I'm trying to conform to Apple's guidelines about responding to low memory warnings in my app. The guidelines say: "Upon receiving (a low memory warning), your application must free up as much memory as possible by releasing objects that it does not need or clearing out memory caches that it can recreate later."
The question is, how to 'free up' memory, using ARC?
For example, suppose I have an image in the view hierarchy of a view that is not on screen. In didReceiveMemoryWarning, I currently remove it from the view by calling removeFromSuperview, but I still have a pointer to the UIImage itself. Should I also try to free up this memory? How? By assigning nil to the pointer?
TIA: John
By the time you get -didReceiveMemoryWarning, your view should have been released automatically by system and -viewDidUnload should be called. So if you don't hold references to subviews of the view you are not responsible to free them.
Otherwise, if you do hold references to some resources you want to free up, you just nullify your references by assigning "nil" to them for NSObjects, and call corresponding release functions for other types of references, e.g, CFRelease for Core Foundation objects.
Make all IBOutlet properties with the strong attribute.
Set all IBOutlet properties all to nil in viewDidUnload.
Related
I just spent an hour trying to fix a retain cycle in my code. It was basically the view controller not getting deallocated after dismiss.
However, when I was using Instruments to check for memory leak, it passed every leak check. Please see the image below.
The problem was when I declared the class protocol, I forgot to mark the delegate as weak...But how come Instruments failed to notice this retain cycle?
I'm quite new to Memory management, if my question is dumb, please understand. Thanks. :)
Instruments detect leaks in fairly simple way - if there are no references leading to the root components (say Application Delegate) from an instance, it means that the instance and all instances retaining it are a memory leak, just like Garbage Collector. So if, say, your parent ViewController merely retains another ViewController (which should have been released getting back), and the parent ViewController is still retained itself by any class that is in chain of references to the root components, it is not considered leak.
I was reading a book, which suggested that declaring my IBOutlets as weak should take care of the issue when my app gets low memory warning. e.g., I would not need to set these outlets to nil now in the viewDidUnload method anymore.
I also heard in iOS6 viewDidUnload is deprecated and instead didReceiveMemoryWarning is called.
Anyway, how shall I go on, shall I declare my IBOutlets as weak, and "forget" about implementing didReceiveMemoryWarnings and viewDidUnloads?
Not all IBOutlets should be made weak. Recommendation from Apple docs (Resource Programming guide)
Outlets should generally be weak, except for those from File’s Owner to top-level objects in a nib file (or, in iOS, a storyboard scene) which should be strong. Outlets that you create should therefore typically be weak, because:
Outlets that you create to, for example, subviews of a view controller’s view or a window controller’s window, are arbitrary references between objects that do not imply ownership.
The strong outlets are frequently specified by framework classes (for example, UIViewController’s view outlet, or NSWindowController’s window outlet).
Example:
Top level objects in your XIB should be declared strong, any other subviews/controls should be made weak properties.
#property (nonatomic, weak) IBOutlet MyView *viewContainerSubview;
#property (nonatomic, strong) IBOutlet MyOtherClass *topLevelObj;
Using ARC weak lifetime qualifier has its own advantages(Refer Apple docs) because,
__weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object.
So you need not worry about setting the IBOutlets nil, its lifetime is automatically bound by its top level instance.
didReceiveMemoryWarning should be implemented to clear any recreatable resources which are hogging the memory. When you receive the didReceiveMemoryWarning call, it should be used to release non-critical resources that are used ex: custom data structures, webservice response used to populate the UI etc. The non-criticality of any resource needs to be decided by the developer.
It is advisable to use IBOutlet as weak. When you declare them as strong, in case of low memory condition, you might need to handle to clear them in viewDidUnload method.
However there are many more objects apart from Outlets which can also be removed from memory in case of memory issues. So, if the program is having outlets as weak, you just need to consider the cached data objects and noting else.
Views are no longer automatically unloaded on memory warnings since iOS6. This doesn't mean that memory use is no longer of any concern of course, you can't just 'forget' about it.
IBOutlets should generally be weak, since they are (directly or indirectly) 'owned' by the viewcontroller's view. And usually you want their lifetimes to be the same as their parent view.
You should still try to clear as much memory as possible in didReceiveMemoryWarning. (memory of assets that are not currently used and can be recreated of course)
According to Apple's documentation:
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.
Therefore, no need to set any of your IBOutlet references to nil anywhere, because the views are no longer purged. It would make no sense to set them to nil in didReceiveMemoryWarning or anything like that.But, in case you were responding to low memory events by releasing easily-recreated model objects, emptying caches, etc., in viewDidUnload, then that stuff should definitely move to didReceiveMemoryWarning.
I have read several Q&A and documentations which state that we should use weak for IBOutlet unless it's top level objects from File's Owner.
But if I still use strong/retained, is there any major downside, or is it just redundant because the subview is already retained with addSubview:?
Note: please do not copy definition of weak / strong here, I don't need that, I want to see real world cases where using strong for IBOutlet could cause problems. Thanks.
With MRC, if you use retain, you will have to release the memory by yourself.
With ARC, if you use strong and the system requests memory from your app (= your view will be unloaded), you will have to release the memory by yourself (note that the controller would be still be active, so no dealloc called there)
For most outlets, weak/assign is appropiate because you don't need to care about releasing the memory.
Exceptions:
IBOutletCollection must be strong/retain. The collection (NSArray) is not retained by the view hierarchy.
You add/remove views dynamically. If you want to remove a view from your view hierarchy and use it again later, the view must be retained somewhere, otherwise it gets deallocated at the time of removal. However, note that you can always retain it in code at the time of removal.
I will mark this as "accepted" until someone provides a better answer.
Apparently the only downside is that when your view receives a memory warning, it would unload the view, and optimally all the subviews should be released. But since your controller still retains them if you use strong, you will have to nil them out manually in viewDidUnload.
From iOS 6, view is not unloaded upon receiving memory warning, so this becomes inconsequential. From a practical point of view there is no major difference between using weak or strong for IBOutlet afaik, unless you have to unload your view manually in your application.
An application that I am working on, that uses ARC and needs to support iOS 4.3 and iOS 5, declares every outlet as #property (strong, nonatomic) IBOutlet in the .h file.
e.g.
// myClass.h
#property (strong, nonatomic) IBOutlet UITextView *myTextview;
I know that with ARC only properties which do not hold a strong reference to an object are released.
As a result, the App relies on - (void)viewDidUnload to set the property myTextview to nil.
i.e.
// myClass.m
- (void)viewDidUnload
{
[super viewDidUnload];
self.myTextview = nil;
}
I know, from Apple's Documentation, that Outlets should generally be weak except those from File's Owner ( i.e. A Runtime Object that owns the contents of the iOS Storyboard scene) to Top-Level Objects (my rule of thumb is to use anything that appears in the window with File's Owner, First Responder and View).
Anything I add to the view will be a subview and thus is retained by it's direct superview, meaning a weak reference should be used.
I am also aware that - (void)viewDidUnload is deprecated in iOS 6 and is not called.
1st Question : What are the issues with taking the approach of declaring every outlet as a strong property and setting it to nil in viewDidUnload, apart from the fact that viewDidUnload is deprecated in iOS 6?
My intuition tells me that it is because situations arise where you can set a pointer to nil, before viewDidUnload is called. So you should, to free up memory on the heap. Is there a noticable performance change if this is the case?
2nd Question : Should I go back throughout the project and change strong to weak? Why? Is it worth the time?
3rd Question : If I was to declare the property in a class extension, to 'hide' it, how does this affect my rule of thumb for deciding on when to use strong or weak.
I know there are many threads here that discuss this issue. But many I've found are out of date, and do not address this issue directly. Thanks.
First, a few of your presumptions need addressing:
I know that ARC only releases properties which do not hold a strong
reference to an object. As a result, the App relies on -
(void)viewDidUnload to set the property myTextview to nil.
Not quite. ARC never retained weak properties in the first place. As for strong properties, ARC still releases them, but not until dealloc is called.
viewDidUnload was never used to prevent leaks. It was essentially an optimization, and one that Apple decided was no longer worth the trouble. To understand, consider the standard lifecycle of a pre-iOS6 view controller:
1. Allocated
2a. View Loaded
2b. View Unloaded
3. Deallocated
Where 2a and 2b could be repeated any number of times. For example, a view controller at the bottom of a navigation stack (its view being hidden) could have its view unloaded in low memory situations. It would then be reloaded the next its view became visible.
The method was essentially saying "Hey view controller programmer, we're running low on memory, and nobody can see me anyways, so I'm releasing my view. If you could do the same for your strong properties, that would be great."
That process had subtleties and was generally confusing. As a result of the tediousness, Apple deprecated it. Views are no longer unloaded, so there's no point in implementing it. The key point is that all your strong properties will still be released in ARC's dealloc method.
I know that Outlets should generally be weak...
Why do you know that? There's nothing special about outlets. The 'IBOutlet' keyword is really just for Xcode's benefit when designing things with its visual tools. It has no effect on the compiled code. So, when thinking about strong vs weak outlets, use the same considerations that you do for any other property, namely "do I need this to exists, or am I okay with it disappearing on me?".
What are the issues with taking the approach of
declaring every outlet as a strong property and setting it to nil in
viewDidUnload, apart from the fact that viewDidUnload is deprecated in
iOS 6?
There are no issues with that. If you want your properties to exists as long as your controller, then use strong. viewDidUnload has nothing to do with this. On older iOS versions, you could release strong outlets in viewDidUnload if you want.
Should I go back throughout the project and change
strong to weak? Why? Is it worth the time?
Again, just use whichever qualifier makes sense for your use case. You're almost always safe using strong for you outlets.
If I was to declare the property in a class extension, to 'hide' it,
how does this affect my rule of thumb for deciding on when to use
strong or weak.
There's no difference.
1st question: The outlets are subviews of the main view which is a property of the view controller. If you declare them as weak, they have a retain count of 1, since they are subviews. If the view is released, they are also released. If you declare them as strong, they have a retain count of 2, since they are subviews, and strong properties of the view controller. So they are only released when the view controller is released (which releases also its view and its subviews). To my understanding, the difference is that when the view of a view controller is released, e.g. when it is not presented and memory is low, the outlets still allocate memory when they have been declared as strong.
2nd question: I believe in most circumstances it does not matter.
3rd question: I believe if you declare properties in a class extension, the simply belong to the class, and thus there is no difference in handling them compared to "real" class properties.
My approach would be to make sure not to declare properties as strong that could result in retain cycles, what happens, i.e., if you declare a delegate as strong.
Questions about the general model of how I should be programming in iOS.
I started before ARC, doing manual memory management. I was originally taught to make every class variable a property and release in dealloc. This model works great, when I push and pop navigation controllers LIVE BYTES alloc and dealloc respectively.
When I switched to ARC however this is not the case. My live bytes never seem to go down, even when popping a navigation controller. I don't get it, am I not supposed to use properties? I generally use a strong property for anything except for an IBOutlet in which I case I'll use a weak property.
Is there something I'm missing? Do I need to do something in viewDidUnload or implement my own dealloc method???
If I use my app for long enough, I'll eventually receive a memory warning and crash. So I know something's not right.