I am fairly new to developing in Swift, and I seem to be struggling with one of the key concepts.
I am looking to pass data between different UIViewControllers, allowing them all to access and manipulate it.
For example, in my application, I have created a simple data store class that contains and array of items. I want this array to be accessible by all ViewControllers.
I initialise the store within AppDelegate:
var itemStore = ItemStore()
I then create the first UIViewController and pass in the store so that it has access to it:
FirstViewController(itemStore: ItemStore)
So to do this I need to make changes to the init of FirstViewController so that it is able to accept itemStore as an argument.
I then want to pass that data to SecondViewController, and then to ThirdDataController.
It seems unnecessary to me that I have to edit every single UIViewController class so that it accepts the itemStore as an argument.
What are my options here? Some people have told me to store the data as a property of AppDelegate so that it is accessible by all. However, it seems that is not the right way of doing it.
Any advice?
You can pass data using a segue like this:
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!) {
if (segue.identifier == “segueTest”) {
var svc = segue.destinationViewController as secondViewController;
svc.toPass = textField.text
}
}
another solution
class AnsViewController: UIViewController {
var theNum: Int
override func viewDidLoad() {
super.viewDidLoad()
println(theNum)
}
}
override func tableView(tableView: UITableView!, didSelectRowAtIndexPath indexPath: NSIndexPath!) {
let viewController = self.storyboard.instantiateViewControllerWithIdentifier("ansView") as AnsViewController
viewController.num = theNum
self.presentViewController(viewController, animated: true, completion: nil)
}
complete Demo Solution with Tutorial might be helpful to you.
Tutorial
edited Added NSUserDefault Solution to Save Data
let highscore = 1000
let userDefaults = NSUserDefaults.standardUserDefaults()
userDefaults.setValue(highscore, forKey: "highscore")
userDefaults.synchronize() // don't forget this!!!!
Then, when you want to get the best highscore the user made, you have to "read" the highscore from the dictionary like this:
if let highscore = userDefaults.valueForKey("highscore") {
// do something here when a highscore exists
}
else {
// no highscore exists
}
I hope this helps!
NSUserDefaults supports the following data types:
NSString NSNumber NSDate NSArray NSDictionary NSData
Here is the Complete Demo Code, might be helpful to you
Related
I want to take user settings details from this view controller and read these details to the previous view controller. I have tried many different ways, but I cannot take values until I visit this view controller
I have tried first method from this page Pass Data Tutorial
This method is also not working. I think it is very simple, but I cannot figure out the right way to do it.
class SetConvViewController: UIViewController {
var engS = "engS"
#IBOutlet weak var swithEnglish: UISwitch!
override func viewDidLoad() {
super.viewDidLoad()
if let eng2 = defaults.value(forKey: engS)
{
swithEnglish.isOn = eng2 as! Bool
}
}
let defaults = UserDefaults.standard
#IBAction func switchEng(_ sender: UISwitch) {
defaults.set(sender.isOn, forKey: engS)
}
}
If I understand you correctly from this part - „but I cannot take values until I visit this view controller” - your problem lies with the fact, that until you visit your settings, there is no value for them in UserDefaults.
If you are reading them using getObject(forKey:) method, I’d recommend you to switch to using getBool(forKey:), since it will return false even if the value has not been set yet for that key ( docs )
Anyhow, if you want to set some default/initial values you can do so in your didFinishLaunching method in AppDelegate :
if UserDefaults.standard.object(forKey: „engS”) == nil {
// the value has not been set yet, assign a default value
}
I’ve also noticed in your code that you used value(forKey:) - you should not do that on UserDefaults - this is an excellent answer as to why - What is the difference between object(forKey:) and value(forKey:) in UserDefaults?.
On a side note, if you are using a class from iOS SDK for the first time, I highly recommend looking through its docs - they are well written and will provide you with general understanding as to what is possible.
I would recommend you to store this kind of data as a static field in some object to be able to read it from any place. e.g.
class AppController{
static var userDefaults = UserDefaults.standard
}
and then you can save it in your SetConvViewController like
#IBAction func switchEng(_ sender: UISwitch) {
AppController.userDefaults.set(sender.isOn, forKey: engS)
}
and after that you can just read it from any other view controller just by calling
AppController.userDefaults
Using segues you can set to any destination whether it be next vc or previous:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "PreviousVC" {
if let prevVC = segue.destination as? PreviousViewController {
//Your previous vc should have your storage variable.
prevVC.value = self.value
}
}
If you're presenting the view controller:
Destination vc:
//If using storyboard...
let destVC = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "DestinationViewController") as! DestinationViewController
destVC.value = self.value
self.present(destVC, animated: true, completion: nil)
Previous vc:
weak var prevVC = self.presentingViewController as? PreviousViewController
if let prevVC = prevVC {
prevVC.value = self.value
}
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 {
}
Context: iOS App written in Swift 3 powered by Firebase 3.0
Challenge: On my app, the user's currentScore is stored on Firebase. This user can complete/un-complete tasks (that will increase/decrease its currentScore) from several ViewControllers.
Overview of the App's architecture:
ProfileVC - where I fetch the currentUser's data from Firebase & display the currentScore.
⎿ ContainerView
⎿ CollectionViewController - users can update their score from here
⎿ DetailsVC - (when users tap on a collectionView cell) - again users can update their score from here.
Question: I need to pass the currentScore to the VCs where the score can be updated. I thought about using prepare(forSegue) in cascade but this doesn't work since it passes "nil" before the query on ProfileVC is finished.
I want to avoid having a global variable as I've been told it's bad practice.
Any thoughts would be much appreciated.
Why don't you create a function that will pull all data before you do anything else.
So in ViewDidLoad call...
pullFirebaseDataASYNC()
Function will look like below...
typealias CompletionHandler = (_ success: Bool) -> Void
func pullFirebaseDataASYNC() {
self.pullFirebaseDataFunction() { (success) -> Void in
if success {
// Perform all other functions and carry on as normal...
Firebase function may look like...
func pullFirebaseDataFunction(completionHandler: #escaping CompletionHandler) {
let refUserData = DBProvider.instance.userDataRef
refUserData.observeSingleEvent(of: .value, with: { snapshot in
if let dictionary = snapshot.value as? [String: AnyObject] {
self.userCurrentScore = dictionary["UserScore"] as! Int
completionHandler(true)
}
})
}
Then when you segue the information across...
In ProfileVC
Create 2 properties
var containerVC: ContainerVC!
var userCurrentScore = Int()
Include the below function in ProfileVC...
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "ProfileToContainerSegue" {
let destination = segue.destination as! ContainerVC
containerVC = destination
containerVC.userCurrentScore = userCurrentScore
}
}
In ContainerVC create a property
var userCurrentScore = Int()
Ways to improve could be an error message to make sure all the information is pulled from Firebase before the user can continue...
Then the information can be segued across the same way as above.
Try instantiation, first embed a navigation controller to your first storyboard, and then give a storyboardID to the VC you are going to show.
let feedVCScene = self.navigationController?.storyboard?.instantiateViewController(withIdentifier: "ViewControllerVC_ID") as! ViewController
feedVCScene.scoreToChange = current_Score // scoreToChange is your local variable in the class
// current_Score is the current score of the user.
self.navigationController?.pushViewController(feedVCScene, animated: true)
PS:- The reason why instantiation is much healthier than a modal segue storyboard transfer , it nearly removes the memory leaks that you have while navigating to and fro, also avoid's stacking of the VC's.
I need to send a username between two view controllers so that the second view controller knows who to send a message to. I have tried prepareForSegue, however I have found that the variable passed cannot be dynamically altered. I decided to use NSUserDefaults, which worked very well for the length of my development process. Today, it stopped working. I do not think I deleted anything or made an changes, but nevertheless NSUserDefaults is no longer reliably carrying the value between the two view controllers. Every once in a while (maybe 20% of the time?) the value will be correctly passed. The rest of the time, nothing comes through.Code:
Set key:
func chooseFriend(sender: UIButton) {
let requestIndex = sender.tag
let friendChosen = self.friends.objectAtIndex(requestIndex) as! String
NSUserDefaults.standardUserDefaults().setValue("thisisatest", forKey: "testKey")
NSUserDefaults.standardUserDefaults().synchronize()
self.performSegueWithIdentifier("toChat", sender: self)
}
Note: In the viewDidLoad I set testKey = ""
Retrieve key on new view controller:
override func viewDidLoad() {
super.viewDidLoad()
let theKey = NSUserDefaults.standardUserDefaults().valueForKey("testKey")
print("The Key: \(theKey)")
refreshTable()
let swipe: UISwipeGestureRecognizer = UISwipeGestureRecognizer(target: self, action: "dismissKeyboard")
swipe.direction = UISwipeGestureRecognizerDirection.Down
self.view.addGestureRecognizer(swipe)
NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardWillShow:"), name: UIKeyboardWillShowNotification, object: nil)
NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("keyboardWillHide:"), name: UIKeyboardWillHideNotification, object: nil)
testLabel.text = ""
self.messages.addObject("Loading......")
}
Every time, the default comes up empty: The Key: Optional()I can successfully print the default after setting it, however it disappears once I am segued to the next view controller...If anyone else has experienced this problem please let me know.Thanks
Randy's code:
func chooseFriend(sender: UIButton) {
let requestIndex = sender.tag
let friendChosen = self.friends.objectAtIndex(requestIndex) as! String
// Instantiate the second view controller via t's identifier in the storyboard
if let secondViewController = self.storyboard?.instantiateViewControllerWithIdentifier("ChatVC") as? chatViewController {
// Set the chosen friend
secondViewController.friendChosen = friendChosen
self.presentViewController(secondViewController, animated: true, completion: nil)
}
}
Added this to destinationviewcontroller:
var friendChosen: String!
The methods for NSUserDefaults are setObject:forKey: and objectForKey:, not setValue:forKey: (Or look at the special methods for specific object types, like setBool:forKey: or stringForKey: (I don't think there's a custom set method for strings.))
The methods with "value" in their names are KVC methods.
But, as Randy says, using your app's model is a better way to go, or passing the information directly to a property in the destination view controller in prepareForSegue. Using NSUserDefaults would not be my first, or my second, choice in this situation.
It looks like you're using storyboards already so it should be pretty easy to pass information using prepareForSegue like this.
class DestinationVC : UIViewController {
var destName : String!
override func viewWillAppear(animated: Bool) {
//configure UI with the destName
self.label.text = destName
}
}
class PresentingVC : UIViewController {
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if let destinationVC = segue.destinationViewController as? DestinationVC {
destinationVC.destName = "Some String to Pass"
}
}
}
As already mentioned NSUserDefaults is not ideal. You will also be loosing type safety and relying on string matching with keys in NSUserDefaults rather than autocompleting and compiler checking with a var on the destinationVC. It's also good practice to limit where your data is kept and where it could be altered. Storing something in NSUserDefaults when the use case is quite confined will make it more difficult to write focussed tests and make it vulnerable to change from any class anywhere in the app. It may be an edge case but starting a pattern like this in your app could expose you to all sorts of side effect bugs in the future.
Ultimately, this type of information should be passed from view controller to view controller in a model via a delegate. That would be the "appropriate" way to achieve this behavior via a true MVC pattern.
Having said that; I think the quickest fix for you would be not to use segues and to avoid NSUserDefaults all together.
Try the following...
func chooseFriend(sender: UIButton) {
let requestIndex = sender.tag
let friendChosen = self.friends.objectAtIndex(requestIndex) as! String
// Instantiate the second view controller via it's identifier in the storyboard
if let secondViewController = self.storyboard?.instantiateViewControllerWithIdentifier("SecondViewControllerIdentifier") as? SecondViewController {
// Set the chosen friend
secondViewController.friendChosen = friendChosen
self.presentViewController(secondViewController, animated: true, completion: nil)
}
}
And in the SecondViewController add the following property.
var friendChosen: String!
Please make sure the value is not nil prior to passing it to the destination view controller
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 {
}