In Screen A I collect data from the user and then display it on Screen B using a prepare for segue function. Then I noticed if the user goes to Screen C and then back to Screen B the data is gone. How do I keep the data there.
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var textInput: UITextField!
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
let a : BViewController = segue.destination as! BViewController
a.honey = textInput.text!
}
}
import UIKit
class BViewController: UIViewController {
#IBOutlet weak var x: UILabel!
var honey:String = "default"
override func viewDidLoad() {
super.viewDidLoad()
x.text = honey
}
}
Then I noticed if the user goes to Screen C and then back to Screen B the data is gone. How do I keep the data there.
I see two problems:
It looks like you're not actually going back to the "screen B" that you left. The storyboard shows a push segue from B to C, and another from C to B. So I think what's happening is that you're creating another Screen B, separate from the first one. You should look into unwinding the push segue instead of adding another one. Remember: the screens you create in a storyboard aren't individual objects -- they're more like patterns or prototypes of objects, and they can be instantiated multiple times.
More generally, it's a good idea to avoid storing your program's data in views or even in view controllers. The M in MVC stands for model, which is meant to be a separate object (or group of objects) that manage the data and business logic for your app. Using a model wouldn't solve problem #1, but it'll still help you avoid a lot of "hey, where did my data go?" issues.
This is a common problem.
You need to pass the data into each new VC when you instantiate it. The data you want to save can be an instance variable in each VC.
Or you need to save the data you want to persist into NSUserDefaults or use Core Data. If you go this route, once you save the data into one of these places, you can access it anytime from anywhere.
Related
I have login few screens and controllers in my app. First screen is screen with button and moves user to next login view with username, password field and login button. On the controller i have function onClickButton and when i have good data i request to the server with this data.
When server give me callback i have many params about user to set in label in next view.
My structure is like this
Login View -> SecondLogin View and LoginViewController -> TabBarController -> NavigationController -> Table View with TableViewController
My code is
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "afterLoginView" {
if let secondVC = segue.destination as? TabBarViewController {
secondVC.finalName = self.username
}
}
}
When i want transfer my data directly to tableViewController i have error
Thread 1: signal SIGABRT
I do not understand what I'm doing wrong
You'll need these values in almost all view controllers. Create a singleton class to store the logged in user values like this
class UserDetails: NSObject, Codable {
static let shared = UserDetails()
private override init() {
super.init()
}
var finalName: String?
var otherDetails: String?
}
Now when you receive the response from the login api, assign the values in this singleton class.
UserDetails.shared.finalName = "something"//Name received from server callback
Now you can access these values from any view controller.
class TableViewController: UITableViewController {
override func viewDidLoad() {
super.viewDidLoad()
print(UserDetails.shared.finalName)
}
}
You have some work to do to get to the right view controller. Since your segue is only pointing at the UITabBarViewController, you should put in another guard or if/let statement to get you to the UINavigationController, and then another to finally get you to the UITableViewController, where you can actually refer to your finalName variable.
That would look something like:
if let secondVC = segue.destination as? TabBarViewController {
if let navCon = secondVC.viewController[0] as? UINavigationController {
if let tableVC = navCon.topViewController as? nameOfYourTableVC {
tableVC.finalName = self.username
}
The code is untested, just typed off the top of my head, so please proceed with due caution. Issues such as which tab is the correct NavController would also need to be addressed.
You need to use the actual name of your tableView class in that last if/let. A generic UITableViewController will not include your custom variables.
When server give me callback i have many params about user to set in label in next view.
This is a great example of why you should keep the M in MVC. When you get a response back from the server, store the returned data in your data model. (If you don't have a data model, you should make one.) When a view controller gets some data from the user, such as a user name, it should store that in the model. There's little reason to pass raw data back and forth between view controllers directly... just make sure that all your view controllers have a reference to the model, and have them get and set values there as needed.
This kind of approach will make your code a lot more flexible. It allows view controllers to worry about what they need to do their job, and it gets them out of the business of caring what other view controllers need.
My structure is like this
Login View -> SecondLogin View and LoginViewController -> TabBarController -> NavigationController -> Table View with TableViewController
It might make more sense to load the tab bar controller and then present the login view controller(s) modally. The view controllers that are managed by the tab bar controller can all be set up to refuse to do anything useful until the data they need is present in the data model, and that lets the tab bar controller be the root view controller. That will make it easy to set the model for each of it's child view controllers when the app starts up, and the app can then present the modal login view controllers, also set up with references to the model.
This question already has answers here:
Passing data between view controllers
(45 answers)
Closed 3 years ago.
I am currently working on an app and have problems with posting events.
I collect data in a sequence of several views (7 screens) and would like to store the data finally at once (or show all the data in the final view).
These data are, for example, location info, user info, event image, comments, event category, ...
I know how to store the data in the database/storage (firebase) if I collect the data in one or two views.
But in my use case I have seven views and I could not find any elegant method.
What's the best way to do that with Xcode 10?
You can use struct as below code. Make all required variable for all screen in this struct (like string, image etc..). And you can access this from any ViewController.
struct InputDetails {
static var details: InputDetails = InputDetails()
var city: String = ""
var lat: String = ""
var long: String = ""
}
Now to add value in this
InputDetails.details.city = textfiels.text
Now to access first screen value in last screen
print(InputDetails.details.city)
And once your API call or above struct usage is over, make sure to reset all details like below.
InputDetails.details = InputDetails()
There are several ways for passing data between View Controllers. For example you could use an instance property or a segue or the delegation method.
I recommend you study this article which paints a complete picture of the different methods and how to apply them:
How To: Pass Data Between View Controllers In Swift
Edit:
Upon examining the picture in your question I figured that using a segue would be the most appropriate solution here. As it seems from the picture you enter data in one View Controller, pass that onto the second View Controller and finally you upload all the data to Firebase.
I assume that you use storyboards (if not then consult the link above for other methods.) In this example below you will pass a string from one VC to another.
Step 1:
Add a segue between two view controllers. Storyboard -> press ctrl and click on VC one and drag your mouse -> you will see a blue arrow, drag that to VC two and release -> select manual segue: show -> click on the segue and give it an identifier
Step 2:
In VC two, make a string variable:
class SecondViewController: UIViewController {
var stringToPass: String = ""
override func viewDidLoad() {
super.viewDidLoad()
print(stringToPass)
}
Step 3:
In VC one, enter the following:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if let vc = segue.destination as? SecondViewController {
vc.stringToPass = "This is the string we pass between two VC"
}
}
Step 4:
Then whenever you want to go to the SecondViewController perform the segue like this:
performSegue(withIdentifier: "identifierYouEntered", sender: self)
Say I have view controllers A, B, C, D & E all embedded in a navigation controller. In view controller B, I have a custom UIImageView object. In C, I have a custom UITextfield object. Both custom classes have a reference to the view controller for various reasons such as I have to perform things like segue when a user taps the image view. To accomplish this, I have this inside each custom class file:
var controller: UIViewController?
And then inside each view controller, inside viewDidLoad I set that variable to self and everything works as expected (segues on tap etc..)
I have an unwind segue from E back to A. However, I noticed that due to these custom objects in view controllers B & C, both were not being deallocated due to a retain cycle caused by having this reference to the view controller. I fixed the issue by setting the controller variable to nil upon segue, however this creates a problem such that if the user goes back (pops the current view controller), because I set the controller variable to nil upon segue, nothing works (it wont segue again because controller var = nil). I thought I might fix this by adding viewWillAppear code as follows:
override func viewWillAppear(_ animated: Bool) {
usernameTextField.controller = self
passwordTextField.controller = self
}
Because I read that viewWillAppear will be called each time the viewcontroller comes into view. This did not fix the problem.
Any ideas on how to go about this? How can I set the controllers to nil during the unwind maybe...?
As the other answers have said you need to make it a weak reference like this:
weak var controller: UIViewControler?
However I would go further and say that you should not be keeping a reference to to a UIViewController inside any UIView based object (UIImageView, UITextField, etc). The UIViews should not need to know anything about their UIViewControllers.
Instead you should be using a delegation pattern. This is a basic example:
1) Create a protocol for the custom UIImageField like this:
protocol MyImageFieldProtocol: class {
func imageTapped()
}
2) Then add a delegate like this:
weak var delegate: MyImageFieldProtocol?
3) Your UIViewController then conforms to the protocol like this:
class MyViewController: UIViewController, MyImageFieldProtocol {
}
4) Somewhere inside the view controller (viewDidLoad is usually a good place you assign the view controller to the image views delegate like this:
func viewDidLoad {
super.viewDidLoad()
myImageView.delegate = self
}
5) Then add the function to respond to the protocol action to the view controller like this:
func imageTapped {
self.performSegue(withIdentifier: "MySegue", sender: nil)
}
var controller: UIViewController? should be a weak reference. Like this:
weak var controller: UIViewController?
To know more about that read about Resolving Strong Reference Cycles Between Class Instances in Swift's documentation.
You should use weak references when you keep some ViewControllers
weak var controller: UIviewControler?
You should check everything link to retain cycle, and referencing in swift :
https://krakendev.io/blog/weak-and-unowned-references-in-swift
https://medium.com/#chris_dus/strong-weak-unowned-reference-counting-in-swift-5813fa454f30
I had similar issues, I advice you to look at those link : How can I manage and free memory through ViewControllers
I have three views, each with its own view controller: VC1, VC2, VC3.
The user will frequently switch back and forth between each of the three views, both forward and backward.
Each view contains data: both shared from the previous view and data unique to that view.
When the user goes back to a View that he has already visited, the data displayed on that view needs to be retained (the same data as he saw the last time he visited that view), and not set to the default values the first time he visited the view.
In the first view controller, VC1, I am using a prepare for segue to push data from VC1 to VC2 or VC3:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "segueToVC2” {
let destinationViewController: VC2 = segue.destination as! VC2;
destinationViewController.passedData1 = firstAmount
destinationViewController.passedData2 = secondAmount
destinationViewController.passedData3 = thirdAmount
} else {
let destinationViewController: VC3 = segue.destination as! VC3;
destinationViewController.passedData1 = firstAmount
destinationViewController.passedData2 = secondAmount
destinationViewController.passedData3 = thirdAmount
destinationViewController.passedData4 = fourthAmount
}
By tapping the GO BACK button on each view, I return to the previous view:
#IBAction func goBackButtonPressed(_ sender: Any) {
print("Back Button Pressed!")
self.view.window?.rootViewController?.dismiss(animated: true, completion: nil)
}
I am having trouble passing data backwards. And when I return to VC2 from VC1, data has been reset to 0. I have no segues going back from VC2 to VC1 or from VC3 to VC2. Would that be the cleanest way to pass the data back: to create another segue in Main.storyboard from VC2 to VC1 and then add another ‘if’ to my prepare for segue that checks for VC1?
I am passing ALL these variables back and forth between view controllers but only using some of them in each view controller. It seems like a waste and I don't think I am on the right track here.
Any help or suggestions?
View controllers should never store data. They are responsible for coordinating between model objects and view objects. That's their whole point. The pattern you're looking for is called MVC (Model-View-Controller) and it's a core part of iOS development.
Move your data out of the view controllers and put it into model classes. Each view controller should fetch data out of the model, and send updates into the model. The only thing the view controllers should pass between themselves is what model objects to work on, and most of the time that only needs to pass in one direction (down the stack).
Delegation can be a useful tool here, and you can also investigate "unwind segues" which are built to help you send data upstream. But again, the data you should be sending is mostly references to the model, and the model itself needs to live outside the view controllers.
It's in Objective-C, but still one of the best simple examples from Apple on MVC design is TheElements, and is worth exploring as a basis. Even without reading the Objective-C, you can see how the various pieces fit together.
I haven't studied it as much as TheElements, but Lister claims to be a good demonstration of MVC patterns in Swift using modern iOS techniques.
Why don't you call a delegate which passes the data to the view controller when you press back button.
Or if the data shared by all view controllers reflect the same value. Make a singleton class and use those values across the app.
example singleton class:
class SomeModel {
static let shared = SomeModel()
private init() {}
}
When we move the a segue from A view to B view or from b view to A view, viewdidload get data from server all the time
So, can we keep the data that viewdidload get at first excute, without starting viewdidload each page?
It's waste of time to get data all the time, when we open each page.
From now we using pageviewcontroller, I think it is inappropriate it.
I using swift language.
If you have good idea, please let me know.
There are very many tutorials on the internet that show how to do what you want. Try a google search on swift pass data between view controllers.
At its very simplest, the second controller needs to have one or properties that can hold data from the first controller. The first controller must implement the following function:
override func prepareForSegue(segue: UIStoryboardSegue!, sender: AnyObject!) {
let secondVC = segue.destinationViewController as! ViewControllerClass
// Pass data to the second view controller.
secondVC.dataFromServer = dataFromServer
secondVC.otherData = otherData // etc.
}
The data will then be passed from the first controller to the second.