What is the iOS equivalent for onRestart() used on Android?
onRestart() is called when current activity is being re-displayed to the user (the user has navigated back to it).
I believe you need viewWillAppear method:
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
//your code here
}
It is called every time right before view is going to be shown. So it will be called when view is shown for the first time as well.
If you want to avoid running your code for the first time viewWillAppear is called, you will have to add a flag property and check if it has been set previously.
If you're trying to capture whenever the scene in question comes into view, there are two cases you might be concerned about:
If you transition to this scene (or dismissing/popping back to this scene) from within the app. In that case, use viewWillAppear:
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
update() // your routine for updating what is displayed to the user
}
If your app is running and you press the "home" button (or go to another app), and then later return to the your app (before it is terminated), viewDidAppear is not called. To detect that scenario, you can observe .UIApplicationDidBecomeActive:
private var observer: NSObjectProtocol?
override func viewDidLoad() {
super.viewDidLoad()
observer = NotificationCenter.default.addObserver(forName: .UIApplicationDidBecomeActive, object: nil, queue: nil) { [weak self] notification in
self?.update()
}
}
deinit {
if let observer = observer {
NotificationCenter.default.removeObserver(observer)
}
}
Related
Why isn't iOS calling viewWillAppear when our application is returning from the background, even when I've set UIModalPresentationStyle.FullScreen?
viewWillAppear is a function that responds to a view controller's state change. Background and foreground states are different; they are done at an app level.
You can still respond app state changes by using notifications:
override func viewDidAppear(_ animated: Bool) {
// ...
NotificationCenter.default.addObserver(self, selector: #selector(didReceiveForegroundNotification), name: UIApplication.willEnterForegroundNotification, object: nil)
}
#objc func didReceiveForegroundNotification() {
// app returned from the background
}
Your view controller will listen to events until it is deallocated or removed as an observer. If you don't want to execute code when the view controller has dissappeared, you can do it on viewDidDisappear:
override func viewDidDisappear(_ animated: Bool) {
// ..
NotificationCenter.default.removeObserver(self)
}
I'm trying to create a timer app. I have a singleton class with a Timer which fires every x minutes. Using custom delegate I pass the data to active view controller and update the value in a label. If the data is when the count is y, I perform push and update the count in another view controller's label.
When application is in foreground I didn't get any problem. If the application is in background state the counter keeps running and label text isn't updated and push isn't performed. Still I'm in first view controller. How to solve this?
like I mentioned in your comment section. when you app is in the background state you shouldn't continuously updating your UI as it is pointless. when user tapped back into your app your view controller will call a function viewDidAppear(animated). In that function you can check timer condition then present second view controller if needed. I'll post a sample code below
class FirstViewController : UIViewController {
var timerExpiration = false
override func viewDidLoad() {
super.viewDidLoad()
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
if timerExpiration {
let vc = SecondViewController()
present(vc, animated: true, completion: nil)
}
}
}
class SecondViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
}
}
You need to wrap all foreground UI tasks in a block like this
DispatchQueue.main.async {
// do your UI stuff here, like
// label.text = "Main thread stuff"
}
Put this in your timer action.
Say you have
var someVC: UIViewController
is it possible to essentially do the following, somehow?
get a notification when {
someVC has a viewWillAppear
self.#selector(wow)
}
#objc func wow() {
print("we spied on that view controller, and it just willAppeared"
}
Is that possible ?
(Or maybe on didLayoutSubviews ?)
(I realize, obviously, you can do this by adding a line of code to the UIViewController in question. That's obvious. I'm asking if we can "add on" to it from elsewhere.)
If I understand your question correctly, you want ViewController B to receive a notification when viewWillAppear is called in ViewController A? You could do this through the Notifications framework. Keep in mind that both VC's have to be loaded for one to receive a notification.
Alternatively, if the two VC's are on the screen at the same time, then I'd recommend a delegate pattern - have VC A tell an overarcing controller class that it's viewWillAppear has been called, and this overarcing controller will then inform ViewController B.
To do this using Notifications:
(This is from memory, so please excuse typos)
class TestClassA: UIViewController {
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
// To improve this code, you'd pull out the Notification name and perhaps put it into an extension, instead of hardcoding it here and elsewhere.
NotificationCenter.default.post(Notification.init(name: Notification.Name.init(rawValue: "viewControllerAppeared")))
}
}
class TestClassB: UIViewController {
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
NotificationCenter.default.addObserver(self, selector: #selector(viewControllerAppeared(notification:)), name: Notification.Name.init(rawValue: "viewControllerAppeared"), object: nil)
}
#objc func viewControllerAppeared(notification: NSNotification) {
print("other viewcontroller appeared")
}
}
Documentation
Normally I can find out when a View Controller appears with
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
}
This won't be called though if the user presses the home button or for some other reason the app goes to the background and then returns to the foreground. To find out when the app comes to the foreground I can add an observer to the Notification Center.
class FirstViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(self, selector: #selector(appWillEnterForeground), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil)
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
print("FirstViewController")
}
#objc func appWillEnterForeground() {
print("app in foreground")
}
}
However, my problem is that I have a tabbed app I want to know which View Controller is active when the app comes back into the foreground. The Notification center just sends a general message. Even though I an setting the notification observer in the first tab, it could be on any tab when the app goes into the background.
NSNotification.Name.UIApplicationWillEnterForeground
is a notification thrown by Notification Center. So obviously that is not related to any specific VC. What you can do rather is,
#objc func appWillEnterForeground() {
if self.viewIfLoaded?.window != nil {
// viewController is visible
}
}
Though notification of App entering foreground gets triggered to every viewController observing it, only the VC which is currently loaded and visible will have its code in if condition executed. That gives you a control to decide which VC is currently visible.
EDIT 1:
All that you want to figure out is the top ViewController in navigation stack of TabBarControllerwhen app comes to foreGround, you can add the observer for NSNotification.Name.UIApplicationWillEnterForeground only in UITabBarControllerand in
#objc func appWillEnterForeground() {
var vc : UIViewController = tabBarController.viewControllers![tabBarController.selectedIndex]
while vc.presentedViewController != nil || self.childViewControllers.count != 0 {
if vc.presentedViewController != nil {
vc = vc.presentedViewController!
}
else {
vc = vc.childViewControllers.last!
}
}
print("\(vc) should be the top most vc")
}
Use the Notification Observer and your appWillEnterForeground() or any event which gets fired from the observer in all view controllers under tab controller. So whichever the view controller you came back to will get your notification event get triggered in that particular VC. If you're looking for a centralized solution, this scattered gun approach may not work.
I have some code I call that changes the language in the viewWillAppear section of a viewcontroller inside a navigation controller.
When I hit the back button the language change doesn't take place even though I have code for it to in the viewWillAppear. The only time it switches is when I hit back all the way to the original screen and then start moving forward it changes. Is there any way to have the function in the viewWillAppear work?
Here is my code, I'm using a language changing pod:
//MARK: Language change
//used to change language text for imediate screens
func setText(){
locationsLabel.text = "Locations".localized()
languageLabel.text = "Languages".localized()
termsOfUseLabel.text = "Terms of Use".localized()
privacyPolicyLabel.text = "Privacy Policy".localized()
pushNotificationsLabel.text = "Push Notifications".localized()
contactUsLabel.text = "Contact Us".localized()
}
// Changes text to current language
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
NSNotificationCenter.defaultCenter().addObserver(self, selector: "setText", name: LCLLanguageChangeNotification, object: nil)
}
// Remove the LCLLanguageChangeNotification on viewWillDisappear
override func viewWillDisappear(animated: Bool) {
super.viewWillDisappear(animated)
NSNotificationCenter.defaultCenter().removeObserver(self)
}
The viewWillAppear method is only adding a notification observer. The observer is removed in viewWillDisappear. This means that setText will only be called if the LCLLanguageChangeNotification notification is sent while the view is visible.
The update stops as soon as the view goes off-screen due to the navigation behaviour.
To ensure that the text is updated, you also need to call setText inside viewWillAppear:
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
setText()
NSNotificationCenter.defaultCenter().addObserver(self, selector: "setText", name: LCLLanguageChangeNotification, object: nil)
}
Implement navigationcontroller delegate methods
navigationController:willShowViewController:animated:
navigationController:didShowViewController:animated:
I have created this little class that solves this problem.
Just set it as a delegate of your navigation controller, and implement simple one or two methods in your view controller - that will get called when the view is about to be shown or has been shown via NavigationController
Here's the GIST showing the code