Handling applicationDidBecomeActive - “How can a view controller respond to the app becoming Active?”
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30-09-2019 - |
Question
I have the UIApplicationDelegate
protocol in my main AppDelegate.m class, with the applicationDidBecomeActive
method defined.
I want to call a method when the application returns from the background, but the method is in another view controller. How can I check which view controller is currently showing in the applicationDidBecomeActive
method and then make a call to a method within that controller?
Solution
Any class in your application can become an "observer" for different notifications in the application. When you create (or load) your view controller, you'll want to register it as an observer for the UIApplicationDidBecomeActiveNotification
and specify which method that you want to call when that notification gets sent to your application.
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(someMethod:)
name:UIApplicationDidBecomeActiveNotification object:nil];
Don't forget to clean up after yourself! Remember to remove yourself as the observer when your view is going away:
[[NSNotificationCenter defaultCenter] removeObserver:self
name:UIApplicationDidBecomeActiveNotification
object:nil];
More information about the Notification Center.
OTHER TIPS
Swift 3, 4 Equivalent:
adding observer
NotificationCenter.default.addObserver(self,
selector: #selector(applicationDidBecomeActive),
name: .UIApplicationDidBecomeActive, // UIApplication.didBecomeActiveNotification for swift 4.2+
object: nil)
removing observer
NotificationCenter.default.removeObserver(self,
name: .UIApplicationDidBecomeActive, // UIApplication.didBecomeActiveNotification for swift 4.2+
object: nil)
callback
@objc func applicationDidBecomeActive() {
// handle event
}
Swift 2 Equivalent:
let notificationCenter = NSNotificationCenter.defaultCenter()
// Add observer:
notificationCenter.addObserver(self,
selector:Selector("applicationWillResignActiveNotification"),
name:UIApplicationWillResignActiveNotification,
object:nil)
// Remove observer:
notificationCenter.removeObserver(self,
name:UIApplicationWillResignActiveNotification,
object:nil)
// Remove all observer for all notifications:
notificationCenter.removeObserver(self)
// Callback:
func applicationWillResignActiveNotification() {
// Handle application will resign notification event.
}
Swift 4.2
Add observer-
NotificationCenter.default.addObserver(self, selector: #selector(handleEvent), name: UIApplication.didBecomeActiveNotification, object: nil)
Remove observer-
NotificationCenter.default.removeObserver(self, name: UIApplication.didBecomeActiveNotification, object: nil)
Handle Event-
@objc func handleEvent() {
}
With Swift 4, Apple advises via a new compiler warning that we avoid the use of #selector
in this scenario. The following is a much safer way to accomplish this:
First, create a lazy var that can be used by the notification:
lazy var didBecomeActive: (Notification) -> Void = { [weak self] _ in
// Do stuff
}
If you require the actual notification be included, just replace the _
with notification
.
Next, we set up the notification to observe for the app becoming active.
func setupObserver() {
_ = NotificationCenter.default.addObserver(forName: .UIApplicationDidBecomeActive,
object: nil,
queue:.main,
using: didBecomeActive)
}
The big change here is that instead of calling a #selector
, we now call the var created above. This can eliminate situations where you get invalid selector crashes.
Finally, we remove the observer.
func removeObserver() {
NotificationCenter.default.removeObserver(self, name: .UIApplicationDidBecomeActive, object: nil)
}