Trying to update UILabel with contents of UITextField on different view controller

I have two view controllers, one is MainViewController, the other is SetupViewController. I want a UILabel on MainViewController to set the text to the contents of a UITextField from the SetupViewController when a button is pressed in the SetupViewController.

In SetupViewController, I have this in the IBAction:

- (IBAction)donePressed:(id)sender {  
   MainViewController *mvc = [[MainViewController alloc] init]; 
  [mvc.testLabelOnMVC setText:testTextFieldOnSVC.text];
  [release mvc];
}

testLabelOnMVC (and testTextFieldOnSCV, with respective terms) is

@property (nonatomic, retain) UILabel *testLabelOnMVC;

and is also synthesized.

Every time I try, it doesn't work. Nothing happens, nothing changes. I have no errors or warnings. Can anyon开发者_运维技巧e help me out?

The view of your MainViewController does not exist until you reference the MainViewController's view property (which forces viewDidLoad to execute). You must reference the view (or otherwise force the view to be constructed) before you attempt to modify any UI objects in the MainViewController.

You are allocating a new MainViewController when you press the button, then you are setting the text of the label on this new controller, not on the MainViewController that your app is showing.

To fix this, create either and IBOutlet or iVar that points to the original MainViewController and set the text on that instead.

Easiest way is to create a @property in the main view controller and write the text in there. Then just read it in the second MVC's viewDidLoad.

The only views that MainViewController should worry about are the ones that it owns; it shouldn't be trying to access the view hierarchy managed by SetupViewController. Likewise, SetupViewController should not directly modify views in MainViewController's view graph.

The right way to do what you're asking is for the two controllers to talk to each other, either directly or via the data model. For example, let's say that your MainViewController instantiates SetupViewController. If that's the case, it'd be natural for mvc to set itself as svc's delegate, so that svc sends it a messages like -setupController:didUpdateTestStringTo:. MainViewController's implementation of that method could then save the new test string somewhere and update it's testLabel field.

Another example: MainViewController instantiates SetupViewController. SetupViewController contains a field where the user can enter a new value for the test string. Before exiting, SetupViewController writes the contents of that field into NSUserDefaults or some other common data storage. When control returns to MainViewController, that object reads the shared data and updates itself as necessary, including setting the new value for testLabel.

There are other variations on the same theme, but the common thread here is that neither view controller has to directly access views that it doesn't own.

You can change the text of the label if the view is already loaded. Instead of initializing the viewcontroller, retrieve it from the view stack if you are using navigation controller. I dont know if your viewController is already loaded or not.