How can I access and change an object that is declared in another class?

In my mainViewController.h I have declared a UIButton *myButton and I synthesize it in mainViewController.m. I also created an 开发者_StackOverflowaccessor method (myButton) that just returns the button. In another view (and therefore a different view controller class) I have:

mainViewController *mainVC = [[mainViewController alloc] init];
UIButton *myButton = [mainVC myButton];
[eventButton setTitle:@"New Title" forState:UIControlStateNormal];

I want this code to change the title label of the button, but it doesn't seem to work. Using a NSLog statement (NSLog(@"%@", myButton.titleLabel);) to see what the current title is, it always returns null. From what I've read, this is because I'm creating an instance of the mainViewController and therefore I'm not actually accessing/changing the button. How can I access the actual view controller and not an instance?

(Before the longer discussion: it looks like you're typing myButton one line 2, and eventButton on line 3. Make sure these match!)

You're correct in thinking that you're creating a new instance of mainViewController. To get at the one you want, You'll either need to pass a reference to mainViewController into the new view controller when you create it, writing a method like:

@synthesize mainViewController;

-(void) initWithMainVC:(MainViewController *)mainVC {
    if ((self = [super init])) {
        self.mainViewController = mainVC;
    }
    return self;
}

to override init, or, if you've got a reference to mainViewController in your App Delegate, you can get it using:

MainViewController *mainVC = [(YourAppDelegate *)[[UIApplication sharedApplication] delegate] mainViewController];

Either way you choose, you'll want to read up on object oriented programming and pointers in objective C. When you call alloc, you're manufacturing a new copy of that class. All copies sit in different chunks of memory, so any new one you make (as in your code above) is actually a totally different entity, like two Priuses, or something like that.

A pointer is like a nametag that identifies, or points the way, to that block of memory. In my init code example above, we're attaching a new nametag to a mainViewController instance with the same guts. Same thing with the second line -- we make a new pointer (see the star?) tell it it's going to be pointing to a MainViewController instance, and slap that nametag, or pointer, on to an instance.

(The second one is a little more complicated, as you're plucking mainViewController from an object called a singleton, or an object that you can get at from anywhere... read more about those and the App Delegate at this post from Cocoa with Love.)

Anyway, I get nervous expounding on this stuff on Stack! Check out Apple's Objective C reference for some more of the basics. The sample code on the iOS Dev Center is really, really good to check out. Download a project that looks nice, or, better yet, just create a project from an Xcode template, and go try to figure out how it's stitched together. Good luck!

What you need to understand is that when you allocate a new mainViewController *mainVc you are actually creating a new instance of the view controller and its associated button, so you are changing the title of the button only it is not the one currently on your screen!

You can either pass a reference to mainViewController as per @Sam Ritchie's suggestion or you can implement a delegate method that gets called when you are ready to change the title of the button.

Say you have a LOGIN button that you want to change to LOGOUT when a user has successfully logged in, you can create a delegate protocol in the view controller that deals with the login logic and implement it in the view controller that "owns" the relevant UIButton, where you can change the button text directly.