Obj-C Sending Messages Between Classes

Question

I'm a newbie in iPhone Programming. I'm trying to send a message from one view controller to another. The idea is that viewControllerA takes information from the user and sends it to viewControllerB. viewControllerB is then supposed to display the information in a label.

viewControllerA.h

#import <UIKit/UIKit.h>
@interface viewControllerA : UIViewController
{
    int num;
}

-(IBAction)do;
@end

viewControllerA.m

#import "viewControllerA.h"
#import "viewControllerB.h"

@implementation viewControllerA

- (IBAction)do {
    //initializing int for example
    num = 2;
    viewControllerB *viewB = [[viewControllerB alloc] init];
    [viewB display:num];
    [viewB release];
    //viewA is presented as a ModalViewController, so it dismisses itself to return to the 
    //original view, i know it is not efficient 
    [self dismissModalViewControllerAnimated:YES];
}

- (void)dealloc {
    [super dealloc];
}

@end

viewControllerB.h

#import <UIKit/UIKit.h>

@interface viewControllerB : UIViewController
{
    IBOutlet UILabel *label;
}

- (IBAction)openA;
- (void)display:(NSInteger)myNum;

@end

viewControllerB.m

#import "viewControllerB.h"
#import "viewControllerA.h"

@implementation viewControllerB

- (IBAction)openA {
    //presents viewControllerA when a button is pressed
    viewControllerA *viewA = [[viewControllerA alloc] init];
    [self presentModalViewController:viewA animated:YES];
}

- (void)display:(NSInteger)myNum {
    NSLog(@"YES");
    [label setText:[NSString stringWithFormat:@"%d", myNum]];
}

@end

YES is logged successfully, but the label's text does not change. I have made sure that all of my connections in Interface Builder are correct, in fact there are other (IBAction) methods in my program that change the text of this very label, and all of those other methods work perfectly...

Any ideas, guys? You don't need to give me a full solution, any bits of information will help. Thanks.

Just to let you know that IBAction methods take a single (id)sender parameter. — Abizern
ps. Not your question really, but also watch out for memory leaks. Any alloc (or copy, new, retain) should be followed by a release or autorelease. — mvds
@Abizern They're not required to. They can match any of the forms specified in developer.apple.com/library/ios/documentation/UIKit/Reference/… — Justin Spahr-Summers

mvds mvds · Accepted Answer · 2011-01-09T19:49:06

With

viewControllerB *viewB = [[viewControllerB alloc] init];

you are instantiating a new viewControllerB. This is not the viewControllerB that (I presume) you have in your nib file. You should make an outlet for that and wire it up.

Otherwise, possibly instantiate it with [... initWithNibName:] from a nib, instead of just [... init], and then (either way) push the instantiated view controller using [self.navigationController pushViewController:viewB animated:YES], or by presenting it modally as you seem to have mastered already.

As a sidenote, maybe name the viewcontroller variable viewConB, since there is a clear and important distinction between views and view controllers. Furthermore, class names tend to start with upper case, and variables with lower case, to keep things clear.

Obj-C Sending Messages Between Classes

1 Answers