I decided to use storyboards in a project I have been doing. When the app launches it does the right thing of awakeFromNib and then viewDidLoad, but when the app has finished segueing to another view, it doesn't call viewDidUnload and, I think, neither does dealloc. I have used Apple's Instruments and doesn't show any memory leaking.
Just to note, I am using custom segues and testing this by inserting NSLogs into the methods. Has anyone else come across this?
Just want to update: dealloc actually is called but not viewDidUnload.
The viewDidUnload method is solely for the purposes of didReceiveMemoryWarning (i.e. when the view is being removed to recover some memory, but the view controller is not). If you want to see viewDidUnload while running in the Simulator, push or presentViewController to a secondary view, then generate a memory warning from the Simulator's menus. I quote from the UIViewController Class Reference:
When a low-memory condition occurs and the current view controller’s views are not needed, the system may opt to remove those views from memory. [The viewDidUnload method] is called after the view controller’s view has been released and is your chance to perform any final cleanup. If your view controller stores separate references to the view or its subviews, you should use this method to release those references. You can also use this method to remove references to any objects that you created to support the view but that are no longer needed now that the view is gone. You should not use this method to release user data or any other information that cannot be easily recreated.
At the time this method is called, the view property is nil.
viewDidUnload is called when the view is actually unloaded. If you want to clean up your resources when the view is not displayed put that in viewDidDisappear.
If you want to see what is happening with viewDidUnload, run your app in the simulator and from the menubar choose Hardware | Simulate Memory Warning.
Under memory pressure, views that are not on screen are removed and that is when the viewDidUnload method is sent.
Related
I have a view controller A being shown modally over another view controller B within a navigation controller container. When simulating memory warnings when this view controller A is shown modally, I receive the didReceiveMemoryWarnings message as expected, and then I receive a message viewDidUnload, thereby making my view controller A's view disappear.
This is surprising to me, and I am not sure how to resolve this. Why is viewDidUnload being called, releasing the visible view in the process?
Here is a stack trace in case it helps pinpoint the problem:
[UIViewController unloadViewForced:] ()
0x01458535 in -[UIViewController unloadViewIfReloadable] ()
0x01463eb8 in -[UINavigationController purgeMemoryForReason:] ()
0x01457b6d in -[UIViewController didReceiveMemoryWarning] ()
Thanks for any help!
According to http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIViewController_Class/Reference/Reference.html
Memory Management
Memory is a critical resource in iOS,
and view controllers provide built-in
support for reducing their memory
footprint at critical times. The
UIViewController class provides some
automatic handling of low-memory
conditions through its
didReceiveMemoryWarning method, which
releases unneeded memory. Prior to iOS
3.0, this method was the only way to release additional memory associated
with your custom view controller class
but in iOS 3.0 and later, the
viewDidUnload method may be a more
appropriate place for most needs.
When a low-memory warning occurs, the
UIViewController class purges its
views if it knows it can reload or
recreate them again later. If this
happens, it also calls the
viewDidUnload method to give your code
a chance to relinquish ownership of
any objects that are associated with
your view hierarchy, including objects
loaded with the nib file, objects
created in your viewDidLoad method,
and objects created lazily at runtime
and added to the view hierarchy.
Typically, if your view controller
contains outlets (properties or raw
variables that contain the IBOutlet
keyword), you should use the
viewDidUnload method to relinquish
ownership of those outlets or any
other view-related data that you no
longer need.
But I tried to do the same experiment on iphone simulator and the method viewDidUnload is not called. I think your behavior can take place when really low memory.
You can simulaute low memory warning from simulator by selecting Hardware>> Simulate memory warning.
And from IOS 6 on low memory viewDidUnload won't be called. So you need to manage what all memory can freed by overriding didreceivememory method warning.
It important to understand, after viewDidUnLoad, viewDidLoad again get called. So you need to assume viewDidLoad will get multiple times in ViewController life cycle.
Say you are not using arc, and allocating array, after some time due to low memory viewDidLoad again get called, this time you again create array than it will cause memory leaks.
There are a few instances where my app crashes when turned back on from sleep and the app is still open. It happens if a UITableViews is open when the iPhone is put into sleep; the table is being filled from an XML being parsed. I understand why it is crashing; in my viewDidUnload method I am releasing the array that is filling the table. How to handle this I am not sure; yes I could simply not release it in viewDidUnload, but then it would never leave memory if you returned to the main menu.
Any help would be appreciated!
The method viewDidUnload is not the right place to release your data if at all. I quote Apple's documentation, which desribes it better than I could:
This method is called as a counterpart
to the viewDidLoad method. It is
called during low-memory conditions
when the view controller needs to
release its view and any objects
associated with that view to free up
memory. Because view controllers often
store references to views and other
view-related objects, you should use
this method to relinquish ownership in
those objects so that the memory for
them can be reclaimed. You should do
this only for objects that you can
easily recreate later, either in your
viewDidLoad method or from other parts
of your application. You should not
use this method to release user data
or any other information that cannot
be easily recreated.
ViewDidUnload is used only to release view related objects. A view controller can release its view because its not shown, still your instance of that controller exists and so does your model.
I'm pretty new to programming and I started learning ObjC and the CocoaTouch Framework.
I've learnt about the delegation pattern and I'm pretty comfortable using it, but maybe I'm using it too much.
There is one thing I do not understand and it's specific to UIKit view controllers, I've read a lot of posts about it but I'm not able to find a clear answer.
Memory management with delegates?
Why are Objective-C delegates usually given the property assign instead of retain?
Let's suppose that I have a navigation controller and I'm pushing view controllers through it, imagine that the visible controller has as a delegate (assign not retain) a non-visible controller in the stack. A memory warning is incoming and all the view controllers (except the visible one ) are unloaded using the viewDidUnload and dealloc method, the delegate will be unloaded and the "callback" never sent.
If the new view controller is not pushed but presented modally the "connection" between delegate and the view controller is never lost, viewDidUnload is never called in the parent view.
Here is my question:
Is it correct to use delegation pattern between two view controllers?
It is certainly a correct design, depending on your controllers semantics. I used this when having a "master" view controller managing (and receiving delegate calls) from "slave" view controllers.
As to your analysis of what happens in case a memory warning is sent, there is possibly a slight misunderstanding, in that the view controller actually receives the viewDidUnload, but what this means is that the view controlled by it has been unloaded to get back some memory, so the controller can do its part of cleaning (as usual). The view controller is not itself "unloaded" or released or whatever. So the callback will always be sent. The only thing is that if the view had been previously unloaded, you would need to restore it.
Keep also in mind that, if is utterly impossible for you to recreate your unloaded views, you can prevent a specific view from being released by not calling super in your didReceiveMemoryWarning override. Take this suggestion "cum granum salis", however!
In the end, no need to use a Modal View.
Indeed, this is what Apple Docs say:
(void)didReceiveMemoryWarning :
The default implementation of this method checks to see if the view controller can safely release its view. This is possible if the view itself does not have a superview and can be reloaded either from a nib file or using a custom loadView method. If the view can be released, this method releases it and calls the viewDidUnload method.
(void)viewDidUnload:
This method is called as a counterpart to the viewDidLoad method. It is called during low-memory conditions when the view controller needs to release its view and any objects associated with that view to free up memory. Because view controllers often store references to views and other view-related objects, you should use this method to relinquish ownership in those objects so that the memory for them can be reclaimed. You should do this only for objects that you can easily recreate later, either in your viewDidLoad method or from other parts of your application. You should not use this method to release user data or any other information that cannot be easily recreated.
Your UIViewController is fairly lightweight and should never be unloaded in a low memory condition. The UIView owned by the view controller is very heavyweight and will definitely be unloaded in low memory conditions. Your controller should be ready to recreate the view if necessary, but you'll never need to regenerate your stack of view controllers.
I've looked through lots of posts, my books and Apple Developer and gleaned most of the understanding I need on use of these. I would be really grateful if some kind person could confirm that I've got it right (or correct me) and also answer the two questions.
Many thanks,
Chris.
Order of Messages
Generally, the messages will appear in the following order:
didReceiveMemoryWarning
viewDidUnload (which can be caused by 1) - obviously only applies to View Controller Classes.
dealloc
didReceiveMemoryWarning
Called when the system is low on memory.
By default, view controllers are registered for memory warning notifications and within the template method, the call to [super didReceiveMemoryWarning] releases the view if it doesn't have a superview, which is a way of checking whether the view is visible or not. It releases the view by setting its property to nil.
Action - Release anything you do not need, likely to be undoing what you might have set up in viewDidLoad. Do not release UI elements as these should be released by viewDidUnload.
Question1 - It seems that this will be called even if the View is visible, so its difficult to see what you could safely release. It would be really helpful to understand this and some examples of what could be released.
viewDidUnload
Called whenever a non visible View Controller's View property is set to nil, either manually or most commonly through didReceiveMemoryWarning.
The viewDidUnload method is there so that you can:
- clean up anything else you would like, to save extra memory or
- if you've retained some IBOutlets, to help free up memory that wouldn't otherwise be released by the view being unloaded.
Action - generally any IBOutlets you release in dealloc, should also be released (and references set to nil) in this method. Note that if the properties are set to retain, then setting them to nil will also release them.
dealloc
Called when the view controller object is de-allocated, which it will be when the retain count drops to zero.
Action - release all objects that have been retained by the class, including but not limited to all properties with a retain or copy.
Popping View Controllers and Memory
Question 2 - Does popping a view remove it from memory?
Some corrections and suggestions:
didReceiveMemoryWarning practices
As you said, the controller's default implementation of didReceiveMemoryWarning releases its view if it is 'safe to do so'. While it's not clear from Apple's documents what 'safe to do so' means, it is generally recognized as it has no superview (thus there is no way that the view is currently visible), and its loadView method can rebuild the entire view without problems.
The best practice when you override didReceiveMemoryWarning is, not to try releasing any view objects at all. Just release your custom data, if it is no longer necessary. Regarding views, just let the superclass's implementation deal with them.
Sometimes, however, the necessity of the data may depend on the state of your view. In most cases, those custom data is set in viewDidLoad method. In these cases, 'safe to release custom data' means that you know that loadView and viewDidLoad will be invoked before the view controller uses the custom data again.
Therefore, in your didReceiveMemoryWarning, call the superclass implementation first, and if its view is unloaded, then release the custom data because you know that loadView and viewDidLoad will be invoked again for sure. For example,
- (void)didReceiveMemoryWarning {
/* This is the view controller's method */
[super didReceiveMemoryWarning];
if (![self isViewLoaded]) {
/* release your custom data which will be rebuilt in loadView or viewDidLoad */
}
}
Be careful not to use self.view == nil, because self.view assumes that the view is needed for someone and will immediately load the view again.
viewDidUnload method
viewDidUnload is called when the view controller unloaded the view due to a memory warning. For example, if you remove the view from the superview and set the view property of the controller to nil, viewDidUnload method will not be invoked. A subtle point is that even if the view of a view controller is already released and set to nil by the time the controller receives didReceiveMemoryWarning, so actually there is no view to unload for the controller, viewDidUnload will be invoked if you call the superclass's implementation of didReceiveMemoryWarning.
That's why it's not a good practice to manually set the view property of a view controller to nil. If you do, you may better send a viewDidUnload message as well. I guess your understanding of viewDidUnload is more desirable, but apparently it's not the current behavior.
Popping view controllers
If you mean 'removing from the superview' by 'popping', it does decrease the retain count of the view, but not necessarily deallocate it.
If you mean popping out from a UINavigationController, it actually decrease the retain count of the view controller itself. If the view controller is not retained by another object, it will be deallocated, desirably with its view. As I explained, viewDidUnload will not be invoked this time.
Others...
Technically, the retain count may not go down to zero. The object is more likely to be just deallocated without setting the count to zero beforehand.
Just to make sure, the view controller itself is normally not deallocated by default behaviors due to the memory warning.
didReceiveMemoryWarning
...
Action - Release anything you do not need, likely to be undoing what you might have set up in viewDidLoad.
This is wrong. Anything that you recreate in viewDidLoad should be released (and set to nil) in viewDidUnload. As you mention below, didReceiveMemoryWarning is also called when the view is visible. In didReceiveMemoryWarning, you should release stuff like caches or other view controllers you are holding on to that can be recreated lazily the next time they are required (i.e., by implementing their getter manually).
viewDidUnload
...
Action - generally any IBOutlets you release in dealloc, should also be released (and references set to nil) in this method. Note that if the properties are set to retain, then setting them to nil will also release them.
Correct. Generally, everything you create in viewDidLoad and all IBOutlets that are declared as retain should be released and set to nil here.
dealloc
...
Action - release all objects that have been retained by the class, including but not limited to all properties with a retain or copy.
Correct. It's worth noting that this includes all objects you handle in viewDidUnload because the latter is not implicitly called in the dealloc process (AFAIK, not entirely sure). That's why it is essential to set all releases objects to nil in viewDidUnload because otherwise you risk releasing something twice (first in viewDidUnload, then again in dealloc; if you set the pointer to nil, the release call in dealloc will have no effect).
Popping View Controllers and Memory
Question 2 - Does popping a view remove it from memory?
Not necessarily. That is an implementation detail that you should not be concerned about. Whatever the current practice is, Apple could change it in the next release.
Just to update this thread to make it iOS6-relevant:
viewDidUnload and viewWillUnload were deprecated in iOS6. These methods are never called.
For this and other deprecated methods, see: http://developer.apple.com/library/ios/#documentation/uikit/reference/UIViewController_Class/DeprecationAppendix/AppendixADeprecatedAPI.html
Form iOS 6 onwards, how we can check whether the view loaded again. Since "viewDidUnload" is deprecated. Are you sure "loadView" and "viewDidload" will call if the view is getting removed after "didReceiveMemoryWarning" warning.
I put an NSlog in my dealloc method of my view controller. It does not get consistently called. I do notice that ViewWillDisappear does get called always. Would it be ok to move all my tidy upcode here? Setting stuff to Nil and release calls.
Anybody got some advice on why dealloc is not getting called? I know it says in docs it may not get called, but if you have a very simple App it gets called always. So something i do must be affecting the dealloc.
This is the code that calls my ViewController than isnt always calling my dealloc.
-(IBAction) playComputerTapped:(id)sender
{
PlayGameViewController *pgvc = [[PlayGameViewController alloc]
initWithNibName:#"PlayGameViewController" bundle:[NSBundle mainBundle]];
pgvc.gameMode = 1;
[self presentModalViewController:pgvc animated:YES];
[pgvc release];
}
The above code takes me from the mailmenu ViewController into the game.
Below is the code to leave the gameViewController and take me back to the menu.
[self.parentViewController dismissModalViewControllerAnimated:YES];
Thanks
-Code
Don't you mean viewDidUnload instead of viewWillDisappear ?
viewWillDisappear is called when the view controller is disappearing. This usually happens when the view controller is being popped out, or other view controller is pushed to the stack. Purpose of viewWillDisappear is to stop active actions - for example stop animations, hide some elements or similar.
viewDidUnload is probably what you meant as this one is called when view controller's view is unloaded. This should never happen for a view controller that is currently visible, only for controllers that are somewhere in the navigation stack (part of UITabBarController or UINavigationController), but not currently visible. Purpose of viewDidUnload is to release any UI elements that are part of the view and that the view controller retained as well.
To understand it's important to realize the reasons why a view for such controller would want to be unloaded. Reason is memory consumption. Views consume considerable amount of memory, even when they are not currently visible. Views are usually very easy to reconstruct - simply by calling the code that constructed them in the first place. That's especially very easy if you are using Interface Builder. So these views are best candidates to be freed to gain more memory.
When system doesn't have enough memory it starts calling didReceiveMemoryWarning method of view controllers which are not currently visible. If you have created your controllers from template in Xcode, this method simply calls super's (UIViewControllers's) implementation [super didReceiveMemoryWarning]. That default implementation will release self.view, which in turn should deallocate it together with all its subviews.
Now let's say that your view-controller needs access to some of the subviews to manipulate it in some way. For example you can have a UILabel element there and you want to change its content accordingly. To access this element you create a member variable (IBOutlet) and connect it to the element. Now your view-controller owns that label, so its retain-count is increased. When controller's view is released, so is the label, but because your view-controller still retains the label, it will not be deallocated. Therefore you should release the label in viewDidUnload method.
I've seen applications that were creating views programmatically (in loadView method), but loading was done in such dirty way that it was not possible to reconstruct the view after it was once deallocated. Therefore each time system was out of memory, it had called didReceiveMemoryWarning which in turn deallocated the view, but after navigating back to that view-controller application had crashed. A fast "bugfix" was to remove calling [super didReceiveMemoryWarning] in view-controllers. Well, system didn't get the memory and some strange effects occurred, but at least the application didn't crash immediately.
Now the third one - dealloc. This is called when object is not owned by anyone and its memory is going to be freed. Here you need to release all objects that you have retained. For view-controllers those are usually references to model classes.
I want to describe one more possible scenario. Let's say you have a view-controller displaying a chat with another person. Let's say it's very fancy chat, with emoticons and buddy-icons being animated. Let's say that each chat-entry is displayed as a cell of UITableView.
When your buddy sends you a message, you want to append a new cell into table-view by reloading it. Therefore your view-controller has an outlet to the table-view.
In viewWillDisappear you should stop the animations of emoticons and icons.
In viewDidUnload you should release the table-view.
In dealloc you want to release chat's history (probably NSArray of all messages sent and received during this conversation).
Now if you navigate away from your chat, viewWillDisappear gets called and you stop the animations.
When system is low on memory, and your view-controller is not visible, didReceiveMemoryWarning gets called and the view is released. Your viewDidUnload gets called and you release UITableView so that it can be really deallocated.
When you navigate back to the chat, loadView gets called again and your view is constructed again, viewDidLoad is called after that. Your model (representation of chat conversation) is still there, so the table-view's datasource has all the data as before, so table-view will display exactly the same thing as before the view was deallocated. After all viewWill/DidAppear is called where you start the animations.
When you finish chatting with your friend, you release the view controller, and it gets deallocated - dealloc is called, you release an array containing the chat messages, and cleanup everything else.
I hope it make things a little clearer.
Well that depends.
If you need to reduce the memory footprint of your app, unloading stuff while the view is not visible via viewWillDisappear is a good way to go. However you'll need to re-initalize everything once the view will be shown again, depending on its content this may produce considerable overhead - maybe even without the need.
For a small app (in terms of memory usage), using dealloc to unload your stuff is fine. It will be called only if the objects retain count drops to zero and is the last method that will be run before the object is destroyed. When using autorelease, that may not be the case right away and of course the object could be retained by some object other than the parentviewcontroller, preventing it from being destroyed and thus dealloc from being called.
You might want to check out the Memory Management Programming Guide, it'll explain things more detailed.