Question

I have two view controllers, firstViewController and secondViewController. I am using this code to switch to my secondViewController (I am also passing a string to it):

secondViewController *second = [[secondViewController alloc] initWithNibName:nil bundle:nil];

second.myString = @"This text is passed from firstViewController!";

second.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;

[self presentModalViewController:second animated:YES];

[second release];

I then use this code in secondViewController to switch back to the firstViewController:

[self dismissModalViewControllerAnimated:YES];

All of this works fine. My question is, how would I pass data to the firstViewController? I would like to pass a different string into the firstViewController from the secondViewController.

Answer 1

You need to use delegate protocols… Here’s how to do it:

Declare a protocol in your secondViewController’s header file. It should look like this:

#import <UIKit/UIKit.h>

@protocol SecondDelegate <NSObject>
-(void)secondViewControllerDismissed:(NSString *)stringForFirst
@end


@interface SecondViewController : UIViewController
{
    id myDelegate;  
}

@property (nonatomic, assign) id<SecondDelegate>    myDelegate;

Don’t forget to synthesize the myDelegate in your implementation (SecondViewController.m) file:

@synthesize myDelegate;

In your FirstViewController’s header file subscribe to the SecondDelegate protocol by doing this:

#import "SecondViewController.h"

@interface FirstViewController:UIViewController <SecondDelegate>

Now when you instantiate SecondViewController in FirstViewController you should do the following:

// If you're using a view controller built with Interface Builder.
SecondViewController *second = [[SecondViewController alloc] initWithNibName:"SecondViewController" bundle:[NSBundle mainBundle]];
// If you're using a view controller built programmatically.
SecondViewController *second = [SecondViewController new]; // Convenience initializer that uses alloc] init]
second.myString = @"This text is passed from firstViewController!";
second.myDelegate = self;
second.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
[self presentModalViewController:second animated:YES];
[second release];

Lastly, in the implementation file for your first view controller (FirstViewController.m) implement the SecondDelegate’s method for secondViewControllerDismissed:

- (void)secondViewControllerDismissed:(NSString *)stringForFirst
{
    NSString *thisIsTheDesiredString = stringForFirst; //And there you have it.....
}

Now when you’re about to dismiss the second view controller you want to invoke the method implemented in the first view controller. This part is simple. All you do is, in your second view controller, add some code before the dismiss code:

if([self.myDelegate respondsToSelector:@selector(secondViewControllerDismissed:)])
{
    [self.myDelegate secondViewControllerDismissed:@"THIS IS THE STRING TO SEND!!!"];
}
[self dismissModalViewControllerAnimated:YES];

Delegate protocols are EXTREMELY, EXTREMELY, EXTREMELY useful. It would do you good to familiarize yourself with them 🙂

NSNotifications are another way to do this, but as a best practice, I prefer using it when I want to communicate across multiple viewControllers or objects. Here’s an answer I posted earlier if you’re curious about using NSNotifications: Firing events accross multiple viewcontrollers from a thread in the appdelegate

EDIT:

If you want to pass multiple arguments, the code before dismiss looks like this:

if([self.myDelegate respondsToSelector:@selector(secondViewControllerDismissed:argument2:argument3:)])
{
    [self.myDelegate secondViewControllerDismissed:@"THIS IS THE STRING TO SEND!!!" argument2:someObject argument3:anotherObject];
}
[self dismissModalViewControllerAnimated:YES];

This means that your SecondDelegate method implementation inside your firstViewController will now look like:

- (void) secondViewControllerDismissed:(NSString*)stringForFirst argument2:(NSObject*)inObject1 argument3:(NSObject*)inObject2
{
    NSString thisIsTheDesiredString = stringForFirst;
    NSObject desiredObject1 = inObject1;
    //....and so on
}