Xcode: Does the delegating object HAVE to send a message to the delegate object?

I'm trying to assign SecondViewController as a delegate object of FirstViewController (if I understand correctly). However FirstViewController doesn't send any messages to SecondViewController.

Am I allowed to pretend as though SecondViewController did get a message from FirstViewController and respond to the FirstViewController? (Note: My SecondViewController is in charge of a view that has a button. When I press开发者_如何学编程 the button on my SecondViewController's view I want it to tell the FirstViewController to update its view)

FirstViewController.h

#import <UIKit/UIKit.h>

@protocol FirstViewControllerDelegate <NSObject>
@optional
- (void) setAnotherLabel;
@end

@interface FirstViewController : UIViewController {
    IBOutlet UILabel *label;
    id <FirstViewControllerDelegate> delegate;
}
@property (nonatomic, retain) IBOutlet UILabel *label;
@property (nonatomic, assign) id <FirstViewControllerDelegate> delegate;
- (void) pretendLabel;
- (void) realLabel;
@end

FirstViewController.m

#import "FirstViewController.h"


@implementation FirstViewController
@synthesize label;
@synthesize delegate;


// Implement viewDidLoad to do additional setup after loading the view, typically from a nib.
- (void) setAnotherLabel;
{
    label.text =@"Real";
    [self.view setNeedsDisplay];
}

- (void) pretendLabel;
{
    label.text =@"Pretend";
    [self.view setNeedsDisplay];
}

- (void) realLabel;
{
    [self setAnotherLabel];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    label.text=@"Load";
    [self pretendLabel];
}
...
@end

SecondViewController.h

#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#import "FirstViewController.h"

@interface SecondViewController : UIViewController <UIImagePickerControllerDelegate, UINavigationControllerDelegate, FirstViewControllerDelegate>
{
    UIImage *image;
    IBOutlet UIImageView *imageView;
}

- (IBAction) sendPressed:(UIButton *)sender;
- (IBAction) cancelPressed:(UIButton *)sender;
@end

SecondViewController.m

#import "SecondViewController.h"

@implementation SecondViewController

- (IBAction) sendPressed:(UIButton *)sender
{
    FirstViewController *fvc = [[FirstViewController alloc] init];
    [fvc setDelegate:self];
    //how do I find out if I'm actually the delegate for FirstViewController at this point?
    [fvc realLabel];
    self.tabBarController.selectedIndex = 0;//switch over to the first view to see if it worked
}

There are a few issues with this and what appears to be a bit of confusion.

I assume that FirstViewController and SecondViewController are in separate tabs in the tab bar controller.

In the sendPressed: method, you're creating a new instance of FirstViewController - this is not the same FirstViewController that is in your tab bar controller and why calling realLabel has no effect.

The second point is that you appear to misunderstand how delegation works - there is no reason for a delegate in the code you posted.

Good read for getting to grips with delegates: http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/CocoaFundamentals/CommunicatingWithObjects/CommunicateWithObjects.html

As for a solution to your problem there are a few options:

1. Post a notification from SecondViewController that FirstViewController is observing (lots of information available on the net regarding notifications).
2. Get the specific instance of FirstViewController within the self.tabBarController.viewControllers array and call the method from there. Something like...

- (IBAction) sendPressed:(UIButton *)sender
{
    for(UIViewController *controller in self.tabBarController.viewControllers)
    {
        if([controller isKindOfClass:[FirstViewController class]])
        {
            FirstViewController *firstViewController = (FirstViewController *)controller;
            [firstViewController realLabel];
        }
    }

    self.tabBarController.selectedIndex = 0;//switch over to the first view to see if it worked
}

There are more options available than this, but the above will give you a good start into researching the best approach for your need.

Hope this helps.