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.
Related
I have seen a lot of posts on stack overflow stating that the viewDidLoad method of controllers is only called the first time the controller is accessed and not necessarily every time but always at least once.
This is not what I am seeing at all! I put together a simple test to highlight this:
https://github.com/imuz/ViewDidLoadTest
It seems for navigation controller segues and modal views viewDidLoad is always called. The only time it is not called is when switching between tabs.
Every explanation of viewDidLoad I can find contradicts this:
When is viewDidLoad called?
UIViewController viewDidLoad vs. viewWillAppear: What is the proper division of labor?
http://www.manning-sandbox.com/thread.jspa?threadID=41506
And apples own documentation indicate that a view is only unloaded when memory is low.
I am currently doing initialisation in viewDidLoad making the assumption that it is called with every segue transition.
Am I missing something here?
Phillip Mills' answer is correct. This is just an enhancement of it.
The system is working as documented.
You are seeing viewDidLoad because the view controller being pushed onto the navigation controller is a new instance. It must call viewDidLoad.
If you investigate a little further, you would see that each of those view controllers are deallocated when they are popped (just put a breakpoint or NSLog in dealloc). This deallocation has nothing to do with the view controller container... it does not control the life of the controller it uses... it is just holding a strong reference to it.
When the controller is popped off the navigation controller stack, the nav controller releases its reference, and since there are no other references to it, the view controller will dealloc.
The navigation controller only holds strong references to view controllers that are in its active stack.
If you want to reuse the same controller, you are responsible for reusing it. When you use storyboard segues, you relinquish that control (to a large extent).
Let's say you have a push segue to view controller Foo as the result of tapping some button. When that button is tapped, "the system" will create an instance of Foo (the destination view controller), and then perform the segue. The controller container now holds the only strong reference to that view controller. Once it's done with it, the VC will dealloc.
Since it creates a new controller each time, viewDidLoad will be called each time that controller is presented.
Now, if you want to change this behavior, and cache the view controller for later reuse, you have to do that specifically. If you don't use storyboard segues, it's easy since you are actually pushing/popping the VC to the nav controller.
If, however, you use storyboard segues, it's a bit more trouble.
There are a number of ways to do it, but all require some form of hacking. The storyboard itself is in charge of instantiating new view controllers. One way is to override instantiateViewControllerWithIdentifier. That is the method that gets called when a segue needs to create a view controller. It's called even for controllers that you don't give an identifier to (the system provides a made-up unique identifier if you don't assign one).
Note, I hope this is mostly for educational purposes. I'm certainly not suggesting this as the best way to resolve your problems, whatever they may be.
Something like...
#interface MyStoryboard : UIStoryboard
#property BOOL shouldUseCache;
- (void)evict:(NSString*)identifier;
- (void)purge;
#end
#implementation MyStoryboard
- (NSMutableDictionary*)cache {
static char const kCacheKey[1];
NSMutableDictionary *cache = objc_getAssociatedObject(self, kCacheKey);
if (nil == cache) {
cache = [NSMutableDictionary dictionary];
objc_setAssociatedObject(self, kCacheKey, cache, OBJC_ASSOCIATION_RETAIN);
}
return cache;
}
- (void)evict:(NSString *)identifier {
[[self cache] removeObjectForKey:identifier];
}
- (void)purge {
[[self cache] removeAllObjects];
}
- (id)instantiateViewControllerWithIdentifier:(NSString *)identifier {
if (!self.shouldUseCache) {
return [super instantiateViewControllerWithIdentifier:identifier];
}
NSMutableDictionary *cache = [self cache];
id result = [cache objectForKey:identifier];
if (result) return result;
result = [super instantiateViewControllerWithIdentifier:identifier];
[cache setObject:result forKey:identifier];
return result;
}
#end
Now, you have to use this storyboard. Unfortunately, while UIApplication holds onto the main storyboard, it does not expose an API to get it. However, each view controller has a method, storyboard to get the storyboard it was created from.
If you are loading your own storyboards, then just instantiate MyStoryboard. If you are using the default storyboard, then you need to force the system to use your special one. Again, there are lots of ways to do this. One simple way is to override the storyboard accessor method in the view controller.
You can make MyStoryboard be a proxy class that forwards everything to UIStoryboard, or you can isa-swizzle the main storyboard, or you can just have your local controller return one from its storyboard method.
Now, remember, there is a problem here. What if you push the same view controller on the stack more than once? With a cache, the exact same view controller object will be used multiple times. Is that really what you want?
If not, then you now need to manage interaction with the controller containers themselves so they can check to see if this controller is already known by them, in which case a new instance is necessary.
So, there is a way to get cached controllers while using default storyboard segues (actually there are quite a few ways)... but that is not necessarily a good thing, and certainly not what you get by default.
I believe the Apple documentation is describing a situation where the view controller is not being deallocated. If you use a segue, then you are causing the instantiation of a new destination controller and, being a new object, it needs to load a view.
In xib-based apps, I have sometimes cached a controller object that I knew I might re-use frequently. In those cases, they behaved in keeping with the documentation in terms of when a view had to be loaded.
Edit:
On reading the links you included, I don't see any contradiction in them. They, too, are talking about things that happen during the lifespan of a view controller object.
It is called every time the controller's view is loaded from scratch (i.e. requested but not yet available). If you deallocate the controller and the view goes along with it, then it will be called again the next time you instantiate the controller (for example when you create the controller to push it modally or via segue). View controllers in tabs are not deallocated because the tab controller keeps them around.
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.
I have a view controller managed in a UINavigationController. My view controller puts up a "loading" screen, and uses ASIHTTP to asynchronously load table data. Then it renders its table, and in each cell there's an image that it uses ASIHTTP to asynchronously load. When it lands each image, it uses [self.tableView reloadRowsAtIndexPaths] to reload that row, inside which the image is fed to the UIImageView in each row.
Works great. But if I back out of the view before it's done loading, it crashes.
Where it crashes is in my -tableView:numberOfRowsInSection method, and NSZombies tells me it dies because it's asking for the -count of an NSArray called self.offers that has been deallocated.
That method looks like this:
-(NSInteger)tableView:(UITableView *)table numberOfRowsInSection:(NSInteger)section
{
return [self.offers count];
}
Wrapping that return in if (self.offers) made no difference.
My -dealloc releases and sets-to-nil every one of these properties, including both self.offers and self.tableView itself. I even tried setting up a BOOL disappearing property, hitting it with YES in -viewWillDisappear, and hanging conditional behavior off that, but it doesn't work because viewWillDisappear doesn't seem to get called! Far as I can tell we're not getting ANY method called when the navigation bar pops us off.
What do I do about this?
EDIT:
Thanks to #cduhn (who's bucking for a check!), I did a bunch more looking at this. The problem has been, my -dealloc just isn't getting called when I pop this viewcontroller (nor my -viewWillDisappear nor -viewDidUnload or anything else I could use to unhook the delegation structure that's at the root of this problem).
Then I realized: THIS viewController isn't the one on the NavController stack! What's at the top of the stack right here is a shell view, just a segmented controller and a big empty UIView. I toggle the contents of that UIView between two other UIViewController subclasses depending on the state of my segmented controller. So when my view with the table on it's PARENT view gets popped from the nav stack, this CHILD I'm working on doesn't seem to get any notice about it. Which is odd, because I'm definitely releaseing it.
I can call its -dealloc from my shell controller. I could call its -viewWillDisappear too, for that matter. Is that how I should be handling this? Probably I should put something into my shell controller's viewWillDisappear like:
[[self.mainView subviews] makeObjectsPerformSelector:#selector(viewWillDisappear)];
...so that message propagates down to my child views.
Am I on the right track here, you think??
(Oh man... and that also explains why actions from inside this child table view can't get to self.navigationController! I've been puzzled about that for weeks!)
Bugs like this, where a method gets called on an object after it's been deallocated, often happen when a method gets called on a delegate after that delegate has been deallocated. The recommended practice to avoid bugs like these is to set any delegate (or delegate-like) properties of an object to nil before you release that object in the delegate's dealloc method. I know that's a confusing sentence, so I'll explain it in the context of your bug.
You have an asynchronous image download that finishes after you've backed out of your table view controller. When this happens, you're calling reloadRowsAtIndexPaths:withRowAnimation:, which results in a call to tableView:numberOfRowsInSection: on the table view's dataSource. This call is failing because that dataSource no longer exists.
The problem is that the table view object still has your controller set as its dataSource and delegate properties, even after your controller has been deallocated. The solution is to simply set these properties to nil in your controller's dealloc, like this:
- (void)dealloc {
self.tableView.dataSource = nil;
self.tableView.delegate = nil;
self.tableView = nil; // Releases as a side-effect due to #property (retain)
[super dealloc];
}
Now when your table view tries to call tableView:numberOfRowsInSection: on its dataSource, it will send the message to the nil object, which swallows all messages silently in Objective C.
You should also do the same thing with your ASIHTTPRequest's delegate, by the way.
Any time you have an asynchronous operation that calls delegate methods upon completion, it's particularly important that you set that delegate to nil. When using UITableViewController under normal circumstances you can typically get away without setting the dataSource and delegate to nil, but in this case it was necessary because you're calling methods on the tableView after its controller has gone away.
As far as I can tell, the user cannot actually "back out" of a view while the UITableView is loading it's data. The methods are not run on a thread and block the main one, also blocking UI interaction. I cannot replicate your results. Even, scrolling the table view quickly and then pressing the back button.
I suggest that the stack popping is not the problem here.
It appears that ViewDidLoad() is sent to a ViewController only after its View is physically displayed (i.e. via NavigationController pushViewController), and not immediately after initWithNibName(). Is this a behavior I can rely on? I would like to get the chance to set the member variables of my view so that all the members are valid by the time ViewDidLoad() is invoked.
You can set up member variables and other such things in initWithNibName:bundle:.
- (id)initWithNibName:(NSString *)nibName bundle:(NSBundle *)nibBundle {
if (self = [super initWithNibName:nibName bundle:nibBundle]) {
// set up ivars and other stuff here.
someIvar = someValue;
}
return self;
}
You are correct that viewDidLoad: is only sent when the view is physically displayed, i.e when it is added to some visible view (which may sometimes be never if the user does not reach that view). So it's useful to split the functionality and think about what you can do at init time and what happens at view load time.
As Marcelo Cantos notes in the comment, viewDidLoad: is generally a fine place to do all sorts of setup work, using the concept of "lazy loading," so that you defer the setup until as late time as possible.
viewDidLoad is called before a view controller is displayed for the first time, not immediately after initWithNibName. For example, if you have a tab bar controller, all of the child view controllers will be initd at launch, but viewDidLoad will only be called when you click on the appropriate tab the first time. It's generally a good idea to initialize memory-intensive items in viewDidLoad, so as to avoid using unnecessary memory.
I found that if I override initiWithNibName in the view controller, the viewDidLoad method is not called. I have to call it manually [self viewDidLoad]. But if I do not override initWithNibName: viewDidLoad is called. I am working with 4 view controllers in tab bar controller. the tab bar controller is loaded from another view.
Sorry to unearth an old thread, but this solved it for me...
-(void)viewDidLoad is only called after -(void)loadView has done its thing. In the docs for loadView:
The view controller calls this method when its view property is requested but is currently nil.
My view controller only has viewDidLoad called after its view is request by a UITabBarItem, meaning viewDidLoad is only called in the viewController once the tab bar button is pressed. I, like the OP, want viewDidLoad to be called directly after the nib is loaded, so it's contents (titles, etc) can be populated before the user clicks the tab button.
So, after calling "self = [super initWithNibName:#"nibName" bundle:nil];" in the view controller's custom initialiser, I immediately called '[self view]' afterwards. As the view is requested earlier than when it is requested by the UITabBarItem (which calls 'addSubview'), the view is initialised fully during initialisation, rather than when requested.
Hope this helps.
Specifically, what am I supposed to do with the view that is now hidden after pushing on a new view controller?
In my situation I have a view with animations going on, that continue to execute after the view is off screen.
Is there some accepted convention?
Do I remove the View Controller and View from memory?
Does Cocoa Touch have a convenient method to "Pause" a view (and controller) and remove it from memory, and bring it back into existence when needed (after a pop)?
Do I have to archive it myself and then un-archive it?
Are there any examples you can point me to?
Thanks
Another possible solution is to implement two of the following methods:
– viewWillAppear:
– viewDidAppear:
– viewWillDisappear:
– viewDidDisappear:
You could potentially stop your animation in viewWillDisappear or viewDidDisappear and then restart it in viewWillAppear or viewDidAppear. You could also store any necessary state information about the animation before you stop it.
Under low memory conditions, your controller's 'view' property will automatically be set to nil, if it's not on screen. Then the view will automatically load again when it is needed - and viewDidLoad should get called at that time.
If your view controller is retaining any subviews of the top-level view, then you may want to override the view controller's setView: method, check if the view is being set to nil, and if so, release subviews that you were retaining. Otherwise, the top-level view may never get deallocated.
- (void)setView:(UIView *)view
{
[super setView:view];
if (view == nil)
{
// Release other views
// self.someView = nil;
}
}
Whenever you call pushViewController, the current viewcontroller is stored in an array by navigation controller (this can be accessed using the viewControllers property of navcontroller).
Think of it as a stack. As you call pushViewController, a new viewcontroller is added to the stack on top of the current viewcontroller. But your rootviewcontroller is still in memory. When you call popViewController, that viewcontroller is removed from the stack and released.
So if you want to stop your animation when the view disappears, use the viewWillDisappear method as suggested by Andy.