I have a two ViewControllers: ViewController and SecondViewController.
I added an observer to this two ViewControllers. In ViewController I also defined an IBAction to post the notification. I handle the notification via a closure in both ViewControllers. But only the closure in the ViewController gets called. The closure (and even the whole code) in the SecondViewController does not get called (I checked with debugger). The closure only contains a print-statement.
Here is my Code
//ViewController
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let nc = NotificationCenter.default
nc.addObserver(forName: Notification.Name(rawValue:"MyNotification"), object: nil, queue: nil) { (notification) in
print("I'm the : \(type(of: self))")
}
}
#IBAction func sendNotification(_ sender: UIButton) {
let nc = NotificationCenter.default
nc.post(name: Notification.Name(rawValue:"MyNotification"), object: nil, userInfo: ["message":"Hello there!", "Date:":Date()])
}
}
The ScondViewController
//SecondViewController
import UIKit
class SecondViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let nc = NotificationCenter.default
nc.addObserver(forName: Notification.Name(rawValue:"MyNotification"), object: nil, queue: nil) { (notification) in
print("I'm the: \(type(of: self))")
}
}
}
The closure in ViewController gets called but the closure in SecondViewController does not. Maybe the reason is that SecondViewController does not get initialized before I post the notification. But how would a solution look like?
Any help is appreciated.
If this is one-to-one relationship you can also use the Delegate pattern instead. Although you are right, the notification observer is not yet called because you do not initialise before the post. So there shouldn't be a reason for a function to be called in the SecondViewController.
So if you only need that to update a variable value, you can do that somewhere else in a Variables class where both ViewControllers have access.
Related
This question already has answers here:
Passing data between view controllers
(45 answers)
Closed 2 years ago.
I am creating a library in IOS/swift that:
takes a user to a scene --> performs a task --> return to the initial scene that called the first while passing a payload back to the user
I have figured out how to take users back to the previous scene that called it, but my issue is how to send a payload back with it using thee code snippet below:
func switchToPreviousPage(){
self.dismiss(animated: true, completion: nil)
}
How do I achieve this?
In your scenario you can use either :
Delegation Pattern
Notification/Observer
Lets discuss each one :
1. Delegation :
If you have idea about Protocol in Swift you can do it easily.
first create a protocol with the required function you want to implement :
protocol FirstControllerDelegate: AnyObject {
func sendData(data: String)
}
Suppose your firstPage is FirstViewController, it has a UILabel and we have to assign a String to it from our secondPage means SecondViewController. the Structure of your FirstViewController may be like this :
class FirstViewController: UIViewController {
#IBOutlet weak var textLabel: UILabel!
#IBAction func gotoSecondPage() {
let secondVC = storyboard?.instantiateViewController(withIdentifier: "SecondViewController") as! SecondViewController
}
}
Now your FirstViewController has to confirm to this protocol and it will implement the sendData(data: ) method :
extension FirstViewController: FirstControllerDelegate {
func sendData(data: String) {
textLabel.text = data
}
}
Now as a feature of Protocol in iOS, Protocols can work as a Type(like Int, String). So just create a variable of type FirstControllerDelegate in your SecondViewController !
class SecondViewController: UIViewController {
weak var delegate: FirstControllerDelegate!
#IBAction func switchToPreviousPage() {
delegate.sendData(data: "Hello")
self.dismiss(animated: true, completion: nil)
}
}
You can now call the sendData(data:) function with the variable you created above !
At last you have to do oneThing just assign the delegate :
secondVC.delegate = self
It should be inside the gotoSecondPage() method !
2. Notification/Observer
With this, our basic idea is to send a Notification inside our app, and it can be observed by any where inside !
So our SecondViewController will send a Notification embedded with required data that we want to pass, and FirstViewController will receive the Notification and it will extract the data from the Notification !!
Each Notification has a specific name, which will differentiate it from other Notifications. we have to create the Name :
Notification.Name(rawValue: "com.app.notificationObserver")
Now the FirstViewController will be Observe to this specific notification :
override func viewDidLoad() {
super.viewDidLoad()
NotificationCenter.default.addObserver(self, selector: #selector(self.changeLabelText(notifcation:)), name: Notification.Name("com.app.notificationObserver"), object: nil)
}
We have to define changeLabelText(notification:) method :
private func changeLabelTExt(notification: NSNotification) {
if let dataDict = notification.userInfo as NSDictionary? {
if let message = dataDict["data"] as? String {
self.textLabel.text = message
}
}
}
Finally, SecondViewController will trigger the Notification :
#IBAction func switchToPreviousPage() {
NotificationCenter.default.post(name: Notification.Name(rawValue: "com.app.notificationObserver"), object: ["data": "hello"])
self.dismiss(animated: true, completion: nil)
}
Thats All .....
Suppose I have a storyboard like so:
Is it possible for me to get a flag or a boolean data from A back to B? I initially thought of using delegation but most of the tutorials about it talks about sending data between UIViewControllers that are part of 1 NavigationController. In my case, the UIViewController I need to get data is outside of the navigation controller. Is there a way for me to send data from A to B despite not being embedded in the same NavigationController?
If you don't want to use delegate between the classes . One possible way is to create separated file , saved in class and fetch required data any where in navigation .
Useful class for your case would be create singleton class FlowEngine . Use getter / setter method for saving and fetching of data. Code is attached for your reference .
class FlowEngine : NSObject{
private let static shared = FlowEngine()
private var data : String
private init(){
}
func savedData(text : String){
data = text
}
func fetchSavedData() -> String{
return data // add checsk for nil values
}
}
Delegation doesn't require the ViewControllers to be in same navigation stack. You can use the same for your case. However, if you choose to go with NotificationCenter, just remember to remove the observer when appropriate.
Other answers seem to accomplish your requirements but for the sake of completeness you could try to use KVC and KVO for modifying values in A and receiving its changes in B (or any other place)
You could see a detailed explanation of how to use them in here.
You have several ways to go, depending on your needs :
Delegation
Declare a protocol in A, and make B conform to it. Set the delegate of A to B. This could be cumbersome if the navigation stack has too many level, as you would need to pass the reference of B to each ViewController between A & B
Notification / KVO
B subscribe to a notification sent by A, no reference needed, thread safe. Don't forget to unsubscribe when done.
Proxy class
Use a proxy singleton class, that will hold your data. A will write to it, and B will read it in viewWillAppear.
UserDefaults
Same concept as a Proxy Class, but the data will persist during your app life cycle and even after killing the app. It's appropriate if you want to change a flag or a setting for your user, not if you have a lot of data to hold.
Cocoa Touch uses the target-action mechanism for communication between a control and another object. More here... If you would like to use it with UIControl objects like buttons, then you can set it in Interface Builder by sending an action to the FirstResponder object.
Target-Action will start searching a VC which responds to a given method from the current first responder and then will move to the next responder and will terminate a search in a current UIWindow. Once a controller which responds to a method signature is found, the search is terminated.
class AViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
}
#IBAction func configure(with dictionary: Dictionary<String, Any>) {
print(dictionary)
}
}
class BViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let a = self.targetViewController(forAction: #selector(ViewController.configure(with:)), sender: self) as? ViewController
a?.configure(with: ["firstName": "Alex", "lastName": "Toto"])
}
}
if your A viewController is not huge, In B viewController do this :
class B : UIViewController {
var a : A! = nil
func viewDidLoad() {
super.viewDidLoad()
a = storyboard?.instantiateViewController(withIdentifier: "StoryBoard ID") as? A
if a.booleanValue == true {
// use your booleanValue
a = nil // deallocate after using your value.
}
}
}
Update (better solution)
We've had to edit a few things to the functionality which presented me with the opportunity to refactor this. I used the NSNotification way, which was way cleaner than using closures.
ViewControllerB
override func viewDidLoad() {
super.viewDidLoad()
//Observe for notification from "myIdentifier"
NotificationCenter.default.addObserver(self, selector: #selector(self.processNotification(notification:)), name: Notification.Name("myIdentifier"), object: nil)
}
//function that gets called when notification is received
//the #objc annotation is required!
#objc func processNotification(notification: Notification) {
//Do something
}
ViewControllerA
#IBAction func didTapButton(_ sender: Any) {
//Process something
// ...
//
//Post a notification to those observing "myIdentifier"
NotificationCenter.default.post(name: Notification.Name("myIdentifier"), object: nil)
self.dismiss(animated: true, completion: nil)
}
Old (but working) solution
This might be an unpopular solution but I managed to solve this with callbacks. I was looking into another possible solution which was commented NSNotification but since someone from the team already had experience with using callbacks in this manner, we decided to ultimately use that.
How we made it work:
ViewControllerB is given the actual code implementation through prepare(for segue: UIStoryboardSegue, sender: Any?) while ViewControllerC (This is the middle UIViewController in the picture) has a callback property and ViewControllerA contains the value to pass when it's about to be dismissed.
ViewControllerB
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "secondSegue" {
let nvc: NavigationController = segue.destination as! NavigationController
let vc = nvc.viewControllers[0] as! ViewControllerC
vc.completion = { hasAgreed in
//Do Something
}
}
}
ViewControllerC
class ViewControllerC: UIViewController {
var completion: ((Bool) -> ())?
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "thirdSegue" {
let nvc: NavigationController = segue.destination as! NavigationController
let vc = nvc.viewControllers[1] as! ViewControllerA
vc.middleController = self
}
}
ViewControllerA
class ViewControllerC: UIViewController {
var middleController: ViewControllerC?
#IBAction func didTapButton(_ sender: Any) {
self.dismiss(animated: true, completion: {
middleController?.completion(true)
})
}
}
With this, we got the data we needed from the diagram picture above.
Your best bet is to make use of NotificationCenter to achieve this.
Post notification like this:
NotificationCenter.default.post(name: Notification.Name("NotificationName"), object: nil, userInfo: ["somekey":"somevalue"])
Observe it like this:
NotificationCenter.default.addObserver(self, selector: #selector(self.dataReceived(notification:)), name: Notification.Name("NotificationName"), object: nil)
Use the following method:
#objc func dataReceived(notification: Notification) {}
I want to push a notification from one(1st) ViewController to (2nd)Another ViewController while this notification takes a function of the first ViewController and prints the result of the function. How can I do that? Please help, I searched all the internet with no result.
Try this
Post Notification: (First ViewController)
NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: nil, userInfo: ["key":"value"])
Get Notification: (Second ViewController)
NotificationCenter.default.addObserver(self, selector: #selector(self.ReceivedNotification(notification:)), name: Notification.Name("NotificationIdentifier"), object: nil)
Method to handle received Notification:
#objc func ReceivedNotification(notification: Notification){
//Take Action on Notification
}
You can easily make use of protocols to pass the data between controller same as observer
Step 1 Create a protocol stub
/// Protcol
#objc protocol showViews : class {
/// An optional func inside protocol that will
/// be used to pass the required Data
/// For now, I am just passing a Bool Value
#objc optional func showToggleMenu(needToShow:Bool)
}
Source ViewController - Controller from which data is to be passed
/// Source Controller
class mainHomeVC: UIViewController
{
/// Create a referencing object for the protocol
var delegate : showViews?
/// To pass data just call the delegate and its function
#IBAction func showToggleMenu(_ sender: UIButton)
{
/// Pass Data
delegate?.showToggleMenu!(needToShow: true)
}
}
Destination Controller Controller in which Data was required to be passed
/// Destination Controller
class homeVC: UIViewController
{
/// Create a reference of class from which data is to be fetched
private lazy var mainHomeVCObjet: mainHomeVC =
{
// Instantiate View Controller
let viewController = self.storyboard?.instantiateViewController(withIdentifier: "mainHomeVC") as! mainHomeVC
viewController.delegate = self
// Add View Controller as Child View Controller
self.addChildViewController(viewController)
return viewController
}()
}
//MARK: Protocol Extension
extension homeVC : showViews
{
//MARK: Show Side Menu ?
func showToggleMenu(needToShow: Bool)
{
if (needToShow)
{
print("time to show a toggle Menu")
}
}
}
I have a UIView similar to the one you can see below:
class ViewTaskViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {
override func viewDidLoad() {
super.viewDidLoad()
subscribeToNotifications()
}
func subscribeToNotifications() {
let notification = NotificationCenter.default
notification.addObserver(forName: Notification.Name(rawValue: "TimerUpdated"), object: nil, queue: nil, using: handleUpdateTimer)
print("Subscribed to NotificationCenter in ViewTaskViewController")
}
override func viewWillDisappear(_ animated: Bool) {
super.viewWillDisappear(animated)
print("TUFU TUFU TUFU")
NotificationCenter.default.removeObserver(self)
}
deinit {
print("DENINT")
}
#objc func handleUpdateTimer(notification: Notification) {
if let userInfo = notification.userInfo, let timeInSeconds = userInfo["timeInSeconds"] as? Int {
withUnsafePointer(to: &self.view) {
print("We got timeeeeee \(timeInSeconds) \($0)")
}
//do something here....
}
}
}
The issue I am having is that I am unable to remove the observers from this particular UIView when the user hits the back button and returns to another viewController.
ViewWillDisppear is called but deinit is not called. The strange thing is that if we remove subscribeToNotifications() from viewDidLoad() then the deinit is called.
The other issue is related to a memory leak. As you can see in the screenshot below, when the view does subscribe to notifications and the user leaves/re-enters the view, the memory usage increase.
Now compare that to when the subscribeToNotifications() is commented out, there is no increase in memory usage and only one instance of the viewController.
The conclusion is that there seems to be a correlation between the notification subscription creation of a new instance of the UIView hence the deinit is not being called.
I'd like to find out if there is a way we can deinitialize the view and unsubscribe from the notification.
Please let me know if you need further information. :)
I've found the removeObserver() only works if you use this version of addObserver()
notification.addObserver(self, selector:#selector(self.handleUpdateTimer), name: Notification.Name(rawValue: "TimerUpdated"), object: nil)
I'm guessing with the original version you aren't actually indicating who the observer is.
As #Spads said you can use
NotificationCenter.default.addObserver(self, selector: #selector(subscribeToNotifications), name: NSNotification.Name(rawValue: "TimerUpdate"), object: nil)
or the one you already have.
you can remove your notification by it's name or it's reference
NotificationCenter.default.removeObserver(self, name: "TimerUpdate", object: nil)
if you declared your notification at the top of your class then you can directly pass the reference of your notification to be removed in your case notification
NotificationCenter.default.removeObserver(notification)
You should store your newly added observer in a opaque object (NSObjectProtocol) and then call NotificationCenter.default.removeObserver(self.nameOfObserver)
I’m trying to trigger a function after dismissing a modal VC (FirstStartVC) back to the main VC. I know that I have to use delegation but it doesn’t work and my debug area stays empty.
In other question topics there were people who had it work exact the same way like below. So I have no idea what I’m doing wrong. Does anyone know what I need to change to the code?
// FirstStartVC.swift
//
import UIKit
import CoreData
import JSSAlertView
protocol NewUser: class {
func newUserAction()
}
class FirstStartVC: UITableViewController, UITextFieldDelegate {
var delegation : NewUser?
func saveNewUser(){
self.delegation?.newUserAction()
self.dismiss(animated: true, completion: nil)
}
}
#IBAction func saveSettings(_ sender: Any) {
self.saveNewUser()
}
override func viewDidLoad() {
super.viewDidLoad()
print (delegation)
}
}
//
// ViewController.swift
//
import UIKit
import UserNotifications
import GoogleMobileAds
import CoreData
import JSSAlertView
class ViewController: UIViewController, UNUserNotificationCenterDelegate, NewUser {
func newUserAction() {
print("Reload some labels")
}
override func viewDidLoad() {
super.viewDidLoad()
var firstStart = FirstStartVC()
firstStart.delegation = self
}
}
Swift 3
In your main VC viewDidLoad add:
NotificationCenter.default.addObserver(self, selector: #selector(mainVc.functionName), name:"NotificationID", object: nil)
and add a function in main VC
func functionName() {
// Do stuff
}
in FirstStartVC call the method with
NotificationCenter.default.postNotificationName("NotificationID", object: nil)
Hope this helps!
A simple edit on Swift 4
NotificationCenter.default.addObserver(self, selector: #selector(self.funcName), name: NSNotification.Name(rawValue: "NotificationID"), object: nil)
Put #objc before the function definition.
#objc func functionName() {
// Do stuff
}
In your code, you have:
func saveNewUser(){
self.delegation?.newUserAction()
self.dismiss(animated: true, completion: nil)
}
}
Simply write the code you want to run after dismissing in completion::
func saveNewUser() {
self.delegation?.newUserAction()
self.dismiss(animated: true, completion: { finished in
// on completion
})
}
}
(You might not even need to say finished in or anything like that.)
If the code you need to execute within newUserAction() is a part of FirstStartVC, you should just call it inside the completion handler of the dismiss(_:animated:) method. However, if you need the code to execute on the VC that presented FirstStartVC, make sure that it conforms to the NewUser protocol. You could do something like this (assuming the presenting VC was named something like PresentingViewController - change it to whatever is the case for your project):
class PresentingViewController: UIViewController {
// However you instantiate the FirstStartVC
let firstStart = FirstStartVC()
// set the delegation property to self
firstStart.delegation = self
}
Then at the bottom of the screen create an extension so it conforms to the protocol:
extension PresentingViewController: NewUser {
func newUserAction() {
// Here you can do whatever you want when the delegation calls this method
}
}
EDIT: - Further recommendation...
I always find it best practice with delegates to use a weak reference to prevent memory problems. To do so you have to make sure to set the protocol as :class, which you've already completed: protocol NewUser: class. So then when you create the property at the top of FirstStartVC you would just say
weak var delegation: NewUser?
Your code will still run the same, I just recommend doing it this way as it's helped me avoid memory issues in numerous instances.