I am trying a very simple code with NotificationCenter. But the addObserver is not getting called. Can any one of you check and let me know what i am missing. There are 2 simple class, one which post notification and another which listens to it. When i run the program, i just see "sending notification" in the console.
Thanks in advance.
Class 1:
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
print("sending notification")
NotificationCenter.default.post(name: Notification.Name("test"), object: nil)
}
}
Class 2:
class secondvc: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
print("second vc")
NotificationCenter.default.addObserver(self,
selector: #selector(doThisWhenNotify(_:)),
name: Notification.Name("test"),
object: nil)
}
#objc func doThisWhenNotify(_ notification: Notification) {
print("inside notification")
}
}
If, at the time ViewController comes into existence, secondvc does not yet exist, then there is no one there to receive the posted notification and that is why you don't see the notification being received later when secondvc does come into existence.
Related
I have 2 view controllers, one with a switch that when toggled should post the following notification. In the other view controller I have Observers which should trigger the following function which just toggles a boolean. I am not able to get the observers to work and make that function call, am I doing something wrong here? I have another Notification (Doesn't trigger with user input) that is being sent in the opposite direction which works fine.
#IBAction func switchAction(_ sender: Any) {
if switchUI.isOn {
print("Collecting Data ")
NotificationCenter.default.post(name:NSNotification.Name(rawValue: "Collect"), object: self)
}
else
{
print("Not Collecting Data")
NotificationCenter.default.post(name:NSNotification.Name(rawValue: "Do Not Collect"), object: self)
}
}
func collectDataObserver () {
//Add an Observer
NotificationCenter.default.addObserver(self, selector: #selector(CentralViewController.toggleData(notification:)), name: Notification.Name(rawValue: "Collect"), object: nil)
NotificationCenter.default.addObserver(self, selector: #selector(CentralViewController.toggleData(notification:)), name: Notification.Name(rawValue: "Do Not Collect"), object: nil)
}
#objc func toggleData(notification: NSNotification) {
let isCollectData = notification.name.rawValue == "Collect"
if(isCollectData){
IsCollectingData = true
}
else
{
IsCollectingData = false
}
}
You need to call collectDataObserver() in viewDidLoad() of CentralViewController, i.e.
override func viewDidLoad() {
super.viewDidLoad()
collectDataObserver()
}
I am using Spotify SDK. I want to change labels in some view controllers when a user changes his/her player state. Here is my scene delegate:
var playerViewController = MatchViewController()
func playerStateDidChange(_ playerState: SPTAppRemotePlayerState) {
playerViewController.stateChanged(playerState)
}
A view controller:
func stateChanged(_ playerState: SPTAppRemotePlayerState) {
// aLabel.text = playerState.track.name
}
The problem is labels or other outlets are nil when the state is changed because the view controllers are not loaded at that time. How can I fix that? (I tried isViewLoaded)
If you have a more than a few places to update according to a change that occurs at one place use observers. Here's how,
Post notification in SceneDelegate like this:
func playerStateDidChange(_ playerState: SPTAppRemotePlayerState) {
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "stateChanged"), object: nil, userInfo: ["playerState": playerState])
}
Observe in ViewControllers like this:
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(self, selector: #selector(stateChanged), name: NSNotification.Name("stateChanged"), object: nil)
}
#objc func stateChanged(_ notification: Notification) {
if let playerState = notification.userInfo?["playerState"] as? SPTAppRemotePlayerState {
print(playerState)
}
}
}
It appears I am getting a Strong Reference Cycle when using the NotifcationCenter.
I am using NotificationCenter to observe the Rotation of the device. (While some would argue this is not the best way to determine the device rotation, this currently seems to be my only route, as no autolayout is being used and storyboard is not being used).
deinit {} is never being called in my ViewController even if I remove the observer in viewWillDisappear and viewDidDisappear.
import UIKit
class TestVC: UIViewController {
deinit {
print("TestClass Deinit") //not being triggered ever
}
#objc private func rotationDetected(sender: Any) {
print("we rotated")
}
override func viewDidDisappear(_ animated: Bool) {
//NotificationCenter.default.removeObserver(UIDevice.orientationDidChangeNotification)
}
override func viewWillDisappear(_ animated: Bool) {
NotificationCenter.default.removeObserver(UIDevice.orientationDidChangeNotification)
//NotificationCenter.default.removeObserver(self) //also doesn't work
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(true)
NotificationCenter.default.addObserver(forName: UIDevice.orientationDidChangeNotification, object: nil, queue: .main, using: rotationDetected)
}
override func viewDidLoad() {
super.viewDidLoad()
}
}
Any ideas as to why this is occurring and how to resolve it?
Also open to any new ideas on how to determine rotation detection else ways (no autolayout or storyboard is being used though).
To reach TestVC() I used self.navigationController?.pushViewController(TestVC(), animated: true) in the previous ViewController and to go back I use the pop.
Without the Observer present, the class will correctly deinit.
RESOLVED
Thanks to the answer marked below the Strong Reference Cycle is removed.
Simply replace NotificationCenter.default.addObserver(forName: UIDevice.orientationDidChangeNotification, object: nil, queue: .main, using: rotationDetected)
with
NotificationCenter.default.addObserver(self, selector: #selector(rotationDetected), name: UIDevice.orientationDidChangeNotification, object: nil)
This should work in viewWillDisappear:
NotificationCenter.default.removeObserver(self, name: UIDevice.orientationDidChangeNotification, object: nil)
in combination with using the following in viewWillAppear:
NotificationCenter.default.addObserver(self, selector: #selector(rotationDetected), name: UIDevice.orientationDidChangeNotification, object: nil)
your method of removing observer is incorrect, you should do like this:
class TestVC {
private var observer: Any
func viewWillAppear() {
observer = NotificationCenter.default.addObserver(forName: UIDevice.orientationDidChangeNotification, object: nil, queue: .main, using: rotationDetected)
}
func viewWillDisappear() {
NotificationCenter.default.removeObserver(observer)
}
}
to eliminate the Strong Reference Cycle, use weak in the closure.
I have an observer that I register in one class as so:
class ViewControllerA: UIViewController {
//create shared class reference
var sharedClassReference_A = SharedClass()
//initialize Notification Observer and store observer reference in sharedClass
override func viewDidLoad() {
super.viewDidLoad()
var observerHandler: Any? = nil
observerHandler = NotificationCenter.default.addObserver(self, selector: #selector(ViewControllerA.appDidTerminate(_:)), name: .UIApplicationWillTerminate, object: nil)
self.sharedClassReference_A.sharedHandler = observerHandler
}
//perform some action when a notification is received
#objc func appDidTerminate(_ notification: NSNotification) {
//perform some action
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "segueA_X" {
let destinationController = segue.destination as! ViewControllerX
destinationController.sharedClassReference_X = self.sharedClassReference_A
}
}
}
I store a reference to the observer in a shared class:
class SharedClass {
var sharedHandler: Any? = nil
}
I attempt to remove the observer once I reach a different view controller as so:
class ViewControllerX: UIViewController {
var sharedClassReference_X = SharedClass()
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
//Attempt to remove observer registered in ViewControllerA
if let observerHandler = self.sharedClassReference_X.sharedHandler {
NotificationCenter.default.removeObserver(observerHandler)
}
}
}
I know that removing the observer using this approach is failing, because the observer is getting called after ViewControllerX is deallocated.
My question is: How can I successfully initialize an observer in one class (ViewControllerA) and be able to remove it later in a different class (ViewControllerX)?
I think it's better to follow the general guidelines of setting the observers inside viewDidLoad/viewWillAppear and removing them inside deinit / viewDidDisappear respectively according to your case , as this
NotificationCenter.default.addObserver(self, selector: #selector(ViewControllerA.appDidTerminate(_:)), name: .UIApplicationWillTerminate, object: nil)
returns void
//
class ViewControllerX: UIViewController {
var aRef:ViewControllerA!
}
in prepareForSegue
destinationController.aRef = self
Then use
NotificationCenter.default.removeObserver(aRef, name: NSNotification.Name.UIApplicationWillTerminate, object: nil)
I have two ViewControllers, FirstViewController and SecondViewController. Both have an own Swift file, FirstViewController.swift and SecondViewController.swift.
FirstViewController.swift contains:
class FirstViewController: UIViewController {
#IBAction func callFunctionInOtherClass(sender: AnyObject) {
// Call "func showAlert" in SecondViewController when clicking the UIButton in FirstViewController
}
}
SecondViewController.swift contains:
class SecondViewController: UIViewController {
#IBOutlet weak var textField: UITextField!
func showAlert(sender: AnyObject) {
let alert = UIAlertView(title: "Working!", message: "This function was called from FirstViewController!\nTextField says: \(textField.text!)", delegate: nil, cancelButtonTitle: "Okay")
alert.show()
}
}
I want to be able to call the func showAlert() in SecondViewController when taping on the UIButton in FirstViewController.
I've already spent many nights to find a solution but none worked. Does anybody know what to do to reach this goal?
I uploaded a sample Xcode project here: CallFuntionInOtherClassX | filedropper.com
P.S.: Of course, I could post some code and explain what error I get, but I think it's not reasonable because I really don't know how to do that.
You may use NSNotificationCentre to accomplish this task.
In viewDidLoad method of your SecondViewController class register self as observer to receive notification broadcasts:-
override func viewDidLoad() {
NotificationCenter.default.addObserver(self, selector: #selector(showAlert), name: NSNotification.Name(rawValue: "callForAlert"), object: nil)
}
and in FirstViewController's button action method you should fire the notification by writing :-
#IBAction func callFunctionInOtherClass(sender: AnyObject) {
//Call "func showAlert" in SecondViewController when clicking the UIButton in FirstViewController
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "callForAlert"), object: nil)
}
Don't forget to call removeObserver in SecondViewController's viewDidUnload method.
EDIT: These functions have been revised in swift 3 as follows:
Code in FirstViewController
override function viewDidLoad(){
NotificationCenter.default.addObserver(self, selector: #selector(showAlert), name: NSNotification.Name(rawValue: "showAlert"), object: nil)
}
Code in SecondViewController:
#IBAction func callFunctionInOtherClass(sender: AnyObject) {
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "showAlert"), object: nil)
}
If you want to show alert view to second viewcontroller from firstview controller when go to it you can do something like,
self.performSegueWithIdentifier("your segue identifier", sender: self) // or whatever way if you are not using storyboard or segue
let alert = UIAlertView(title: "Working!", message: "This function was called from FirstViewController!", delegate: nil, cancelButtonTitle: "Okay")
alert.show()
If you want to set any variables of secondviewcontroller then you need to implement prepareForSegue method. (if you are using segue).
Second thing you can show alertview in viewDidload of secondViewController also.
Try this:
SecondViewController().showAlert(self)
In your second view controller
if let text = textField?.text {
dispatch_async(dispatch_get_main_queue(),{
let alert = UIAlertView(title: "Working!", message: "This function was called from FirstViewController!\nTextField says: \(text)", delegate: nil, cancelButtonTitle: "Okay")
alert.show()
})
}