how to access previous view elements in UINavigationController

Question

I have a UINavigationController in which I am loading different view controllers. I want to know how can i access the elements (like labels etc) of my previous view. Here is an eg.

View A myLabel.text = @"first view";

(User moves to view B)

View B (user entered a message, that i need to display in View A) something like ViewA.myLabel.text = @"user entered message"

I tried many things but was not able to find anything very useful. Please help..

I am using Xcode 4 without ARC and without storyboard.

Thanks Sam

---

Edited:

I want to update the property declared in viewController of View A and not the labels directly. My labels get updated using that property. Like while pushing the viewController we can pass the values as below.

ViewA *myView = [[ViewA alloc] init];
myView.title = @"View B" ;
myView.tableView.tag = 3;
myView.myTextView.text = @"Some Text";
[self.navigationController pushViewController:myView animated:YES];
[myView release];

Is there any way to pass these values to properties of ViewController of ViewA while popping ViewB and returning back to ViewA ?

The actual scenario is as follows: the user gets and option to write a message in textView or he can use the predefined templates. If he clicks on the templates button he is taken to a list of predefined templates where he can select any of the predefined message. Now I want that when the user click on any of the predefined message the view containing the list of predefined message gets popped of and the message he selected gets automatically populated in the textView of main view. what is the best approach to achieve this ?

TIA Sam

Answer 1

You can get the navigation controller's viewControllers property and use it, perhaps like this:

UILabel *label = ((SomeViewController *)[self.navigationController.viewControllers objectAtIndex:1]).myLabel;

However, that is not reliable. Since the “previous” view is off the screen, the system can unload it to free up memory. Then label will be nil.

You could force that other view controller to reload its view (if it has been unloaded) by accessing the view controller's view property.

But really this smells like bad design. You should almost never try to access the views of a view controller when that view controller's view is not on screen. Remember how the system can unload a view controller's view if the view is off-screen? If some UILabel under that view contained the only copy of important data, that data is now gone!

Any important data needs to be stored somewhere other than a view - perhaps in a property of the view controller, or in a model object. You should ask the view controller for the data, or for the model object that contains the data. A view controller's view objects should almost always be considered a private implementation detail of the view controller, not exposed to other classes.

EDIT

Your question is puzzling because you talk about popping ViewB and returning to ViewA, but your code only creates and pushes a ViewA. ViewB is not mentioned in the code.

I will assume that your ViewA creates and pushes a ViewB. So you should give ViewB a property of type ViewA, like this:

@class ViewA;  // forward declaration to avoid circular imports

@interface ViewB

@property (weak, nonatomic) ViewA *aView;

Then, when your ViewA creates a ViewB instance, you set the aView property:

@implementation ViewA

- (void)pushViewB {
    ViewB *bView = [[ViewB alloc] init];
    bView.aView = self;
    [self.navigationController pushViewController:bView animated:YES];
}

Now your ViewB has access to the ViewA that created it, and can set the properties of that ViewA.

Answer 2

You should set your AViewController as the delegate of your BViewController so you can message it back after a particular event. Using a delegate will also allow better decoupling of your ViewControllers.

In your BViewController, define a protocol like this :

BViewController.h :

@protocol BViewControllerDelegate <NSObject>
- (void)viewB:(UIViewController *)didEnterMessage:(NSString *)message;
@end

and add a delegate property :

@property (nonatomic, weak) id <BViewControllerDelegate> delegate;

When the user enter the message in your BViewController and hit the button that pops the BViewController to show to AViewController do this :

- (IBAction)messageEntered {
  if ([self.delegate respondsToSelector:@selector(viewB:didEnterMessage:)]) {
    [self.delegate viewB:self didEnterMessage:self.yourTextField.text];
  }
}

Your AViewController should implement the BViewControllerDelegate protocol like this :

AViewController.h :

@interface AViewController <BViewControllerDelegate>

When your AViewController creates the BViewController, it should set itself as its delegate before presenting it. Might look like this :

BViewController *bvc = [[BViewController alloc] init…];
bvc.delegate = self;

And finally, your AViewController should implement the viewB:didEnterMessage: method :

- (void)viewB:(UIViewController *)didEnterMessage:(NSString *)message {
  self.myLabel.text = message;
}

That's the cleanest way to do that, IMHO.

Answer 3

If you want to write a good code you should follow the Model-View-Controller pattern. Here's rather good tutrial http://www.cocoalab.com/?q=node/24 In a couple of words it means that you should not store data in View (and also a view should not act as controller). I suggest you to write a custom class that will do this management(store data and pass it from one view to another). If it's just a test app then you can use viewControllers property of UINavigationController to access the controllers which are in navigation stack or just create a variable to store this data for example, in View B

- (void)textFieldDidEndEditing:(UITextField *)textField {
    stringToDisplayInFirstController = textField.text;
    NSArray * arrayOfControllers =  self.navigationController.viewControllers;
    UIViewController * viewControllerA = [arrayOfControllers objectAtIndex:[arrayOfControllers count]-1];
    viewControllerA.label.text = stringToDisplayInFirstController;

}