransition:(BOOL)isAppearing animated:(BOOL)animated;
Description:
Tells a child controller its appearance is about to change.
If you are implementing a custom container controller, use this method to tell the child that its views are about to appear or disappear. Do not invoke viewWillAppear:
, viewWillDisappear:
, viewDidAppear:
, or viewDidDisappear:
directly.
(void)endAppearanceTransition;
Description:
Tells a child controller its appearance has changed. If you are implementing a custom container controller, use this method to tell the child that the view transition is complete.
Sample code:
(void)applicationDidEnterBackground:(UIApplication *)application
{
[self.window.rootViewController beginAppearanceTransition: NO animated: NO]; // I commented this line
[self.window.rootViewController endAppearanceTransition]; // I commented this line
}
Question: How I fixed?
Ans: I found this piece of lines in application. This lines made my app not recieving any ViewWillAppear notification's. When I commented these lines it's working fine.
In your viewDidLoad
, register at notification center to listen to this opened from background action
NotificationCenter.default.addObserver(self, selector:#selector(doSomething), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil)
Then add this function and perform needed action
func doSomething(){
//...
}
Finally add this function to clean up the notification observer when your view controller is destroyed.
deinit {
NotificationCenter.default.removeObserver(self)
}
Using Objective-C
You should register a UIApplicationWillEnterForegroundNotification
in your ViewController
's viewDidLoad
method and whenever app comes back from background you can do whatever you want to do in the method registered for notification. ViewController
's viewWillAppear or viewDidAppear won't be called when app comes back from background to foreground.
-(void)viewDidLoad{
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doYourStuff)
name:UIApplicationWillEnterForegroundNotification object:nil];
}
-(void)doYourStuff{
// do whatever you want to do when app comes back from background.
}
Don't forget to unregister the notification you are registered for.
-(void)dealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
}
Note if you register your viewController
for UIApplicationDidBecomeActiveNotification
then your method would be called every time your app becomes active, It is not recommended to register viewController
for this notification .
Using Swift
For adding observer you can use the following code
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(self, selector: "doYourStuff", name: UIApplication.willEnterForegroundNotification, object: nil)
}
func doYourStuff(){
// your code
}
To remove observer you can use deinit function of swift.
deinit {
NotificationCenter.default.removeObserver(self)
}
Just have your view controller register for the UIApplicationWillEnterForegroundNotification
notification and react accordingly.
I think registering for the UIApplicationWillEnterForegroundNotification is risky as you may end up with more than one controller reacting to that notification. Nothing garanties that these controllers are still visible when the notification is received.
Here is what I do: I force call viewDidAppear on the active controller directly from the App's delegate didBecomeActive method:
Add the code below to - (void)applicationDidBecomeActive:(UIApplication *)application
UIViewController *activeController = window.rootViewController;
if ([activeController isKindOfClass:[UINavigationController class]]) {
activeController = [(UINavigationController*)window.rootViewController topViewController];
}
[activeController viewDidAppear:NO];
try adding this in AppDelegate applicationWillEnterForeground.
func applicationWillEnterForeground(_ application: UIApplication) {
// makes viewWillAppear run
self.window?.rootViewController?.beginAppearanceTransition(true, animated: false)
self.window?.rootViewController?.endAppearanceTransition()
}
Source: Stackoverflow.com