Passing data between view controllers
This question seems to be very popular here on Stack Overflow so I thought I would try and give a better answer to help out people starting in the world of iOS like me.
Passing Data Forward
Passing data forward to a view controller from another view controller. You would use this method if you wanted to pass an object/value from one view controller to another view controller that you may be pushing on to a navigation stack.
For this example, we will have ViewControllerA
and ViewControllerB
To pass a BOOL
value from ViewControllerA
to ViewControllerB
we would do the following.
in
ViewControllerB.h
create a property for theBOOL
@property (nonatomic, assign) BOOL isSomethingEnabled;
in
ViewControllerA
you need to tell it aboutViewControllerB
so use an#import "ViewControllerB.h"
Then where you want to load the view, for example, didSelectRowAtIndex
or some IBAction
, you need to set the property in ViewControllerB
before you push it onto the navigation stack.
ViewControllerB *viewControllerB = [[ViewControllerB alloc] initWithNib:@"ViewControllerB" bundle:nil];
viewControllerB.isSomethingEnabled = YES;
[self pushViewController:viewControllerB animated:YES];
This will set isSomethingEnabled
in ViewControllerB
to BOOL
value YES
.
Passing Data Forward using Segues
If you are using Storyboards you are most likely using segues and will need this procedure to pass data forward. This is similar to the above but instead of passing the data before you push the view controller, you use a method called
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
So to pass a BOOL
from ViewControllerA
to ViewControllerB
we would do the following:
in
ViewControllerB.h
create a property for theBOOL
@property (nonatomic, assign) BOOL isSomethingEnabled;
in
ViewControllerA
you need to tell it aboutViewControllerB
, so use an#import "ViewControllerB.h"
Create the segue from
ViewControllerA
toViewControllerB
on the storyboard and give it an identifier. In this example we'll call it"showDetailSegue"
Next, we need to add the method to
ViewControllerA
that is called when any segue is performed. Because of this we need to detect which segue was called and then do something. In our example, we will check for"showDetailSegue"
and if that's performed, we will pass ourBOOL
value toViewControllerB
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{ if([segue.identifier isEqualToString:@"showDetailSegue"]){ ViewControllerB *controller = (ViewControllerB *)segue.destinationViewController; controller.isSomethingEnabled = YES; } }
If you have your views embedded in a navigation controller, you need to change the method above slightly to the following
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
if([segue.identifier isEqualToString:@"showDetailSegue"]){
UINavigationController *navController = (UINavigationController *)segue.destinationViewController;
ViewControllerB *controller = (ViewControllerB *)navController.topViewController;
controller.isSomethingEnabled = YES;
}
}
This will set isSomethingEnabled
in ViewControllerB
to BOOL
value YES
.
Passing Data Back
To pass data back from ViewControllerB
to ViewControllerA
you need to use Protocols and Delegates or Blocks, the latter can be used as a loosely coupled mechanism for callbacks.
To do this we will make ViewControllerA
a delegate of ViewControllerB
. This allows ViewControllerB
to send a message back to ViewControllerA
enabling us to send data back.
For ViewControllerA
to be a delegate of ViewControllerB
it must conform to ViewControllerB
's protocol which we have to specify. This tells ViewControllerA
which methods it must implement.
In
ViewControllerB.h
, below the#import
, but above@interface
you specify the protocol.@class ViewControllerB; @protocol ViewControllerBDelegate <NSObject> - (void)addItemViewController:(ViewControllerB *)controller didFinishEnteringItem:(NSString *)item; @end
Next still in the
ViewControllerB.h
, you need to set up adelegate
property and synthesize inViewControllerB.m
@property (nonatomic, weak) id <ViewControllerBDelegate> delegate;
In
ViewControllerB
we call a message on thedelegate
when we pop the view controller.NSString *itemToPassBack = @"Pass this value back to ViewControllerA"; [self.delegate addItemViewController:self didFinishEnteringItem:itemToPassBack];
That's it for
ViewControllerB
. Now inViewControllerA.h
, tellViewControllerA
to importViewControllerB
and conform to its protocol.#import "ViewControllerB.h" @interface ViewControllerA : UIViewController <ViewControllerBDelegate>
In
ViewControllerA.m
implement the following method from our protocol- (void)addItemViewController:(ViewControllerB *)controller didFinishEnteringItem:(NSString *)item { NSLog(@"This was returned from ViewControllerB %@", item); }
Before pushing
viewControllerB
to navigation stack we need to tellViewControllerB
thatViewControllerA
is its delegate, otherwise we will get an error.ViewControllerB *viewControllerB = [[ViewControllerB alloc] initWithNib:@"ViewControllerB" bundle:nil]; viewControllerB.delegate = self [[self navigationController] pushViewController:viewControllerB animated:YES];
References
- Using Delegation to Communicate With Other View Controllers in the View Controller Programming Guide
- Delegate Pattern
NSNotification center
It's another way to pass data.
// Add an observer in controller(s) where you want to receive data
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleDeepLinking:) name:@"handleDeepLinking" object:nil];
-(void) handleDeepLinking:(NSNotification *) notification {
id someObject = notification.object // Some custom object that was passed with notification fire.
}
// Post notification
id someObject;
[NSNotificationCenter.defaultCenter postNotificationName:@"handleDeepLinking" object:someObject];
Passing Data back from one class to another (A class can be any controller, Network/session manager, UIView subclass or any other class)
Blocks are anonymous functions.
This example passes data from Controller B to Controller A
Define a block
@property void(^selectedVoucherBlock)(NSString *); // in ContollerA.h
Add block handler (listener)
Where you need a value (for example, you need your API response in ControllerA or you need ContorllerB data on A)
// In ContollerA.m
- (void)viewDidLoad {
[super viewDidLoad];
__unsafe_unretained typeof(self) weakSelf = self;
self.selectedVoucherBlock = ^(NSString *voucher) {
weakSelf->someLabel.text = voucher;
};
}
Go to Controller B
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
ControllerB *vc = [storyboard instantiateViewControllerWithIdentifier:@"ControllerB"];
vc.sourceVC = self;
[self.navigationController pushViewController:vc animated:NO];
Fire block
-(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:
(NSIndexPath *)indexPath {
NSString *voucher = vouchersArray[indexPath.row];
if (sourceVC.selectVoucherBlock) {
sourceVC.selectVoucherBlock(voucher);
}
[self.navigationController popToViewController:sourceVC animated:YES];
}
Another Working Example for Blocks
Swift
There are tons and tons of explanations here and around Stack Overflow, but if you are a beginner just trying to get something basic to work, try watching this YouTube tutorial (It's what helped me to finally understand how to do it).
- YouTube tutorial: How to send data through segue (Swift)
Passing data forward to the next View Controller
The following is an example based on the video. The idea is to pass a string from the text field in the First View Controller to the label in the Second View Controller.
Create the storyboard layout in the Interface Builder. To make the segue, you just Control click on the button and drag over to the Second View Controller.
First View Controller
The code for the First View Controller is
import UIKit
class FirstViewController: UIViewController {
@IBOutlet weak var textField: UITextField!
// This function is called before the segue
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
// Get a reference to the second view controller
let secondViewController = segue.destination as! SecondViewController
// Set a variable in the second view controller with the String to pass
secondViewController.receivedString = textField.text!
}
}
Second View Controller
And the code for the Second View Controller is
import UIKit
class SecondViewController: UIViewController {
@IBOutlet weak var label: UILabel!
// This variable will hold the data being passed from the First View Controller
var receivedString = ""
override func viewDidLoad() {
super.viewDidLoad()
// Used the text from the First View Controller to set the label
label.text = receivedString
}
}
Don't forget
- Hook up the outlets for the
UITextField
and theUILabel
. - Set the first and second View Controllers to the appropriate Swift files in Interface Builder.
Passing data back to the previous View Controller
To pass data back from the second view controller to the first view controller, you use a protocol and a delegate. This video is a very clear walk though of that process:
- YouTube tutorial: iOS Swift Basics Tutorial: Protocols and Delegates But also read this post to make sure you don't get into a strong reference cycle.
The following is an example based on the video (with a few modifications).
Create the storyboard layout in the Interface Builder. Again, to make the segue, you just Control drag from the button to the Second View Controller. Set the segue identifier to showSecondViewController
. Also, don't forget to hook up the outlets and actions using the names in the following code.
First View Controller
The code for the First View Controller is
import UIKit
class FirstViewController: UIViewController, DataEnteredDelegate {
@IBOutlet weak var label: UILabel!
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "showSecondViewController" {
let secondViewController = segue.destination as! SecondViewController
secondViewController.delegate = self
}
}
func userDidEnterInformation(info: String) {
label.text = info
}
}
Note the use of our custom DataEnteredDelegate
protocol.
Second View Controller and Protocol
The code for the second view controller is
import UIKit
// Protocol used for sending data back
protocol DataEnteredDelegate: AnyObject {
func userDidEnterInformation(info: String)
}
class SecondViewController: UIViewController {
// Making this a weak variable, so that it won't create a strong reference cycle
weak var delegate: DataEnteredDelegate? = nil
@IBOutlet weak var textField: UITextField!
@IBAction func sendTextBackButton(sender: AnyObject) {
// Call this method on whichever class implements our delegate protocol
delegate?.userDidEnterInformation(info: textField.text!)
// Go back to the previous view controller
_ = self.navigationController?.popViewController(animated: true)
}
}
Note that the protocol
is outside of the View Controller class.
That's it. Running the app now, you should be able to send data back from the second view controller to the first.