In Swift using MVC a common way to send data forward is to use prepare(for:sender:). Inside that method you get a reference to the destination VC and access its properties to send the data. But isn't that considered coupling the view controllers? I'm hoping the answer isn't considered a matter of opinion because I'd really like to understand how segues fit into MVC.
Passing data in a segue is completely compatible with MVC. The destinationVC is considered as a View of the sourceVC. When a controller communicates with a View, it configures the View with the data it needs. Writing to the public interface (properties) of the destinationVC is how you set it up. This is what is happening in prepare(for segue:sender).
The concern about coupling relates to reuse. The more tightly coupled the viewControllers are, the harder it is to reuse them. This is only a problem if the destinationVC knows details of the sourceVC. If the destinationVC needs to pass data back to the sourceVC, it should do so using delegation (where a protocol is used to define the methods that the sourceVC implements).
If view controller A segues to view controller B and assumes it is of type ViewControllerB then yes, that is tight coupling:
prepare(for: segue, sender: sender) {
if let viewControllerB = segue.destination as? ViewControllerB {
viewControllerB.property = value
}
}
That code only works if the destination is a specific class, ViewControllerB, and the first view controller has to know the properties of ViewControllerB.
That's not usually a problem because usually when you segue to another view controller you know what you've asked for, and hence what to expect.
However, you might use the same prepare(for:sender:) method to segue to any of several different types of view controllers that have common properties.
In that case you can use protocols to make the coupling looser:
Approach 2: Loosely coupled using a protocol
protocol DestProtocol {
var property: String
}
You might have a ViewControllerB that conforms to DestProtocol
class ViewControllerB: UIViewController, DestProtocol {
var property: string
//The rest of ViewControllerB
}
And you might also have a ViewControllerC that conforms to DestProtocol
class ViewControllerC: UIViewController, DestProtocol {
var property: string
//The rest of ViewControllerC
}
Then in the first view controller's prepare(for:sender:):
prepare(for: segue, sender: sender) {
if let destination = segue.destination as? DestProtocol {
destination.property = value
}
}
With the second approach the first view controller's prepare(for:sender) doesn't know that the destination is an instance of ViewControllerB. It just checks to see if the destination conforms to the DestProtocol. The segue could be loading an instance of ViewControllerB or ViewControllerC.
Related
I have a UIViewController inside a container view on my main view controller. I'm trying to un-hide the child view controller and perform a function on the child view but I don't know how to properly access it.
I've tried this:
let ChildVC = children.first as! ChildView
ChildVC.RunFunction()
But this does not work as I have multiple child views.
let ChildVC = storyboard?.instantiateViewController(withIdentifier: "Child") as! ChildView
ChildVC.RunFunction()
I've then tried this, but when the RunFunction() function is executed, it throws a lot of errors saying the things inside it are nil. (The error is not with the run function as it works fine when accessing via the children.first method)
How do I specify the exact childViewController without using the children[1] (or what ever number the childViewController is) method?
Any help would be much appreciated!
Since you mention a "container view" I am assuming you invoke the child view controller using an embed segue.
Assuming that's the case you should implement a prepare(for:sender) method that saves a pointer to the child:
Give the parent view controller an instance variable:
var childView: ChildView?
And then in your prepare(for:sender) method
func prepare(for segue: NSStoryboardSegue, sender: Any?) {
if let dest = segue.destination as? ChildView {
childView = dest
}
}
Then when you need to send messages to the childView:
childView?.runFunction()
(Method names should start with lower-case letters)
I am new to Swift and Xcode. I am building an Financial Expense ios app.
In my first view controller, I created a referencing outlet for a label called expenseNum.
In my second view controller, I have a function for a button called Add Expense. When it is clicked, I need it to update the expenseNum variable with the amount of the expense.
What is the best way to go about this? I had created an object of the first view controller class and accessed it like "firstviewcontroller.expenseNum" but this will create a new instance of the class and I need it to be all the same instance so it can continuously add to the same variable. Thanks for the help!
You need a delegate
protocol SendManager {
func send(str:String)
}
In first
class FirstVc:UIViewcontroller , SendManager {
func send(str:string) {
self.expenseNum.text = str
}
}
when you present SecondVc
let sec = SecondVc()
sec.delegate = self
// present
In second
class SecondVc:UIViewcontroller {
var delegate:SendManager?
#IBAction func btnClicked(_ sender:UIButton) {
delegate?.send(str:"value")
}
}
// setting delegate
in viewDidLoad of SecondVc
if let first = self.tabBarController.viewControllers[0] as? FirstVc {
self.delegate = first
}
There are several ways you can pass data from ViewController2 to another ViewController1
The best way here is Protocol Delegates
Please follow below steps to pass data
In Your SecondViewController from where you want to send data back declare a protocol at the top of class declaration
protocol SendDataBack: class {
func sendDataFromSecondVCtoFirstVC(myValue: String)
}
Now in the class , declare a object of your protocol in same ViewController
weak var myDelegateObj: SendDataBack?
And now in your Add Expense button action just call the delegate method
myDelegateObj?.sendDataFromSecondVCtoFirstVC(myValue: yourValue)
Now go to your first ViewController
the place from where you have pushed/present to SecondViewController you must have taken the object of SecondVC to push to push from first
if let secondVC = (UIStoryboard.init(name: "Main", bundle: nil)).instantiateViewController(withIdentifier: "secondVCID") as? SecondViewController {
vc?.myDelegateObj = self
self.navigationController?.pushViewController(secondVC, animated: true)
**OR**
self.present(secondVC, animated: true, completion: nil)
}
now in your FirstViewController make an extension of FirstViewVC
extension FirstViewController: SendDataBack {
func sendDataFromSecondVCtoFirstVC(myValue: String) {
}
}
I think you can make a variable in your properties in second ViewController (before viewDidLoad method)
var delegate: FirstViewController? = nil
and use from the properties of the first view controller anywhere of the second view controller.
delegate!.mainTableView.alpha=1.0
//for example access to a tableView in first view controller
The simplest way to achieve this is to use a public var. Add a new Swift file to your project, call it Globals. Declare the public variable in Globals.swift like so:
public var theValue: Int = 0
Set its required value in the first ViewController, and you'll find you can read it in the second with ease.
I'm trying to pass data from a SecondViewController to my FirstViewController when I click on my back button (UINaviagtionController).
For pass my data from FirstViewController to the SecondViewController I do this:
if segue.identifier == "showSecondVC" {
let vc = segue.destination as! SecondViewController
vc.rows = rows[pathForAVC]
vc.lap = lapTime[pathForAVC]
vc.indexPath = pathForAVC
}
But I have no idea how to pass data from SecondViewController to the FirstViewController and I really don't understand topics about it on Stack Overflow.
I want to transfer it when I click here:
Thanks.
You can use delegate pattern for that. You can grab the back button press event like this and update the data
override func viewWillDisappear(_ animated: Bool) {
if self.isMovingFromParentViewController {
self.delegate.updateData( data)
}
}
For more information on delegates you can go through this.
Actually things depend on your requirement, if you want data to be updated in first view controller as soon as it is updated in second view controller, you would need to call delegate as soon as the data is updated. But as in the question you have mentioned that you want it to be updated on back button only, above is the place to do it.
Another way would be to have Datasource as singleton so that it is available to all the view controllers and the changes are reflected in all view controllers. But create singleton if absolutely necessary, because these nasty guys hang around for entire time your application is running.
You should have a custom protocol such as:
public protocol SendDataDelegate: class {
func sendData(_ dataArray:[String])
}
Here I suppose you want to send a single array back to FirstViewController
Then make your first view controller to conform to the custom protocol, such as:
class FirstViewController: UIViewController, SendDataDelegate
In the second view controller, create a delegate a variable for that protocol, such as:
weak var delegate: SendDataDelegate?
and then you catch the back action and inside it you call your custom protocol function, such as:
self.delegate?.sendData(arrayToSend)
In the first viewController, in the prepare for segue function just set the delegate like
vc.delegate = self
I'm having trouble wrapping my head around delegation in Swift. After reading some guides, I was able to set it up delegation between two ViewControllers, but I'm not understanding how it works. In my first view controller, I have a a label that displays what has been entered in the second view controller which contains a text field and a button (that returns to the first view controller). Here is the code for the first view controller:
#IBOutlet weak var labelText: UILabel!
func userDidEnterInformation(info: String) {
labelText.text = info;
}
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if(segue.identifier == "transition"){
let secondVC: SecondViewController = segue.destinationViewController as! SecondViewController;
secondVC.delegate = self;
}
}
Here's the code for the second view controller:
protocol DataEnteredDelegate{
func userDidEnterInformation(info: String);
}
#IBOutlet weak var userText: UITextField!
var delegate: DataEnteredDelegate? = nil;
#IBAction func buttonPressed(sender: AnyObject) {
let information = userText.text!;
delegate!.userDidEnterInformation(information);
self.navigationController?.popToRootViewControllerAnimated(true);
}
My understanding is that in the text inside the text field gets stored in the information constant, then the userDidEnterInformation method from the protocol is called, with the method being defined inside the first view controller. This method then changes the label inside the first view controller. The thing is, I'm not sure what is happening in the prepareForSegue function. Specifically, I'm not sure what's the purpose of secondVC.delegate = self.
I would appreciate any sort of clarity on delegation.
The diagram is simple but can help you understand what's going on.
FirstViewController must conform to the DataEnteredDelegate protocol you have defined (see Sumit's answer). When using secondVC.delegate = self, you are saying that for the segue transition with the destination being a SecondViewController, the attribute delegate of that SecondViewController instance will be set to this instance of FirstViewController, thus delegating things from SecondViewController to your FirstViewController as made possible by the DataEnteredDelegate protocol.
The protocol you created in second viewcontroller is an Interface. You must implement your first view controller with the DataEnteredDelegate protocol.
class FirstViewController:UIViewController, DataEnteredDelegate{
func userDidEnterInformation(info: String) {
//stub
}
}
If the delegate of the second VC is not set in prepareForSegue() it remains nil. The second VC is then unable to call the first VC.
On a side note, if the delegate is nil your code will crash because delegate! is trying to unwrap an optional binding with the value of nil. It's better to first unwrap the delegate variable:
if let handler = delegate {
handler.userDidEnterInformation(information)
}
Alternatively, you could use Swift's Optional Chaining, calling userDidEnterInformation only if delegate is not nil.
delegate?.userDidEnterInformation(information);
In addition it is recommended to declare the delegate weak, to prevent retain cycles:
weak var delegate: DataEnteredDelegate?
Delegates and Protocols
Do not try to figure out how the dictionary definition of “delegate” fits with the concept of delegation in Swift. It doesn't.
Delegation in Swift is an agreement between two players—a sensing object and a requesting object. The “delegate” is the “requesting object.” Just think “asker” or “requester” every time you see “delegate” and it will make a lot more sense. Here is their agreement...
The Sensing Object (Second View Controller):
I have data from some event that took place. I will publish instructions (a protocol) on how you may access that data. If you want it, you must do three things.
You must declare in your class type that your class abides by my protocol.
You must write the functions that I describe in my protocol. I don't care what those functions do but the function type must match what I publish.
In YOUR code, you must set MY “delegate” (think “asker”) property to point to you. {secondVC.delegate = self} That way I can call one of YOUR functions to deliver the data.
After that, when I get some data, I will call one of the functions in your object that I told you to write. My call to your function will contain the data you are looking for as one of the arguments. {delegate!.userDidEnterInformation(information)} Note: delegate! (asker!) is YOU.
The Delegate (Requesting) Object (First View Controller):
O.K. You've got a deal.
I have an optional bool variable called showSettings on my first view controller which is called ViewController, and I'm popping from SecondViewController back to ViewController.
Before I pop, I want to set the bool to true. Seems wrong to instantiate another view controller since ViewController is in memory.
What's the best way to do this? I'm not using storyboards, if that's important for your answer.
Thanks for your help
So I figured it out, based mostly from this post – http://makeapppie.com/2014/09/15/swift-swift-programmatic-navigation-view-controllers-in-swift/
In SecondViewController, above the class declaration, add this code:
protocol SecondVCDelegate {
func didFinishSecondVC(controller: SecondViewController)
}
Then inside of SecondViewContoller add a class variable:
var delegate: MeditationVCDelegate! = nil
Then inside of your function that your button targets, add this:
self.navigationController?.popViewControllerAnimated(true)
delegate.didFinishSecondVC(self)
What we're doing here is doing the pop in SecondViewController, and not passing any data, but since we've defined a protocol, we're going to use that in ViewController to handle the data.
So next, in ViewController, add the protocol you defined in SecondViewController to the list of classes ViewController inherits from:
class ViewController: UIViewController, SecondVCDelegate { ... your code... }
You'll need to add the function we defined in the new protocol in order to make the compiler happy. Inside of ViewController's class, add this:
func didFinishSecondVC(controller: SecondViewController) {
self.myBoolVar = true
controller.navigationController?.popViewControllerAnimated(true)
}
In SecondViewController where we're calling didFinishSecondVC, we're calling this method inside of the ViewController class, the controller we're popping to. It's similar to if we wrote this code inside of SecondViewController but we've written it inside of ViewController and we're using a delegate to manage the messaging between the two.
Finally, in ViewController, in the function we're targeting to push to SecondViewController, add this code:
let secondVC = secondViewController()
secondVC.delegate = self
self.navigationController?.pushViewController(secondVC, animated: true)
That's it! You should be all set to pass code between two view controllers without using storyboards!
_ = self.navigationController?.popViewController(animated: true)
let previousViewController = self.navigationController?.viewControllers.last as! PreviousViewController
previousViewController.PropertyOrMethod
I came across this while looking for a way to do it. Since I use Storyboards more often, I found that I can get the array of controllers in the navigation stack, get the one just before the current one that's on top, check to see if it's my delegate, and if so, cast it as the delegate, set my methods, then pop myself from the stack. Although the code is in ObjC, it should be easily translatable to swift:
// we need to get the previous view controller
NSArray *array = self.navigationController.viewControllers;
if ( array.count > 1) {
UIViewController *controller = [array objectAtIndex:(array.count - 2)];
if ( [controller conformsToProtocol:#protocol(GenreSelectionDelegate)]) {
id<GenreSelectionDelegate> genreDelegate = (id<GenreSelectionDelegate>)controller;
[genreDelegate setGenre:_selectedGenre];
}
[self.navigationController popViewControllerAnimated:YES];
}
Expanding upon the answer by Abdul Baseer Khan:
For cases where the current view controller may have been loaded by different types of previous view controller, we can use the safer as? call instead of as!, which will return nil if the controller is not what we were looking for:
let previousVC = self.navigationController?.viewControllers.last as? AnExampleController
previousVC?.doSomething()
Although, you would need to repeat that for each different view controller that could load the current view controller.
So, you may want to, instead, implement a protocol to be assigned to all the possible previous view controllers:
protocol PreviousController: UIViewController {
func doSomething()
}
class AnExampleController: UIViewController, PreviousController {
// ...
func doSomething() {}
}
class AnotherController: UIViewController, PreviousController {
// ...
func doSomething() {}
}
class CurrentController: UIViewController {
// ...
func goBack() {
let previousVC = self.navigationController?.viewControllers.last as? PreviousController
previousVC?.doSomething()
}
}