A colleague and I have been designing our code so that our view controller objects are being allocated within the viewDidLoad method of our controller objects. We did this a lot and the object graph seemed to come out just fine. After a while we found the need to add another layer above our current view controller hierarchy that would own, allocate, and release our previous parent view controller.
This got me thinking about low memory warnings and view did load. Before, our views were always visible and thus wouldn't be released if a low memory warning occurred, but now there is a chance of that happening. If it does, it seems like when the views are reloaded, they will allocated new memory for new view controllers.
Will this cause widespread memory leaks? Or will the fact that we used retained declared properties save us, since the old controllers will be released automatically? Are there styles or conventions I should consider? Thanks for the help!
If you do create your view controllers in viewDidLoad, you should release them in viewDidUnload. True for pretty much any object, not just view controllers.
But I would not be surprised if you say "Wut!?" If your view controllers need to keep their state through memory warnings, then you don't want to release them. But it will be no better if you replace them with new ones that have fresh state.
It probably doesn't make sense to create view controllers in viewDidLoad. Create them in initWithNibName:bundle: (and release them in dealloc). The design pattern is that view controllers stay around, their views can come and go. If your subordinate view controllers have anything memory-intensive, release just that object in their viewDidUnload. Then on a memory warning, you will still free up a lot of memory, but all your view controllers will stay around in a low-memory usage state, retaining only a few flags, lists of indexes, etc., that they will need to restore their views when requested.
If you are explicitly releasing each view controller that is not needed any more you will not cause any memory leaks. Usually when you present a view controller the object that is doing the presenting will retain the controller for you, so you can release it if you don't need access to it anymore. But if you are using a UINavigationController and you are pushing new view controller's then you should be implementing the viewDidUnload method. This is straight from Apple's documentation:
After it is loaded into memory, a view controller’s view remains in memory until a low-memory condition occurs or the view controller itself is deallocated. In the case of a low-memory condition, the default UIViewController behavior is to release the view object stored in the view property if that view is not currently being used. However, if your custom view controller class stores outlets or pointers to any views in the view hierarchy, you must also release those references when the top-level view object is released. Failure to do so prevents those objects from being removed from memory right away and could potentially cause memory leaks later if you subsequently overwrite any pointers to them.
There are two places where your view controller should always clean up any references to view objects:
The dealloc method
The viewDidUnload method
If you use a declared property to store a reference to your view, and that property uses retain semantics, assigning a nil value to it is enough to release the view. Properties are by far the preferred way to manage your view objects because of their convenience. If you do not use properties, you must send a release message to any view that you explicitly retained before setting the corresponding pointer value to nil
Related
I have two view controller in swift "A" and "B". Let I have some variable in A. So after initialization, it stored in Memory area. Then what happen, when I moved to B view controller. The memory gets released or still there in the memory space ?
As long as view controller A is still present in the view controller hierarchy, it will still be strongly referenced and any variables that it strongly references will still be in memory.
If you had previously presented A and then dismissed it, then A and its variables would potentially have no strong references and would no longer be in memory.
I have an application where the user can push infinitely deep into views (pushing onto nav stack). At some point, there will be a memory warning. I'd like to remove view controllers from the navigation controller and dealloc them. If the user goes back to those view controllers, I'd like to recreate the view controllers and push them back into the correct position in the stack.
Is this the right way to be thinking about this problem?
Is there an open source project that does this? It doesn't seem like an uncommon issue and I'd rather not reinvent the wheel.
There was a time where you needed to make sure that calls to viewDidLoad and viewDidUnload matched properly so that memory warnings could be handled this way. Since iOS 6 this is no longer necessary.
As it says in the documentation for viewDidUnload:
In iOS 5 and earlier, when a low-memory condition occurred and the
current view controller’s views were not needed, the system could opt
to call this method after the view controller’s view had been
released. This method was your chance to perform any final cleanup. If
your view controller stored separate references to the view or its
subviews, you could use this method to release those references. You
could 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 would not use this method to release user data
or any other information that cannot be easily recreated.
In iOS 6 and later, clearing references to views and other objects in
your view controller is unnecessary.
If you're manually keeping large objects in memory, such as image or video data, then you can override didReceiveMemoryWarning to release those objects where necessary.
I am pretty new to IOS programming and I am working on my first app that I want to submit to app store.
That's why I made a mistake initializing all objects in viewDidLoad() function. I am using viewDidUnload() function to free the memory once the user exits from the view controller. Even though, x-code memory checking tool shows that the objects has not been deleted (the RAM usage does not decrease).
Does anyone know how to fix that or I will have to rewrite the whole code?
DeprecationAppendix
viewDidUnload
Called when the controller’s view is released from memory. (Deprecated in iOS 6.0. Views are no longer purged under low-memory conditions and so this method is never called.)
- (void)viewDidUnload
Discussion
In iOS 5 and earlier, when a low-memory condition occurred and the current view controller’s views were not needed, the system could opt to call this method after the view controller’s view had been released. This method was your chance to perform any final cleanup. If your view controller stored separate references to the view or its subviews, you could use this method to release those references. You could 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 would not use this method to release user data or any other information that cannot be easily recreated.
In iOS 6 and later, clearing references to views and other objects in your view controller is unnecessary.
At the time this method is called, the view property is nil.
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 6, when a low-memory warning occurred, the UIViewController class purged its views if it knew it could reload or recreate them again later. If this happens, it also calls the viewWillUnload and viewDidUnload methods to give your code a chance to relinquish ownership of any objects that are associated with your view hierarchy, including objects loaded from the nib file, objects created in your viewDidLoad method, and objects created lazily at runtime and added to the view hierarchy. On iOS 6, views are never purged and these methods are never called. If your view controller needs to perform specific tasks when memory is low, it should override the didReceiveMemoryWarning method.
viewDidUnload is deprecated function. (Link 1) It is hard to use and not called >= iOS 6.
The view controllers are automatically freed when they are not used. But if there is any reference to the view controllers, they will still reside in the memory. You should check all the strong reference to the view controller.
iOS programming has ARC now, so you dont need to worry about memory problem if you use ARC. ARC tutorial
viewDidUnload was deprecated in iOS 6. It's no longer being called. If you're using ARC, don't worry about memory management, as everything is taken care of for you. If you aren't using ARC, put your stuff in dealloc
I faced a very interesting situation and do not know how to resolve it. I'll outline first the architecture of my program. I have a UITableViewController derived class that also implements a my delegate protocol. The cells in this table view are custom cells, and each of them have a strong (assign) type property to the delegate (the table view controller). The delegate handle some UI action.
To reproduce the crash, I load the table view, and then navigate away from it. Normally here the table view would be dealloced but in my case the cells still hold the strong reference to it, so it remains in the memory. The problem is that I get a crash when a memory warning arrives to the device after this. I deducted that the following happens:
the table view controller receives a memory warning
the it releases all their (reusable) cells
in the dealloc of the cells I nil the delegate property, thus they send a release to the table view controller.
when the last cell nils its property, the ref count of the table view reaches zero, so it will dealloc itself
after freeing the cells, the default implementation of didReceiveMemoryWarning of the table view continues, but already on a dealloced, zombie object
sometimes later it calls viewDidUnload on the zombie and it crashes the app.
How can I resolve this situation?
PS: obviously I don't use ARC
When you're setting a delegate, you should assign it, not retain it (and so the deallocation of the cells should not send a release message to the table view controller).
Delegating objects do not (and should not) retain their delegates.
However, clients of delegating objects (applications, usually) are
responsible for ensuring that their delegates are around to receive
delegation messages. To do this, they may have to retain the delegate
in memory-managed code. This precaution applies equally to data
sources, notification observers, and targets of action messages. Note
that in a garbage-collection environment, the reference to the
delegate is strong because the retain-cycle problem does not apply.
From Concepts in Objective-C Programming
I set up a detail view, do I have several Nib files that get loaded depending on what item is selected in the root view controller's table.
What I've discovered is that for the Nibs' classes, viewDidUnload is never called, whereas viewWillDisappear is called and dealloc of course is also called.
Anybody know why this would be?
Thanks.
I believe viewDidUnload isn't normally called when a view disappears from view. The reason for this is because dealloc will typically take care of all the memory dumping, so it doesn't need to call viewDidUnload first.
I think an example would help identify when viewDidUnload is called. Let's say you have a UINavigationController and you've pushed on a new view. This new view is very heavy on memory usage, so the app tries to shore up some resources. It does this by seeing if any Views are loaded that aren't currently on the screen. If so, it calls viewDidUnload where ideally you remove things that you built in loadView or viewDidLoad. Then when you go back to that view, it calls loadView or viewDidLoad again to re-build what it dumped off in viewDidUnload.
But if it doesn't need to free up memory to show your detail view, it won't call it in the normal processing of it. That's why viewWillDisappear is called (and dealloc) but never viewDidUnload.
From Apple's documentation:
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.
UIViewController Class Reference