After adding a view by pushViewController method, there will be a back button in the navigation bar to pop the view off the stack. However, it seems that iOS won't destroy the view after popping it off the stack so when will it be destroyed? Can we destroy it manually when popping out the view?
Generally the pattern is like this:
- (void)pushSomeViewControllerOnStack
{
SomeViewController* someViewController = [[SomeViewController alloc] initWithNibName:#"SomeView" bundle:nil];
[self.navigationController pushViewController:someViewController animated:YES];
[someViewController release];
}
In other words, the navigation controller will do its own retain of the view controller, which means you also need to release it yourself, since there's an init. The navigation controller will also take care of releasing this controller when appropriate.
You should implement the viewDidUnload and dealloc methods within your UIViewController subclasses.
When a UINavigationController pops a view controller off its stack the code within those methods will be executed.
You should read the View Controller Programming Guide for iOS: Navigation Controllers documentation from Apple's iOS Developer Library as well as the class reference documentation for the UINavigationController and UIViewController classes so that you will better understand the view controller life cycle and what to expect when various application events occur.
Related
I am a newbie to iOS world and have started building custom code on top of a templated code.
So excuse me for the obvious.
The View chain starts with a MainWindow.xib which contains a App Delegate Object, a Window Object and Application ViewController. I dont understand why those objects are needed over there. But what I understand, I need to mention starting ViewController in the "Nib Name" Property to initiate my custom View Controller (called "EmptyViewController"). Its a dummy view controller, just there to avoid crash to happen as a result of missing valid viewcontroller.
I initiate a separate Modal View Controller(MainViewController) inside didFinishLaunchingWithOptions.
Code for initiating modal View Controller --
self.window.rootViewController = self.viewController;
mainView = [[MainViewController alloc] initWithNibName:#"MainViewController" bundle:nil];
// present the viewcontroller
UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:mainView];
[self.viewController presentModalViewController:navController animated:NO];
// release it, because it's retained as modalViewController
[navController release];
I do not put this MainViewController inside MainWindow.xib as I want to have navigation at the root of MainViewController.
Inside MainViewController, I push HelpViewController when "help" button is pressed.
But HelpViewController does not show any navigation bar. I do not understand why?
Code for Pushing Navigation bar --
HelpViewController *helpVC = [[HelpViewController alloc] init];
[self.navigationController pushViewController:helpVC animated:YES];
[helpVC release];
So I would like to understand --
1) Why is MainWindow.xib needed? Can I remove it? (Note: I tried to remove it, but then I get blank screen)
1.a) Why are all the controls/objects App Delegate Object, a Window Object and Application ViewController objects needed?
2) Why doesnt HelpViewController show Navigation bar?
3) Another thing I noticed, if I say self.presentingViewController, EmptyViewController handle is returned while popViewController returns me back to MainViewController.
Thanks
The App Delegate simply implements some app-level 'callbacks' by which iOS communicates with your own code. In main.m you can see how iOS is told which of your classes implements UIApplicationDelegate. iOS creates an instance of this class and call these delegate methods ('callback') whenever appropriate (e.g. when the app goes to background).
The Window is something iOS provides, your app needs to tell what to display on it. And, as you saw, this is usually done in didFinishLaunchingWithOptions (which is called by iOS to inform your app things are ready to get started).
A View Controller is a class that handles states of stuff you show on the Window. You don't show stuff directly on the Window, but instead use Views. Every View Controller has a View with UI elements.
The XIB or NIB is a UI description/layout file. A XIB and View are linked together; you need to tell the XIB to which View Controller member (e.g. a UILabel) a UI element belongs, and you tell the XIB which View Controller method to call on a certain UI event (e.g. user taps on a button).
These are the basics. I'm aware it does not answer all your questions; I suggest you read the very good Apple documentation. Don't try to understand everything immediately as things, as you're experiencing, indeed can seem illogical at start.
First: I ported my App to ARC and everything seemed to work. But now I discovered a problem: I have a UINavigationController that is presented modally with some UIViewControllers on its stack. But when I dismiss the modal view controller, the view controllers from the stack don't seem to be deallocated. Here is what I do:
UIViewController* root = [[UIViewController alloc] init];
UINavigationController* navi = [[UINavigationController alloc] initWithRootViewController:root];
[self presentModalViewController:navi animated:TRUE];
Then from the root I push some more view controllers, but that doesn't really matter. The fact is when I later call
[self dismissModalViewControllerAnimated:TRUE];
root doesn't get deallocated. Of course in my code root is a subclass of UIViewController, and I track dealloc and viewDidUnload, but nothing gets called.
Any ideas?
What's inside your navigation controller? It could be that something else (perhaps a view controller inside your navigation controller) is the culprit, which is leading up the chain meaning the navigation controller doesn't get released.
Either way, the code you posted is correct, so if your navigation controller isn't being released after calling dismissModalViewController it would suggest that something else still has an active reference to it or one of its dependencies. I know that doesn't answer your question, but you will probably have to hunt around.
Since you aren't showing actual code, it's hard to tell what is going on with your root view controller.
But, with ARC, if you have a strong pointer to an object, it won't get released. I suspect that you are holding on to this controller after adding it to your navigation controller.
But, without seeing your code, I can't tell.
I have a UIViewController with this method:
-(void)prepareToShowVault {
...
UINavigationController *passcodeNavigationController = [[UINavigationController alloc] initWithRootViewController:passcodeViewController];
[self.navigationController presentModalViewController:passcodeNavigationController animated:YES];
[passcodeViewController release];
[passcodeNavigationController release];
}
When called from another method in this UIViewController, it works just fine. But when called from another UIViewController, this method fires but the code doesn't display a new modalViewController, presumably because the UIViewController with the above method isn't active. How can i make this work?
In the UIViewController docs it says
modalViewController
The controller for the active modal view—that is, the view that is temporarily displayed on top of the view managed by the receiver. (read-only)
So I would guess that unless your view controller is the active one you will not see the modal view that it presents.
Therefore you need to either be in that view controller or add the method to the other view controller as well possibly using a category that you add to both.
Why are you pushing a navigation controller? A navigation controller is already present. Don't create a navigation controller. I think you should simply be doing-
[self.navigationController presentModalViewController:passcodeViewController animated:YES];
HTH,
Akshay
My small app is growing more sprawling and I'm looking for some best practice advice on the management/ownership of view controllers and navigation controllers.
Here's what I'm doing now:
AppController is a singleton that creates and owns a UINavigationController instance. The app controller, and thus the navigation controller, can be globally accessed via a +sharedController like method.
Every view controller in the app that wishes to push a new view controller, basically does this:
NextViewController * nextViewController = [[NextViewController alloc] init];
[[[AppController sharedController] navigation] pushViewController:nextViewController ...];
[nextViewController release];
In this way, all "leaf" views are responsible for creating the next view over and pushing it, and the navigation controller lives in one place that everyone can get to.
But I cooked this up myself. Since navigation through view controllers is such a critical piece of architecture, I'm wondering if anyone has a better or more thoughtful approach here.
Thanks.
Every view controller has a navigationController property. If the UIViewController is part of a navigation stack, this property is set so you can grab a reference to the UINavigationController. So, instead of having to reference the AppController (or perhaps even having one at all - you can just put this in the AppDelegate), you can just do something like this:
NextViewController * nextViewController = [[NextViewController alloc] init];
[self.navigationController pushViewController:nextViewController ...];
[nextViewController release];
Why not use self.navigationController in those views that need to push? The only thing you need to do in the AppController is push the initial view on the navigation controller.
I was kind of scratching my head at this a week ago, and now with a little bit more Cocoa experience under my belt I feel like I have an inkling as to what might be going on.
I'm making an application that is driven by a UINavigationController. In the AppDelegate, I create an instance of this class, using "page 1" as the Root View Controller.
UINavigationController *aNavigationController = [[UINavigationController alloc]
initWithRootViewController:page1ViewController];
Now here's where I'm having the problem. From "page 1" I'd like to use a modal view controller that slides over the interface and then disappears once the user has made an edit. I do that using code like this, inside of Page1ViewController:
[self presentModalViewController:myModalViewController animated:YES];
When the Modal View Controller is gone, I want a value on "Page 1" to change based on what the user entered in the Modal View Controller. So, I wrote some code like this, which resides in the Modal View Controller:
[self.parentViewController dismissModalViewControllerAnimated:YES];
[self.parentViewController doSomethingPleaseWithSomeData:someData];
The update to page 1 wasn't happening, and it took me a long time to realize that the "doSomethingPleaseWithSomeData" message was not being sent to Page1ViewController, but the Navigation Controller.
Is this always to be expected when using Navigation Controllers? Did I perhaps configure something improperly? Is there an easy way to get at the View Controller that I want (in this case, Page1ViewController).
I would recommend using the delegation pattern to solve your problem. Create a property
#property (nonatomic, assign) id <MyModalViewDelegate> delegate;
And a corresponding protocol
#protocol MyModalViewDelegate
#optional
- (void)myModalViewControllerDidFinish:(MyModalViewController *)aModalViewController;
#end
When the user finishes with your view (e.g. taps the save button), send this message:
if ([self.delegate respondsToSelector:#selector(myModalViewControllerDidFinish:)])
[self.delegate myModalViewControllerDidFinish:self];
Now, set the delegate to the view controller that should manage the whole thing, and it will be notified when the view controller is finished. Note that you'll need your view controller to dismiss the modal view controller. But, logically, that makes sense, since it was the object that presented the modal view controller in the first place.
This is how Apple solves this problem in, for example, the UIImagePickerController and UIPersonPickerController.
There are a couple of ways you can handle this. The simplest is probably just to add a UIViewController property into myModalViewController and set it to page1Controller before you present it:
myModalViewController.logicalParent = self; //page1Controller
[self presentModalViewController:myModalViewController animated:YES];
Just make sure you add the appropriate instance variable #property, and #synthesize for logicalParent to myModalViewController, then you will have a way to communicate data back to the ViewController that triggered the modal dialog. This is also for passing data back and forth between different levels of navigation before you push and pop them on the stack.
The one important thing to worry about when doing this is that it is easy to get retain loops if you are not careful. Depending on exactly how you structure this you might need to use assign properties.
I just ran into this same problem. It definitely seems that if you put a UIViewController embedded in a NavigationController, then when, from that UIViewController you present another UIViewController modally, the presentee thinks that the presenter is the NavigationController. In other words, parentViewController is incorrect.
I bet this is a bug: either that, or the documentation seems incomplete. I will inquire.
Just ran into the same problem. I believe this is a bug. My scenario is the following:
A navigation hierarchy with A, B and C view controllers in this order. On C there's a button that would open a modal view controller called D. Once D is presented the navigation controller drops C from its hierarchy which is a terrible behavior. Once D gets dismissed, the navigation controller instantiates a new C type view controller and pushes it into its hierarchy to recover the original one. Terrible. My solution is hacking the navigation hierarchy this way (a very bad solution but works well. with a 2 dimension array you could implement stacking modals):
- (void)presentModalViewController:(UIViewController *)c {
[self.navigationHierarchy removeAllObjects];
[self.navigationHierarchy addObjectsFromArray:[navigation viewControllers]];
[navigation setViewControllers:[NSArray array] animated:YES];
[navigation presentModalViewController:c animated:YES];
}
- (void)dismissModalViewController {
[navigation dismissModalViewControllerAnimated:YES];
[navigation setViewControllers:[NSArray arrayWithArray:self.navigationHierarchy] animated:YES];
}
These two methods are defined where I maintain the main navigation hiererchy: the app delegate. navigation and navigationhierarchy are defined this way:
NSMutableArray *navigationHierarchy;
UINavigationController *navigation;