Why is selector being sent to previous view controller? - ios

I have an app with a data model class that declares a protocol, and two view controllers embedded in a navigation controller. The data model class is a shared instance. Both view controllers are delegates of the data model. The second view controller has a UITableView.
On start, calls to data model functions from the first view controller work as expected. When I segue from the first view controller to the second, calls to data model functions work as expected as well.
However, when I navigate back to the first view controller and a data model function is called, the app crashes with this error:
2017-04-03 09:48:12.623027 CoreDataTest[3207:1368182]
-[CoreDataTest.PracticesViewController noDupeWithResult:]: unrecognized selector sent to instance 0x15fe136e0
That PracticesViewController is the second view controller. I don't understand why a selector is being sent to what I am thinking of as the previous view controller. My expectation is that the selector should be sent to the first view controller that has just been navigated back to,
I am self-taught, so I presume there is something basic that I am missing, but I don't know what I don't know. Can someone explain why the crash is happening?
Code for the data model
import Foundation
import CoreData
import UIKit
#objc protocol PracticesDataDelegate {
#objc optional func practicesLoadError(headline:String,message:String)
#objc optional func practicesLoaded(practices:[NSManagedObject])
#objc optional func practiceStored()
#objc optional func practiceDeleted()
#objc optional func noDupe(result:String)
}
class PracticesDataModel {
static let sharedInstance = PracticesDataModel()
private init () {}
var delegate: PracticesDataDelegate?
var practices: [NSManagedObject] = []
// some code omitted . . .
/// Check for a duplicate exercise
func checkForDupe(title:String,ngroup:String,bowing:String,key:String){
guard let appDelegate =
UIApplication.shared.delegate as? AppDelegate else {
return
}
let managedContext = appDelegate.persistentContainer.viewContext
let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: "Practice")
let predicate = NSPredicate(format: "exTitle == %# AND notegroup == %# AND bowing == %# AND key == %#", title, ngroup, bowing, key)
fetchRequest.predicate = predicate
do {
practices = try managedContext.fetch(fetchRequest)
if practices.count == 0 {
self.delegate?.noDupe!(result:"none") // exception happens here
} else {
self.delegate?.noDupe!(result:"dupe")
}
} catch let error as NSError {
// to come
}
}
The top of the first view controller
import UIKit
class galamianSelection: UIViewController, ExcerciseDataModelDelegate, PracticesDataDelegate {
let exerciseModel = ExerciseDataModel.sharedInstance
let pModel = PracticesDataModel.sharedInstance
// some code omitted . . .
//// THE VIEW ////
override func viewDidLoad() {
super.viewDidLoad()
exerciseModel.delegate = self
pModel.delegate = self
exerciseModel.loadExercises()
}
//// RESPOND TO PRACTICE DATA MODEL ////
func practiceStored() {
print("exercise stored")
}
func noDupe(result: String) {
if result == "none" {
let d = Date()
pModel.storePractice(date: d, exTitle: theExerciseTitle.text!, notegroup: theNoteGroup.text!, bowing: theBowings.text!, rythem: theRhythms.text!, key: theKey.text!, notes: "")
} else {
print("dupe")
}
}
The top of the second view controller
import UIKit
class PracticesViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, PracticesDataDelegate {
#IBOutlet weak var tableView: UITableView!
let pModel = PracticesDataModel.sharedInstance
// some code omitted . . .
//// THE VIEW ////
override func viewDidLoad() {
super.viewDidLoad()
pModel.delegate = self
pModel.getPractices()
}
delegate is properly set to self in both view controllers.
I am happy to provide more code is needed, but I suspect someone who knows can diagnose from what I've provided.

There are a few issues here.
You are trying to reuse a delegate but only setting it in viewDidLoad. viewDidLoad is only called when, as the name implies, the view initially loads. That means that if you have VC1 in a navigation controller, then you push to VC2, then pop back to VC1, viewDidLoad will not be called a second time. The view has already loaded. If you want a piece of code to be called every time a view controller comes back into focus, you should put it into viewWillAppear: or viewDidAppear:
You are force unwrapping an optional protocol method which you haven't actually implemented (a bug which you're seeing as a result of the first issue, but it's still an issue on its own). Change the force unwrap to an optional self.delegate?.noDupe?(result:"none")
You also declared your delegate like this var delegate: PracticesDataDelegate?. This makes your class retain its delegate and is generally not the right behavior. In your case it actually causes a retain cycle (until you change the delegate). You should change this declaration to weak var delegate: PracticesDataDelegate?

What appears to be happening is:
In PracticesViewController you set the model's delegate to self, as in pModel.delegate = self.
You navigate back to your first view controller. This means that PracticesViewController gets deallocated. But the model's delegate has not been changed, so it's still pointing to the memory location where the view controller used to be.
Later (but soon), your model tries to call a method on its delegate, but it can't because it was deallocated. This crashes your app.
You could reassign the value of the delegate, for example in viewDidAppear. Or you could have your model check whether its delegate implements a method before attempting to call it. That's a standard practice for optional protocol methods-- since they don't have to be implemented, you check first before calling them.
In general, don't use ! in Swift unless you want your code to crash there if something goes wrong.

Related

Refactoring CNContactPicker UI Code using the delegate pattern in Swift

I have implemented the CNContactPickerViewController ContactsUI provided by iOS in iOS10 succesfully in a view controller so I can have a user select multiple contacts to invite to an event. I am trying to reduce the size of this single view controller by implementing the delegate pattern, and am stuck on a black screen. I have looked at a few resources, and think I am calling the delegate and defining the protocol accordingly. I have a view controller, CreateEventViewController and it implements my self defined ContactsToInviteDelegate. This protocol is as follows:
protocol ContactsToInviteDelegate : class {
//array of array of KV-pairs where inner array is {"email":"email#gmail.com", "phone": "+18965883371"}
//array of JSON objects to upload
func contactsToInvite(_ contactsStructure: [[String:String]])
}
My ContactPickerViewController self defined class is as follows:
class ContactPickerViewController: UIViewController, CNContactPickerDelegate {
//class variables
let phoneNumberKit = PhoneNumberKit()
weak var delegate: ContactsToInviteDelegate?
var contactsToSendInvitesTo = [[String:String]]()
func contactPicker(_ picker: CNContactPickerViewController, didSelect contacts: [CNContact]) {
contacts.forEach { contact in
let phoneNum = contact.phoneNumbers.first
var stringPhoneNumber = String()
do{
let phoneNumber = try self.phoneNumberKit.parse((phoneNum?.value.stringValue)!, withRegion: "US", ignoreType:true)
stringPhoneNumber = "+1\(phoneNumber.adjustedNationalNumber())"
print(stringPhoneNumber)
}
catch {
print("phone number parsing error")
}
let contactDisplayName = contact.givenName
print("displayName: \(contactDisplayName)" )
let contactEmail = contact.emailAddresses.first?.value ?? ""
print("email: \(contactEmail)")
self.contactsToSendInvitesTo.append(["email":contactEmail as String, "phone":stringPhoneNumber])
}
delegate?.contactsToUpload(self.contactsToSendInvitesTo)
}
func contactPickerDidCancel(_ picker: CNContactPickerViewController) {
print("cancel contact picker")
}
func contactPicker(_ picker: CNContactPickerViewController,didSelectContactProperties contactProperties: [CNContactProperty]) {
}
}
And in the CreateEventViewController I am calling the delegate when i click the invite users button and implementing the method of the protocol to just attempt to print the final structure displaying contacts emails and phone numbers to send invitations to:
func selectContactsPicker() {
let cnPicker = ContactPickerViewController()
cnPicker.delegate = ContactPickerViewController() as? ContactsToInviteDelegate
self.present(cnPicker, animated:true, completion:nil)
}
func contactsToInvite(_ contactsStructure: [[String : String]]) {
print(contactsStructure)
}
This code without refactoring to try to use the delegate pattern worked before. I had all these functions within one single view controller, but with all the logic required this file itself is extending beyond 400+ lines. My problem now is that after attempting to refactor using the delegate pattern, when i click the button to trigger selectContactsPicker all I see is a black screen. I don't know what I am doing wrong, but I have a feeling it is this function itself. I am not quite sure what the body of this function should be in order to delegate the responsibility to the correct controller, or how to display it properly. Examples I saw used storyboards and segues, such as this. I looked at other examples for using delegates but I think my problem is a bit too specific and I don't know how to ask in a more general sense. If I did, I would probably not have this problem to begin with, as then I would probably properly understand how to implement the delegate pattern.
A delegate does not have to be a view controller. This is a convenient pattern when a view controller manages elements requiring delegates - rather than instantiate separate objects just let the view controller implement the protocol.
There are a number of ways to manage unruly view controllers which grow too large.
One simple way is to use extensions. To add a delegate protocol to an existing view controller:
extension SomeViewController : CNContactPickerDelegate {
... implement contact picker delegate methods
}
This can nicely compartmentalise your source code making it easier to read.
If you want to use a separate class instance as the delegate, that can be done quite easily too.
Declare your delegate class, either in the same source file or another:
class MyPickerDelegate : NSObject, CNContactPickerDelegate {
... implement contact picker delegate methods
}
note the class must inherit from NSObject, but does not need to be a UIViewController.
In the code where you fire up the contact picker:
picker = CNContactPickerViewController()
self.pickerDelegate = MyPickerDelegate()
picker.delegate = self.pickerDelegate
self.present(picker, animated: true)
Note picker view controller only keeps a weak reference to the delegate, so you must make sure to keep a strong reference to the object somewhere. Here I am using a property pickerDelegate

In Swift, how do I access data in second ViewController from third ViewController? [duplicate]

Say I have multiple view controllers in my Swift app and I want to be able to pass data between them. If I'm several levels down in a view controller stack, how do I pass data to another view controller? Or between tabs in a tab bar view controller?
(Note, this question is a "ringer".) It gets asked so much that I decided to write a tutorial on the subject. See my answer below.
Your question is very broad. To suggest there is one simple catch-all solution to every scenario is a little naïve. So, let's go through some of these scenarios.
The most common scenario asked about on Stack Overflow in my experience is the simple passing information from one view controller to the next.
If we're using storyboard, our first view controller can override prepareForSegue, which is exactly what it's there for. A UIStoryboardSegue object is passed in when this method is called, and it contains a reference to our destination view controller. Here, we can set the values we want to pass.
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "MySegueID" {
if let destination = segue.destination as? SecondController {
destination.myInformation = self.myInformation
}
}
}
Alternatively, if we're not using storyboards, then we're loading our view controller from a nib. Our code is slightly simpler then.
func showNextController() {
let destination = SecondController(nibName: "SecondController", bundle: nil)
destination.myInformation = self.myInformation
show(destination, sender: self)
}
In both cases, myInformation is a property on each view controller holding whatever data needs to be passed from one view controller to the next. They obviously don't have to have the same name on each controller.
We might also want to share information between tabs in a UITabBarController.
In this case, it's actually potentially even simpler.
First, let's create a subclass of UITabBarController, and give it properties for whatever information we want to share between the various tabs:
class MyCustomTabController: UITabBarController {
var myInformation: [String: AnyObject]?
}
Now, if we're building our app from the storyboard, we simply change our tab bar controller's class from the default UITabBarController to MyCustomTabController. If we're not using a storyboard, we simply instantiate an instance of this custom class rather than the default UITabBarController class and add our view controller to this.
Now, all of our view controllers within the tab bar controller can access this property as such:
if let tbc = self.tabBarController as? MyCustomTabController {
// do something with tbc.myInformation
}
And by subclassing UINavigationController in the same way, we can take the same approach to share data across an entire navigation stack:
if let nc = self.navigationController as? MyCustomNavController {
// do something with nc.myInformation
}
There are several other scenarios. By no means does this answer cover all of them.
This question comes up all the time.
One suggestion is to create a data container singleton: An object that gets created once and only once in the life of your application, and persists for the life of your app.
This approach is well suited for a situation when you have global app data that needs to be available/modifiable across different classes in your app.
Other approaches like setting up one-way or 2-way links between view controllers are better suited to situations where you are passing information/messages directly between view controllers.
(See nhgrif's answer, below, for other alternatives.)
With a data container singleton, you add a property to your class that stores a reference to your singleton, and then use that property any time you need access.
You can set up your singleton so that it saves it's contents to disk so that your app state persists between launches.
I created a demo project on GitHub demonstrating how you can do this. Here is the link:
SwiftDataContainerSingleton project on GitHub
Here is the README from that project:
SwiftDataContainerSingleton
A demonstration of using a data container singleton to save application state and share it between objects.
The DataContainerSingleton class is the actual singleton.
It uses a static constant sharedDataContainer to save a reference to the singleton.
To access the singleton, use the syntax
DataContainerSingleton.sharedDataContainer
The sample project defines 3 properties in the data container:
var someString: String?
var someOtherString: String?
var someInt: Int?
To load the someInt property from the data container, you'd use code like this:
let theInt = DataContainerSingleton.sharedDataContainer.someInt
To save a value to someInt, you'd use the syntax:
DataContainerSingleton.sharedDataContainer.someInt = 3
The DataContainerSingleton's init method adds an observer for the UIApplicationDidEnterBackgroundNotification. That code looks like this:
goToBackgroundObserver = NSNotificationCenter.defaultCenter().addObserverForName(
UIApplicationDidEnterBackgroundNotification,
object: nil,
queue: nil)
{
(note: NSNotification!) -> Void in
let defaults = NSUserDefaults.standardUserDefaults()
//-----------------------------------------------------------------------------
//This code saves the singleton's properties to NSUserDefaults.
//edit this code to save your custom properties
defaults.setObject( self.someString, forKey: DefaultsKeys.someString)
defaults.setObject( self.someOtherString, forKey: DefaultsKeys.someOtherString)
defaults.setObject( self.someInt, forKey: DefaultsKeys.someInt)
//-----------------------------------------------------------------------------
//Tell NSUserDefaults to save to disk now.
defaults.synchronize()
}
In the observer code it saves the data container's properties to NSUserDefaults. You can also use NSCoding, Core Data, or various other methods for saving state data.
The DataContainerSingleton's init method also tries to load saved values for it's properties.
That portion of the init method looks like this:
let defaults = NSUserDefaults.standardUserDefaults()
//-----------------------------------------------------------------------------
//This code reads the singleton's properties from NSUserDefaults.
//edit this code to load your custom properties
someString = defaults.objectForKey(DefaultsKeys.someString) as! String?
someOtherString = defaults.objectForKey(DefaultsKeys.someOtherString) as! String?
someInt = defaults.objectForKey(DefaultsKeys.someInt) as! Int?
//-----------------------------------------------------------------------------
The keys for loading and saving values into NSUserDefaults are stored as string constants that are part of a struct DefaultsKeys, defined like this:
struct DefaultsKeys
{
static let someString = "someString"
static let someOtherString = "someOtherString"
static let someInt = "someInt"
}
You reference one of these constants like this:
DefaultsKeys.someInt
Using the data container singleton:
This sample application makes trival use of the data container singleton.
There are two view controllers. The first is a custom subclass of UIViewController ViewController, and the second one is a custom subclass of UIViewController SecondVC.
Both view controllers have a text field on them, and both load a value from the data container singlelton's someInt property into the text field in their viewWillAppear method, and both save the current value from the text field back into the `someInt' of the data container.
The code to load the value into the text field is in the viewWillAppear: method:
override func viewWillAppear(animated: Bool)
{
//Load the value "someInt" from our shared ata container singleton
let value = DataContainerSingleton.sharedDataContainer.someInt ?? 0
//Install the value into the text field.
textField.text = "\(value)"
}
The code to save the user-edited value back to the data container is in the view controllers' textFieldShouldEndEditing methods:
func textFieldShouldEndEditing(textField: UITextField) -> Bool
{
//Save the changed value back to our data container singleton
DataContainerSingleton.sharedDataContainer.someInt = textField.text!.toInt()
return true
}
You should load values into your user interface in viewWillAppear rather than viewDidLoad so that your UI updates each time the view controller is displayed.
Another alternative is to use the notification center (NSNotificationCenter) and post notifications. That is a very loose coupling. The sender of a notification doesn't need to know or care who's listening. It just posts a notification and forgets about it.
Notifications are good for one-to-many message passing, since there can be an arbitrary number of observers listening for a given message.
Swift 4
There are so many approaches for data passing in swift. Here I am adding some of the best approaches of it.
1) Using StoryBoard Segue
Storyboard segues are very much useful for passing data in between Source and Destination View Controllers and vice versa also.
// If you want to pass data from ViewControllerB to ViewControllerA while user tap on back button of ViewControllerB.
#IBAction func unWindSeague (_ sender : UIStoryboardSegue) {
if sender.source is ViewControllerB {
if let _ = sender.source as? ViewControllerB {
self.textLabel.text = "Came from B = B->A , B exited"
}
}
}
// If you want to send data from ViewControllerA to ViewControllerB
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.destination is ViewControllerB {
if let vc = segue.destination as? ViewControllerB {
vc.dataStr = "Comming from A View Controller"
}
}
}
2) Using Delegate Methods
ViewControllerD
//Make the Delegate protocol in Child View Controller (Make the protocol in Class from You want to Send Data)
protocol SendDataFromDelegate {
func sendData(data : String)
}
import UIKit
class ViewControllerD: UIViewController {
#IBOutlet weak var textLabelD: UILabel!
var delegate : SendDataFromDelegate? //Create Delegate Variable for Registering it to pass the data
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
textLabelD.text = "Child View Controller"
}
#IBAction func btnDismissTapped (_ sender : UIButton) {
textLabelD.text = "Data Sent Successfully to View Controller C using Delegate Approach"
self.delegate?.sendData(data:textLabelD.text! )
_ = self.dismiss(animated: true, completion:nil)
}
}
ViewControllerC
import UIKit
class ViewControllerC: UIViewController , SendDataFromDelegate {
#IBOutlet weak var textLabelC: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
}
#IBAction func btnPushToViewControllerDTapped( _ sender : UIButton) {
if let vcD = self.storyboard?.instantiateViewController(withIdentifier: "ViewControllerD") as? ViewControllerD {
vcD.delegate = self // Registring Delegate (When View Conteoller D gets Dismiss It can call sendData method
// vcD.textLabelD.text = "This is Data Passing by Referenceing View Controller D Text Label." //Data Passing Between View Controllers using Data Passing
self.present(vcD, animated: true, completion: nil)
}
}
//This Method will called when when viewcontrollerD will dismiss. (You can also say it is a implementation of Protocol Method)
func sendData(data: String) {
self.textLabelC.text = data
}
}
Instead of creating a data controller singelton I would suggest to create a data controller instance and pass it around. To support dependency injection I would first create a DataController protocol:
protocol DataController {
var someInt : Int {get set}
var someString : String {get set}
}
Then I would create a SpecificDataController (or whatever name would currently be appropriate) class:
class SpecificDataController : DataController {
var someInt : Int = 5
var someString : String = "Hello data"
}
The ViewController class should then have a field to hold the dataController. Notice that the type of dataController is the protocol DataController. This way it's easy to switch out data controller implementations:
class ViewController : UIViewController {
var dataController : DataController?
...
}
In AppDelegate we can set the viewController's dataController:
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
if let viewController = self.window?.rootViewController as? ViewController {
viewController.dataController = SpecificDataController()
}
return true
}
When we move to a different viewController we can pass the dataController on in:
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
...
}
Now when we wish to switch out the data controller for a different task we can do this in the AppDelegate and do not have to change any other code that uses the data controller.
This is of course overkill if we simply want to pass around a single value. In this case it's best to go with nhgrif's answer.
With this approach we can separate view form the logic part.
As #nhgrif pointed out in his excellent answer, there are lots of different ways that VCs (view controllers) and other objects can communicate with each other.
The data singleton I outlined in my first answer is really more about sharing and saving global state than about communicating directly.
nhrif's answer lets you send information directly from the source to the destination VC. As I mentioned in reply, it's also possible to send messages back from the destination to the source.
In fact, you can set up an active one-way or 2-way channel between different view controllers. If the view controllers are linked via a storyboard segue, the time to set up the links is in the prepareFor Segue method.
I have a sample project on Github that uses a parent view controller to host 2 different table views as children. The child view controllers are linked using embed segues, and the parent view controller wires up 2-way links with each view controller in the prepareForSegue method.
You can find that project on github (link). I wrote it in Objective-C, however, and haven't converted it to Swift, so if you're not comfortable in Objective-C it might be a little hard to follow
SWIFT 3:
If you have a storyboard with identified segues use:
func prepare(for segue: UIStoryboardSegue, sender: Any?)
Although if you do everything programmatically including navigation between different UIViewControllers then use the method:
func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool)
Note: to use the second way you need to make your UINavigationController, you are pushing UIViewControllers on, a delegate and it needs to conform to the protocol UINavigationControllerDelegate:
class MyNavigationController: UINavigationController, UINavigationControllerDelegate {
override func viewDidLoad() {
self.delegate = self
}
func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool) {
// do what ever you need before going to the next UIViewController or back
//this method will be always called when you are pushing or popping the ViewController
}
}
It depends when you want to get data.
If you want to get data whenever you want, can use a singleton pattern. The pattern class is active during the app runtime. Here is an example of the singleton pattern.
class AppSession: NSObject {
static let shared = SessionManager()
var username = "Duncan"
}
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
print(AppSession.shared.username)
}
}
If you want to get data after any action, can use NotificationCenter.
extension Notification.Name {
static let loggedOut = Notification.Name("loggedOut")
}
#IBAction func logoutAction(_ sender: Any) {
NotificationCenter.default.post(name: .loggedOut, object: nil)
}
NotificationCenter.default.addObserver(forName: .loggedOut, object: nil, queue: OperationQueue.main) { (notify) in
print("User logged out")
}
The way that I would do it would be instead of passing data between view controllers, I would just declare a variable globally. You can even do this with a function!
For example:
var a = "a"
func abc() {
print("abc")
}
class ViewController: UIViewController {
}

I'm passing data from UITableViewController to UIViewController through protocols and instead of int value I'm getting nil. Why?

I have two UITableViewController, the first one:
protocol FetchUserProfileData {
func getNumberOfRequests()
}
class ListEvents: UITableViewController{
var fetchInfo:FetchUserProfileData?
func getNumberOfRequests() -> Int{
return 12
}
and the UIViewController:
class UserProfileDetails:UIViewController, FetchUserProfileData {
var listEvents: UserListEvents?
func getNumberOfRequests(){
}
override func viewDidLoad(){
listEvents?.fetchInfo = self
print(listEvents?.getNumberOfRequests())
and this line: print(listEvents?.getNumberOfRequests()) gives me a nil value instead of 12... What's wrong here?
---- edit
Ok, now I see that listEvents is empty... So my question is how can I pass that data from ListEvents to UserProfileDetails?
In this code, listEvents is probably nil.
But, the way you use the protocol looks odd to me. I would expect:
getNumberOfRequests in the protocol to return Int
ListEvents should be implementing the protocol, not UserProfileDetails
The empty getNumberOfRequests() in UserProfileDetails should be deleted
You did not set listEvents. When you are using story boards then you should set the fetchInfo not earlier than in (overwriting) prepareForSegue. Google for examples, the web is full of them. When you segue programmatically then you can set the property not before you actually instanticated the new view controller. You are better of using listEvents!.fetchInfo = self because in that case you'll get an exception when listEvents is nil.
I made some change your code and this will pass data from ListEvents to UserProfileDetails.
protocol FetchUserProfileDelegate {
func getNumberOfRequests()->Int
}
class ListEvents: UITableViewController,FetchUserProfileDelegate{
var userProfile: UserProfileDetails?
override func viewDidLoad() {
userProfile = UserProfileDetails()
userProfile?.delegate = self
}
// MARK: FetchUserProfileDelegate
func getNumberOfRequests() -> Int{
return 12 // return as your target Int
}
}
class UserProfileDetails:UIViewController {
var delegate:FetchUserProfileDelegate?
override func viewDidLoad() {
if let _ = delegate{
let resultInt = delegate?.getNumberOfRequests() // get the Int form ListEvents
print(resultInt)
}
}
}
The idea of moving data from one controller to another is very common. Most of the time this is done using a segue. A controller can have a function called prepareForSegue. This function gets called before the transition happens. Inside the prepareForSegue function, the system gives you destination controller object. You take that object and set your data in it. When the transition happens, and your destination controller comes up, it already has the data you want to give to it.
Use Xcode and make a new project. Choose "Master-Detail Application". This will generate the code for you and it is a good example of how to pass data between controllers.

How to make data visible for all view controllers?

Let's consider the following case:
I have a tab bar application where tapping each tab bar item takes user to other view that is handled by different view controller(typical pattern).
In one of my controllers I have method that downloads the important data and I want them to be global for whole application. What design pattern should I use?
One way to do that is to store this data using persistence such as core data, but is it the only way to make data visible for all view controllers? Maybe app delegate is able to perform such actions?
How in general you solve such situation where you have some data or variable which should be visible for all view controllers in your project?
Note that I'm not asking about persisting data across launches of the app, I just wonder how to make some data global in terms of the whole project.
Dont (emphasize DON'T) use following:
Singletons
AppDelegate (just another Singleton)
NSUserDefaults
Rather Don't:
Core Data
Do:
pass in either during instantiation or via properties
Why?
The DON'Ts messes up your memory
the Rather Don't messes with several principals of SOLID.
How would you do it correctly:
Create a base view controller that has a property that takes your data, make all your view controller inherit from it.
subclass UITabBarController
if a new view controller is selected, set the data to the view controller
the implementation is a bit tricky, this is from a real world app
class ContentTabBarController : UITabBarController {
private var kvoSelectedViewControllerContext: UInt8 = 1
required init(coder aDecoder: NSCoder) {
self.addObserver(self, forKeyPath: "selectedViewController", options: .New | .Old | .Initial , context: &kvoSelectedViewControllerContext)
}
deinit{
self.removeObserver(self, forKeyPath: "selectedViewController")
}
override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
if context == &kvoSelectedViewControllerContext {
var targetVC : UIViewController?
if let viewController = change["new"] as? UIViewController{
if let oldViewController = change["old"] as? UIViewController{
if viewController != oldViewController {
targetVC = viewController
}
}
} else {
targetVC = self.viewControllers![0] as? UIViewController
}
self.configureTargetViewController(targetVC)
}
}
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
self.navigationController?.navigationBar.translucent = false
}
func configureTargetViewController(viewController: UIViewController?){
//setup data
}
}
How does the tab bar controller get the data.
Well, that is up to you. It could fetch core data, it could actually pass a fetching manager as data. It could read from disc or network. But for a sane design it should not do it itself but use another class for it. and an instance of this fetcher class should be set from outside the tab bar controller, i.e. from the App Delegate.
One easy way would be to make a struct and make it hold variables. Then, you can edit it anytime you would want to. For example:
struct Variables {
static var number = 4
}
Then you can edit the data inside Variables in any view controller you want by doing this code.
Variables.number = 6 //or any other number you want
A cleaner and efficient, although not necessarily different, way to do this is to create a singleton class, e.g. AppData, which you can access in a variety of ways, and which would be available to all your other classes. It has the benefit of separating your app-specific stuff from the app delegate stuff. You might define the class this way:
#interface AppData : NSObject
// Perhaps you'll declare some class methods here & objects...
#end
you can define ivars for the AppData class, and then manage a singleton instance of AppData. Use a class method, e.g. +sharedInstance, to get a handle to the singleton on which you could then call mehods. For example,
[[AppData sharedInstance] someMethod:myArgument];
Your implementation of +sharedInstance can be where you manage the actual creation of the singleton, which the method ultimately returns.
Try this simple method,
1) Create a variable in appdelegate.swift that could be visible to all viewcontroller.
import UIKit
#UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
...
...
var Check:String!="" // to pass string
...
...
}
2) Create appdelegate instance in any viewcontroller
viewcontroller1.swift
import UIKit
class ViewController: UIViewController {
let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
...
...
override func viewDidLoad() {
super.viewDidLoad()
...
var tmp = "\(appDelegate.Check)"
appDelegate.Check="Modified"
}
}
Viewcontroller2.swift
import UIKit
class ViewController: UIViewController {
let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
...
...
override func viewDidLoad() {
super.viewDidLoad()
...
var getdata = "\(appDelegate.Check)"
println("Check data : \(getdata)") // Output : Check data : Modified
}
}

How do you share data between view controllers and other objects in Swift?

Say I have multiple view controllers in my Swift app and I want to be able to pass data between them. If I'm several levels down in a view controller stack, how do I pass data to another view controller? Or between tabs in a tab bar view controller?
(Note, this question is a "ringer".) It gets asked so much that I decided to write a tutorial on the subject. See my answer below.
Your question is very broad. To suggest there is one simple catch-all solution to every scenario is a little naïve. So, let's go through some of these scenarios.
The most common scenario asked about on Stack Overflow in my experience is the simple passing information from one view controller to the next.
If we're using storyboard, our first view controller can override prepareForSegue, which is exactly what it's there for. A UIStoryboardSegue object is passed in when this method is called, and it contains a reference to our destination view controller. Here, we can set the values we want to pass.
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "MySegueID" {
if let destination = segue.destination as? SecondController {
destination.myInformation = self.myInformation
}
}
}
Alternatively, if we're not using storyboards, then we're loading our view controller from a nib. Our code is slightly simpler then.
func showNextController() {
let destination = SecondController(nibName: "SecondController", bundle: nil)
destination.myInformation = self.myInformation
show(destination, sender: self)
}
In both cases, myInformation is a property on each view controller holding whatever data needs to be passed from one view controller to the next. They obviously don't have to have the same name on each controller.
We might also want to share information between tabs in a UITabBarController.
In this case, it's actually potentially even simpler.
First, let's create a subclass of UITabBarController, and give it properties for whatever information we want to share between the various tabs:
class MyCustomTabController: UITabBarController {
var myInformation: [String: AnyObject]?
}
Now, if we're building our app from the storyboard, we simply change our tab bar controller's class from the default UITabBarController to MyCustomTabController. If we're not using a storyboard, we simply instantiate an instance of this custom class rather than the default UITabBarController class and add our view controller to this.
Now, all of our view controllers within the tab bar controller can access this property as such:
if let tbc = self.tabBarController as? MyCustomTabController {
// do something with tbc.myInformation
}
And by subclassing UINavigationController in the same way, we can take the same approach to share data across an entire navigation stack:
if let nc = self.navigationController as? MyCustomNavController {
// do something with nc.myInformation
}
There are several other scenarios. By no means does this answer cover all of them.
This question comes up all the time.
One suggestion is to create a data container singleton: An object that gets created once and only once in the life of your application, and persists for the life of your app.
This approach is well suited for a situation when you have global app data that needs to be available/modifiable across different classes in your app.
Other approaches like setting up one-way or 2-way links between view controllers are better suited to situations where you are passing information/messages directly between view controllers.
(See nhgrif's answer, below, for other alternatives.)
With a data container singleton, you add a property to your class that stores a reference to your singleton, and then use that property any time you need access.
You can set up your singleton so that it saves it's contents to disk so that your app state persists between launches.
I created a demo project on GitHub demonstrating how you can do this. Here is the link:
SwiftDataContainerSingleton project on GitHub
Here is the README from that project:
SwiftDataContainerSingleton
A demonstration of using a data container singleton to save application state and share it between objects.
The DataContainerSingleton class is the actual singleton.
It uses a static constant sharedDataContainer to save a reference to the singleton.
To access the singleton, use the syntax
DataContainerSingleton.sharedDataContainer
The sample project defines 3 properties in the data container:
var someString: String?
var someOtherString: String?
var someInt: Int?
To load the someInt property from the data container, you'd use code like this:
let theInt = DataContainerSingleton.sharedDataContainer.someInt
To save a value to someInt, you'd use the syntax:
DataContainerSingleton.sharedDataContainer.someInt = 3
The DataContainerSingleton's init method adds an observer for the UIApplicationDidEnterBackgroundNotification. That code looks like this:
goToBackgroundObserver = NSNotificationCenter.defaultCenter().addObserverForName(
UIApplicationDidEnterBackgroundNotification,
object: nil,
queue: nil)
{
(note: NSNotification!) -> Void in
let defaults = NSUserDefaults.standardUserDefaults()
//-----------------------------------------------------------------------------
//This code saves the singleton's properties to NSUserDefaults.
//edit this code to save your custom properties
defaults.setObject( self.someString, forKey: DefaultsKeys.someString)
defaults.setObject( self.someOtherString, forKey: DefaultsKeys.someOtherString)
defaults.setObject( self.someInt, forKey: DefaultsKeys.someInt)
//-----------------------------------------------------------------------------
//Tell NSUserDefaults to save to disk now.
defaults.synchronize()
}
In the observer code it saves the data container's properties to NSUserDefaults. You can also use NSCoding, Core Data, or various other methods for saving state data.
The DataContainerSingleton's init method also tries to load saved values for it's properties.
That portion of the init method looks like this:
let defaults = NSUserDefaults.standardUserDefaults()
//-----------------------------------------------------------------------------
//This code reads the singleton's properties from NSUserDefaults.
//edit this code to load your custom properties
someString = defaults.objectForKey(DefaultsKeys.someString) as! String?
someOtherString = defaults.objectForKey(DefaultsKeys.someOtherString) as! String?
someInt = defaults.objectForKey(DefaultsKeys.someInt) as! Int?
//-----------------------------------------------------------------------------
The keys for loading and saving values into NSUserDefaults are stored as string constants that are part of a struct DefaultsKeys, defined like this:
struct DefaultsKeys
{
static let someString = "someString"
static let someOtherString = "someOtherString"
static let someInt = "someInt"
}
You reference one of these constants like this:
DefaultsKeys.someInt
Using the data container singleton:
This sample application makes trival use of the data container singleton.
There are two view controllers. The first is a custom subclass of UIViewController ViewController, and the second one is a custom subclass of UIViewController SecondVC.
Both view controllers have a text field on them, and both load a value from the data container singlelton's someInt property into the text field in their viewWillAppear method, and both save the current value from the text field back into the `someInt' of the data container.
The code to load the value into the text field is in the viewWillAppear: method:
override func viewWillAppear(animated: Bool)
{
//Load the value "someInt" from our shared ata container singleton
let value = DataContainerSingleton.sharedDataContainer.someInt ?? 0
//Install the value into the text field.
textField.text = "\(value)"
}
The code to save the user-edited value back to the data container is in the view controllers' textFieldShouldEndEditing methods:
func textFieldShouldEndEditing(textField: UITextField) -> Bool
{
//Save the changed value back to our data container singleton
DataContainerSingleton.sharedDataContainer.someInt = textField.text!.toInt()
return true
}
You should load values into your user interface in viewWillAppear rather than viewDidLoad so that your UI updates each time the view controller is displayed.
Another alternative is to use the notification center (NSNotificationCenter) and post notifications. That is a very loose coupling. The sender of a notification doesn't need to know or care who's listening. It just posts a notification and forgets about it.
Notifications are good for one-to-many message passing, since there can be an arbitrary number of observers listening for a given message.
Swift 4
There are so many approaches for data passing in swift. Here I am adding some of the best approaches of it.
1) Using StoryBoard Segue
Storyboard segues are very much useful for passing data in between Source and Destination View Controllers and vice versa also.
// If you want to pass data from ViewControllerB to ViewControllerA while user tap on back button of ViewControllerB.
#IBAction func unWindSeague (_ sender : UIStoryboardSegue) {
if sender.source is ViewControllerB {
if let _ = sender.source as? ViewControllerB {
self.textLabel.text = "Came from B = B->A , B exited"
}
}
}
// If you want to send data from ViewControllerA to ViewControllerB
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.destination is ViewControllerB {
if let vc = segue.destination as? ViewControllerB {
vc.dataStr = "Comming from A View Controller"
}
}
}
2) Using Delegate Methods
ViewControllerD
//Make the Delegate protocol in Child View Controller (Make the protocol in Class from You want to Send Data)
protocol SendDataFromDelegate {
func sendData(data : String)
}
import UIKit
class ViewControllerD: UIViewController {
#IBOutlet weak var textLabelD: UILabel!
var delegate : SendDataFromDelegate? //Create Delegate Variable for Registering it to pass the data
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
textLabelD.text = "Child View Controller"
}
#IBAction func btnDismissTapped (_ sender : UIButton) {
textLabelD.text = "Data Sent Successfully to View Controller C using Delegate Approach"
self.delegate?.sendData(data:textLabelD.text! )
_ = self.dismiss(animated: true, completion:nil)
}
}
ViewControllerC
import UIKit
class ViewControllerC: UIViewController , SendDataFromDelegate {
#IBOutlet weak var textLabelC: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
}
#IBAction func btnPushToViewControllerDTapped( _ sender : UIButton) {
if let vcD = self.storyboard?.instantiateViewController(withIdentifier: "ViewControllerD") as? ViewControllerD {
vcD.delegate = self // Registring Delegate (When View Conteoller D gets Dismiss It can call sendData method
// vcD.textLabelD.text = "This is Data Passing by Referenceing View Controller D Text Label." //Data Passing Between View Controllers using Data Passing
self.present(vcD, animated: true, completion: nil)
}
}
//This Method will called when when viewcontrollerD will dismiss. (You can also say it is a implementation of Protocol Method)
func sendData(data: String) {
self.textLabelC.text = data
}
}
Instead of creating a data controller singelton I would suggest to create a data controller instance and pass it around. To support dependency injection I would first create a DataController protocol:
protocol DataController {
var someInt : Int {get set}
var someString : String {get set}
}
Then I would create a SpecificDataController (or whatever name would currently be appropriate) class:
class SpecificDataController : DataController {
var someInt : Int = 5
var someString : String = "Hello data"
}
The ViewController class should then have a field to hold the dataController. Notice that the type of dataController is the protocol DataController. This way it's easy to switch out data controller implementations:
class ViewController : UIViewController {
var dataController : DataController?
...
}
In AppDelegate we can set the viewController's dataController:
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
if let viewController = self.window?.rootViewController as? ViewController {
viewController.dataController = SpecificDataController()
}
return true
}
When we move to a different viewController we can pass the dataController on in:
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
...
}
Now when we wish to switch out the data controller for a different task we can do this in the AppDelegate and do not have to change any other code that uses the data controller.
This is of course overkill if we simply want to pass around a single value. In this case it's best to go with nhgrif's answer.
With this approach we can separate view form the logic part.
As #nhgrif pointed out in his excellent answer, there are lots of different ways that VCs (view controllers) and other objects can communicate with each other.
The data singleton I outlined in my first answer is really more about sharing and saving global state than about communicating directly.
nhrif's answer lets you send information directly from the source to the destination VC. As I mentioned in reply, it's also possible to send messages back from the destination to the source.
In fact, you can set up an active one-way or 2-way channel between different view controllers. If the view controllers are linked via a storyboard segue, the time to set up the links is in the prepareFor Segue method.
I have a sample project on Github that uses a parent view controller to host 2 different table views as children. The child view controllers are linked using embed segues, and the parent view controller wires up 2-way links with each view controller in the prepareForSegue method.
You can find that project on github (link). I wrote it in Objective-C, however, and haven't converted it to Swift, so if you're not comfortable in Objective-C it might be a little hard to follow
SWIFT 3:
If you have a storyboard with identified segues use:
func prepare(for segue: UIStoryboardSegue, sender: Any?)
Although if you do everything programmatically including navigation between different UIViewControllers then use the method:
func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool)
Note: to use the second way you need to make your UINavigationController, you are pushing UIViewControllers on, a delegate and it needs to conform to the protocol UINavigationControllerDelegate:
class MyNavigationController: UINavigationController, UINavigationControllerDelegate {
override func viewDidLoad() {
self.delegate = self
}
func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool) {
// do what ever you need before going to the next UIViewController or back
//this method will be always called when you are pushing or popping the ViewController
}
}
It depends when you want to get data.
If you want to get data whenever you want, can use a singleton pattern. The pattern class is active during the app runtime. Here is an example of the singleton pattern.
class AppSession: NSObject {
static let shared = SessionManager()
var username = "Duncan"
}
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
print(AppSession.shared.username)
}
}
If you want to get data after any action, can use NotificationCenter.
extension Notification.Name {
static let loggedOut = Notification.Name("loggedOut")
}
#IBAction func logoutAction(_ sender: Any) {
NotificationCenter.default.post(name: .loggedOut, object: nil)
}
NotificationCenter.default.addObserver(forName: .loggedOut, object: nil, queue: OperationQueue.main) { (notify) in
print("User logged out")
}
The way that I would do it would be instead of passing data between view controllers, I would just declare a variable globally. You can even do this with a function!
For example:
var a = "a"
func abc() {
print("abc")
}
class ViewController: UIViewController {
}

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