hello i'm studying iOS programming
i created a project, which is an empty application
and i created table view controller without xib file.
and i inserted follow code in didFinishLaunchingWithOptions
TableViewController *tvc = [[TableViewController alloc] init];
[self.window addSubView:tvc.view];
[tvc release];
this code was crashed when i scrolled down. why is that?
when i comment this code
[tvc release];
program doesn't crash.
i didn't write dealloc in AppDelegate file.
why is that??
i think i created table view controller with alloc
so retain count is 1.
and add sub view to window and table view controller retain count is 2.
so i release table view controller
but it crash when i scrolled down.
i don't know why..
help me please
Simple, adding tvc.view as a subview of the window causes tvc.view to be retained but does not retain tvc itself. In essence, your TableViewController instance becomes invalid as soon as you call release on it. The app crashes when you scroll presumably because the TableViewController instance is configured as a delegate or datasource for a UITableView or UIScrollView or any other thing associated with tvc.view.
Also note that the way you are displaying the view is not the recommended way to go about it. Really you should be calling presentModalViewController: or pushViewController: and passing the TableViewController instance. This will cause the TableViewController to be retained until it is dismissed/popped, making it safe for you to call release as in your example code.
Or, since you are doing this setup manually as part of didFinishLaunchingWithOptions, you can also set window.rootViewController directly, though again that's not really recommended. XCode allows you to specify the app's default/root view controller and will automatically set it up for you when the app launches.
Actually you have just added the view alone. So the view alone will be retained. The viewcontroller will be released. But the viewcontroller needs to be the datasource and delegate for the tableview. Since it has been deallocated, that datasource will not have any valid reference and so it crashes.
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.
If a new iOS project is created with an Empty App template in Xcode 4.3.2, and in AppDelegate.m:
self.window.rootViewController = [[FooViewController alloc] init];
and in FooViewController's viewDidLoad, the following:
NSLog(#"self.view is %p", self.view);
NSLog(#"self.view is %#", self.view);
will print out the view, so it looks like the default loadView will instantiate a view and assign it to self.view.
So if I override loadView with an all empty method, and comment out the second NSLog statement above, I expect the first NSLog statement to print out 0x0, but instead the app crashed due to bad memory access right at that NSLog line. Why would that be?
Okay, after a knee-jerk and obviously wrong answer, I tried this. The Empty App template would not have a rootViewController, so I used a single screen template. After running, I see that you are getting a stack overflow. In trying to access self.view, you are calling the view property on the superclass, which is then trying to load the view in order to return it, which is calling viewDidLoad, etc., as far as I can see. The other NSLog statement does the same.
The documentation for the view property in UIViewController states:
Because accessing this property can cause the view to be loaded automatically, you can use the isViewLoaded method to determine if the view is currently in memory.
It also has a link to The View Controller Life Cycle, which states:
The steps that occur during the load cycle are as follows:
The load cycle is triggered when the view controller's view property is accessed and the view is not currently in memory.
The view controller calls its loadView method. The default implementation of the loadView method does one of two things:
If the view controller is associated with a storyboard, it loads the views from the storyboard.
If the view controller is not associated with a storyboard, an empty UIView object is created and assigned to the view property.
The view controller calls its viewDidLoad method to allow your subclass to perform any additional load-time tasks.
So when you say:
So if I override loadView with an all empty method
You're deliberately breaking the life cycle, because when your overridden version of loadView finishes, it should have loaded a view. Because it didn't, you get a crash.
I have been developing iphone applications for around 3months now and theres a few things that stump me and i don't really have an idea how to work round them.
I have a navigation controller controlling the views in my application however every screen that is loaded, used then pushed back loses all the information as it seems to be reinstantiated... I believe this is a possible memory management issue?
But how to i create an app that navigates and retains all information in its views until the application is closed.
Thanks :)
Possible you didn't keep a reference to the view controller, the issue is for UIVIewController not to be released.
Make the view controller an ivar you will instanciate only one time when you push it on stack.
// in .h
MyViewController *mVC;
// in .m
// maybe when the user selects a row in a tableview
if(mVC == nil) {
// first time use, alloc/init
mVC = [[MyViewController ....];
}
// then push on the stack
[self.navigationController ....];
Of course don't forget to release it later.
In this part:
MyViewController *myViewController=[MyViewController alloc] initWithNibName:#"myView" bundle:nil];
[[self navigationController] pushViewController:myViewController animated:YES];
[myViewController release];
You will probably have something like this... Instead, make your myViewController a class's property so you have a reference to it. And drop the [myViewController release]; statement.
Possibly your app is receiving a didReceiveMemoryWarning.
In such cases, when the super class is called, the framework does memory cleaning by unloading all the views that are not currently displayed. This could explain the behavior you are seeing.
To check it further, override didReceiveMemoryWarning in one of your view controllers or applicationDidReceiveMemoryWarning in your app delegate, and put a breakpoint in it. Don't forget to call [super...] appropriately, otherwise pretty soon your app will be killed. What you should see in this way is that the views do not disappear before hitting the breakpoint, and do disappear after that.
If the hypothesis is correct, you should find a way to save the state of your view in viewDidUnload and restore it in viewDidLoad. Look also at didReceiveMemoryWarning reference.
Try to save data in NSUserDefaults it its small or use plist or it its too small like 5-10 objects save in in some variable in appDelegate, and if its too large use sqlite and for saving something like images of files like xml use Document directory
The UINavigationController works like a stack: you push and pop UIViewControllers on it. That means when a UIViewController get popped, it will have its retain count decremented by 1, and if no other object holds a reference to it, it will be deallocated. You can avoid the UIViewControllers getting dealloced by keeping a reference to them yourself by calling -retain on the objects, for instance in your appDelegate.
You can use NSUserDefaults to save the states of the UIControls in the view.
So whenever u r loading a view, set the values to the controls so that it looks like it resume from the place where we left.
I want to implement view transition without using navigation controller like pushviewController mechanism. So, I created the view hierarchy like below.
window is superview and contains three subviews.
[window] - aViewController - bViewController - cViewController
If I want to go back to the aVewiController from cViewController, I simply allocate aViewController again like this: [[aViewController alloc] init].
Then, after 4 circulations, I got didReceiveMemoryWarning and "Program exited with status value:0" messages. Its obviously memory issue but no memory leak. Allocating viewcontroller over and over is a problem. I have no idea how to transit view in this case.
If you want your hierarchy of UIViewControllers to behave as they do in UINavigationController case you should
A) show your new ViewController with this call
//This code should be implemented in viewControllerA
[self presentModalViewController:viewControllerB animated:YES]
B) go back one step by calling
//This code should be implemented in viewControllerA
[self dismissModalViewControllerAnimated:YES]
Notes: The way you would tell viewControllerA that you want to close viewControllerB is by sending a NSNotification. The good thing about this is that if you want to go from viewControllerC to viewControllerA you simply send a notification to viewControlerA to dissmisModalViewController and it will recursively dismiss viewControllerC and viewControllerB for you.
Hope this helps
You should allocate them only once and transition between the views itself by hiding/unhiding them:
viewController.view.hidden = YES/NO;
You will need to make sure you are releasing your object after you add them to their superview. and Make sure that you are removing them from their superview when they are sent off screen....
If you want to reuse the object then you can keep them in a list and try to dequeue them like the table view does. This way you wont have to remake them each time.
If you add them to the subview and never remove them. then you will have multiple views that are no longer used that are still being referenced by their super view. therefore they will still be in memory.
i am using UIviewcontroller subclasses. In my main view i have 3 buttons, each button will load a different nib. and each new nib is having one back button to come back to main view.
when i click one the back button of any view to move to the main view the dealloc of that view is not getting called? i didnt understood this.
can anyone explain when those views dealloc will be called?
if the dealloc method hasn't been called, it means that your retained your viewController object by hands. for example, in this case dealloc will not be called after clicking back button to return
MyViewController *controller = [[MyViewController alloc] init];
[self.navigationController pushViewController:controller animated:YES];
You should add
[controller release];
to this code to be sure that your instance of viewController will be deallocated. If you are absolutely sure, that you had sent equal number of alloc(or any message that increases object's retainCount) and release messages for your object and dealloc method doesn't be called anyway, it will be more complex. I hope that this answer will help. If you will find that your situation is "more complex", post a comment, then I'll try to explain with more details.
I too would like to dive deeper into understanding memory management details (below surface level) where it comes to controllers being pushed on and off of the stack. I built my framework from the text, "Beginning iPhone 3 Development" by Mark and LaMarche, but that text effectively re-uses sub-controllers and their dealloc methods never get called.
I have noticed that repeated use of a sub-controller with a NIB containing a UIWebView that calls Google's web directions url ... eventually results in a memory warning and my data is lost. This involves repeated "reuse" of the sub-controller.
If you can point me as well to in depth text that goes into nav controller and sub view memory management, that would be excellent.