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I am developing exclusively for iOS 5 using ARC. Should IBOutlets to UIViews (and subclasses) be strong or weak?
The following:
#property (nonatomic, weak) IBOutlet UIButton *button;
Would get rid of all of this:
- (void)viewDidUnload
{
// ...
self.button = nil;
// ...
}
Are there any problems doing this? The templates are using strong as are the automatically generated properties created when connecting directly to the header from the 'Interface Builder' editor, but why? The UIViewController already has a strong reference to its view which retains its subviews.
WARNING, OUTDATED ANSWER: this answer is not up to date as per WWDC 2015, for the correct answer refer to the accepted answer (Daniel Hall) above. This answer will stay for record.
Summarized from the developer library:
From a practical perspective, in iOS and OS X outlets should be defined as declared properties. 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 will therefore typically be weak by default, 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).
#property (weak) IBOutlet MyView *viewContainerSubview;
#property (strong) IBOutlet MyOtherClass *topLevelObject;
The current recommended best practice from Apple is for IBOutlets to be strong unless weak is specifically needed to avoid a retain cycle. As Johannes mentioned above, this was commented on in the "Implementing UI Designs in Interface Builder" session from WWDC 2015 where an Apple Engineer said:
And the last option I want to point out is the storage type, which can
either be strong or weak. In general you should make your outlet
strong, especially if you are connecting an outlet to a subview or to
a constraint that's not always going to be retained by the view
hierarchy. The only time you really need to make an outlet weak is if
you have a custom view that references something back up the view
hierarchy and in general that's not recommended.
I asked about this on Twitter to an engineer on the IB team and he confirmed that strong should be the default and that the developer docs are being updated.
https://twitter.com/_danielhall/status/620716996326350848
https://twitter.com/_danielhall/status/620717252216623104
While the documentation recommends using weak on properties for subviews, since iOS 6 it seems to be fine to use strong (the default ownership qualifier) instead. That's caused by the change in UIViewController that views are not unloaded anymore.
Before iOS 6, if you kept strong links to subviews of the controller's view around, if the view controller's main view got unloaded, those would hold onto the subviews as long as the view controller is around.
Since iOS 6, views are not unloaded anymore, but loaded once and then stick around as long as their controller is there. So strong properties won't matter. They also won't create strong reference cycles, since they point down the strong reference graph.
That said, I am torn between using
#property (nonatomic, weak) IBOutlet UIButton *button;
and
#property (nonatomic) IBOutlet UIButton *button;
in iOS 6 and after:
Using weak clearly states that the controller doesn't want ownership of the button.
But omitting weak doesn't hurt in iOS 6 without view unloading, and is shorter. Some may point out that is also faster, but I have yet to encounter an app that is too slow because of weak IBOutlets.
Not using weak may be perceived as an error.
Bottom line: Since iOS 6 we can't get this wrong anymore as long as we don't use view unloading. Time to party. ;)
I don't see any problem with that. Pre-ARC, I've always made my IBOutlets assign, as they're already retained by their superviews. If you make them weak, you shouldn't have to nil them out in viewDidUnload, as you point out.
One caveat: You can support iOS 4.x in an ARC project, but if you do, you can't use weak, so you'd have to make them assign, in which case you'd still want to nil the reference in viewDidUnload to avoid a dangling pointer. Here's an example of a dangling pointer bug I've experienced:
A UIViewController has a UITextField for zip code. It uses CLLocationManager to reverse geocode the user's location and set the zip code. Here's the delegate callback:
-(void)locationManager:(CLLocationManager *)manager
didUpdateToLocation:(CLLocation *)newLocation
fromLocation:(CLLocation *)oldLocation {
Class geocoderClass = NSClassFromString(#"CLGeocoder");
if (geocoderClass && IsEmpty(self.zip.text)) {
id geocoder = [[geocoderClass alloc] init];
[geocoder reverseGeocodeLocation:newLocation completionHandler:^(NSArray *placemarks, NSError *error) {
if (self.zip && IsEmpty(self.zip.text)) {
self.zip.text = [[placemarks objectAtIndex:0] postalCode];
}
}];
}
[self.locationManager stopUpdatingLocation];
}
I found that if I dismissed this view at the right time and didn't nil self.zip in viewDidUnload, the delegate callback could throw a bad access exception on self.zip.text.
IBOutlet should be strong, for performance reason. See Storyboard Reference, Strong IBOutlet, Scene Dock in iOS 9
As explained in this paragraph, the outlets to subviews of the view
controller’s view can be weak, because these subviews are already
owned by the top-level object of the nib file. However, when an Outlet
is defined as a weak pointer and the pointer is set, ARC calls the
runtime function:
id objc_storeWeak(id *object, id value);
This adds the pointer
(object) to a table using the object value as a key. This table is
referred to as the weak table. ARC uses this table to store all the
weak pointers of your application. Now, when the object value is
deallocated, ARC will iterate over the weak table and set the weak
reference to nil. Alternatively, ARC can call:
void objc_destroyWeak(id * object)
Then, the object is
unregistered and objc_destroyWeak calls again:
objc_storeWeak(id *object, nil)
This book-keeping associated
with a weak reference can take 2–3 times longer over the release of a
strong reference. So, a weak reference introduces an overhead for the
runtime that you can avoid by simply defining outlets as strong.
As of Xcode 7, it suggests strong
If you watch WWDC 2015 session 407 Implementing UI Designs in Interface Builder, it suggests (transcript from http://asciiwwdc.com/2015/sessions/407)
And the last option I want to point out is the storage type, which can either be strong or weak.
In general you should make your outlet strong, especially if you are connecting an outlet to a sub view or to a constraint that's not always going to be retained by the view hierarchy.
The only time you really need to make an outlet weak is if you have a custom view that references something back up the view hierarchy and in general that's not recommended.
So I'm going to choose strong and I will click connect which will generate my outlet.
In iOS development NIB loading is a little bit different from Mac development.
In Mac development an IBOutlet is usually a weak reference: if you have a subclass of NSViewController only the top-level view will be retained and when you dealloc the controller all its subviews and outlets are freed automatically.
UiViewController use Key Value Coding to set the outlets using strong references. So when you dealloc your UIViewController, the top view will automatically deallocated, but you must also deallocate all its outlets in the dealloc method.
In this post from the Big Nerd Ranch, they cover this topic and also explain why using a strong reference in IBOutlet is not a good choice (even if it is recommended by Apple in this case).
One thing I wish to point out here, and that is, despite what the Apple engineers have stated in their own WWDC 2015 video here:
https://developer.apple.com/videos/play/wwdc2015/407/
Apple keeps changing their mind on the subject, which tells us that there is no single right answer to this question. To show that even Apple engineers are split on this subject, take a look at Apple's most recent
sample code, and you'll see some people use weak, and some don't.
This Apple Pay example uses weak:
https://developer.apple.com/library/ios/samplecode/Emporium/Listings/Emporium_ProductTableViewController_swift.html#//apple_ref/doc/uid/TP40016175-Emporium_ProductTableViewController_swift-DontLinkElementID_8
As does this picture-in-picture example:
https://developer.apple.com/library/ios/samplecode/AVFoundationPiPPlayer/Listings/AVFoundationPiPPlayer_PlayerViewController_swift.html#//apple_ref/doc/uid/TP40016166-AVFoundationPiPPlayer_PlayerViewController_swift-DontLinkElementID_4
As does the Lister example:
https://developer.apple.com/library/ios/samplecode/Lister/Listings/Lister_ListCell_swift.html#//apple_ref/doc/uid/TP40014701-Lister_ListCell_swift-DontLinkElementID_57
As does the Core Location example:
https://developer.apple.com/library/ios/samplecode/PotLoc/Listings/Potloc_PotlocViewController_swift.html#//apple_ref/doc/uid/TP40016176-Potloc_PotlocViewController_swift-DontLinkElementID_6
As does the view controller previewing example:
https://developer.apple.com/library/ios/samplecode/ViewControllerPreviews/Listings/Projects_PreviewUsingDelegate_PreviewUsingDelegate_DetailViewController_swift.html#//apple_ref/doc/uid/TP40016546-Projects_PreviewUsingDelegate_PreviewUsingDelegate_DetailViewController_swift-DontLinkElementID_5
As does the HomeKit example:
https://developer.apple.com/library/ios/samplecode/HomeKitCatalog/Listings/HMCatalog_Homes_Action_Sets_ActionSetViewController_swift.html#//apple_ref/doc/uid/TP40015048-HMCatalog_Homes_Action_Sets_ActionSetViewController_swift-DontLinkElementID_23
All those are fully updated for iOS 9, and all use weak outlets. From this we learn that A. The issue is not as simple as some people make it out to be. B. Apple has changed their mind repeatedly, and C. You can use whatever makes you happy :)
Special thanks to Paul Hudson (author of www.hackingwithsift.com) who gave me the clarification, and references for this answer.
I hope this clarifies the subject a bit better!
Take care.
From WWDC 2015 there is a session on Implementing UI Designs in Interface Builder. Around the 32min mark he says that you always want to make your #IBOutlet strong.
Be aware, IBOutletCollection should be #property (strong, nonatomic).
It looks like something has changed over the years and now Apple recommends to use strong in general. The evidence on their WWDC session is in session 407 - Implementing UI Designs in Interface Builder and starts at 32:30. My note from what he says is (almost, if not exactly, quoting him):
outlet connections in general should be strong especially if we connect a subview or constraint that is not always retained by the
view hierarchy
weak outlet connection might be needed when creating custom views that has some reference to something back up in the view hierarchy
and in general it is not recommended
In other wards it should be always strong now as long as some of our custom view doesn't create a retain cycle with some of the view up in the view hierarchy
EDIT :
Some may ask the question. Does keeping it with a strong reference doesn't create a retain cycle as the root view controller and the owning view keeps the reference to it? Or why that changed happened?
I think the answer is earlier in this talk when they describe how the nibs are created from the xib. There is a separate nib created for a VC and for the view. I think this might be the reason why they change the recommendations. Still it would be nice to get a deeper explanation from Apple.
I think that most important information is:
Elements in xib are automatically in subviews of view. Subviews is NSArray. NSArray owns it's elements. etc have strong pointers on them. So in most cases you don't want to create another strong pointer (IBOutlet)
And with ARC you don't need to do anything in viewDidUnload
Hi I am a newbie to iOS programming. I know what a strong and weak reference is. But I get confused which type of reference to use when I have to deal with outlets. After going through the documentation which states that
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.
So what I understood after going through the above statement is that the Outlets that we create should typically be weak by default.
But while studying some tutorials I have come across the code where people have declared an outlet as strong reference. For example consider the following code :
#interface AboutViewController : UIViewController
#property (nonatomic, strong) IBOutlet UIWebView *webView;
#end
The code :
#property (nonatomic, strong) IBOutlet UIWebView *webView;
says that our AboutViewController has an UIWebView object.
But why we need a strong reference here for the UIView object?? As the document states shouldn't this be an weak reference ?
Also please explain in the documentation statement which I have quoted above what does the File’s Owner to top-level objects mean?.
I have gone through many of the similar questions on this website but none of them helped me to clear my doubt. So please help. Thanks in advance :)
What to use for non top level GUI elements - strong or weak - depends on how you are going to use their outlets. If you have a weak reference
#property (nonatomic, weak) IBOutlet UIWebView *webView;
then after calling method
[webView removeFromSupeview];
your webView will be nil and it will be impossible to restore UIWebView just by adding
[self.view addSubview:webView];
If this is appropriate for you - it is better to use weak because you will free webView's memory when you do not need it.
On the other hand, in case of a strong reference after
[webView removeFromSupeview];
webView will still have referenceCount > 0 and webView will be deallocated only if owner will free it explicitly
self.webView = nil;
or in the owner's
- (void)dealloc
together with the owner itself.
Usually there is no difference if you have static GUI. If you want to remove (not hide) some views add be able to add them later - strong references should be used.
Top level objects should be retained strong. Like
#property(nonatomic,retain) UIView *view;
in UIViewController.
It usually doesn't hurt to use a strong reference in place of a weak one in the case of outlets like this. And in some cases, you do need a strong reference.
The idea is that something has to keep a strong reference to the object at all times or it could vanish. If the object is a view that is a subview of another view, then that superview will keep a strong reference to it and so you can use a weak reference. But, if you're going to do something else with that view, such as remove it from it's superview for some reason (maybe to reuse it elsewhere, or something), then you'll want to use a strong property so that there's always something holding it strongly.
Regarding the File Owner issue, that's because the top level object (most likely a view) does not have a superview holding on to it, so you need to use a strong property so that you're holding on to it.
The simple answer is that unless you are supporting iOS 5, outlets should always be strong.
The purpose of weak outlets was so that in iOS5, if the system unloaded the view controller's view to save memory, any outlets pointing to subviews would be automatically released.
In iOS 6 and later, the system never unloads the view controller's view (viewDidUnload is never called) because Apple found a way to release most of the memory used by a view without releasing the view itself.
Consequently, the outlets in a view controller will never normally need to be released until the view controller itself is released, at which point ARC will clean up all the outlets anyway.
So just use strong for all your outlets and you won't have to worry about obscure bugs or compiler warnings due to using the wrong reference type.
Quoting from Apple's Resource Programming Guide,
Each time you ask the NSBundle or NSNib class to load a nib file, the underlying code creates a new copy of the objects in that file and returns them to you. You need to ensure that you maintain the new object graph as long as necessary, and disown it when you are finished with it. You typically need strong references to top-level objects to ensure that they are not deallocated; you don’t need strong references to objects lower down in the graph because they’re owned by their parents, and you should minimize the risk of creating strong reference cycles.
In case of framework classes like UIViewController the top-level object for the NIB file is the view property. If you check in the documentation it is declared as retain(similar to strong).
#property(nonatomic, retain) UIView *view
So any subviews to this container view should be automatically owned by it. If you now declare these subview outlets as strong they will create a strong cycle and cause memory leaks when the framework tries to cleanup the container view. To avoid these strong cycles all subviews (or non top level objects) should be declared as weak properties.
When can you declare IBOutlet's as strong
Outlets should be changed to strong when the outlet should be considered to own the referenced object:
As indicated previously, this is often the case with File’s Owner—top level objects in a nib file are frequently considered to be owned by the File’s Owner.
You may in some situations need an object from a nib file to exist outside of its original container. For example, you might have an outlet for a view that can be temporarily removed from its initial view hierarchy and must therefore be maintained independently.
You need to check in your code if webView object qualifies for case2 as above. If not the tutorial has got this one wrong and it actually should be weak.
Hope that helps!
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 am a little confused about where to use strong and where to use weak. Are there any tools to monitor reference counts in ios ?
Strong reference is used when you want the class to keep ownership to an object and hence prevent the object from being deallocated until the class doesn't need it anymore. It is usually used for objects that cannot be cheaply reproduced (e.g. data downloaded from the server) or things that are crucial during the operation of the class.
Weak reference is used when you need a reference to the object. The object will be gone if it is deallocated by its owner. This is usually used to for cycle referencing without worrying about the object not deallocated.
Just to add to the other comments, while strong reference cycles (previously known as retain cycles) are important to understand, they're relatively uncommon unless you're passing around pointers to parent objects to their children. If you're doing that sort of thing, you should make sure you understand the discussion of strong reference cycles (a.k.a. retain cycles) as outlined in some Apple docs, specifically Acquire Basic Programming Skills, Advance Memory Management, and Transitioning to ARC. As a practical matter, the common example of retain cycles would be if you are passing pointers to parent controllers to a child controllers. In these cases, should generally make these pointers weak, otherwise strong is generally pretty safe.
The other common situation for weak references is IBOutlets for controls in a NIB or Storyboard, which should also generally be weak. (If you make the outlets by control dragging them from Interface Builder to your header file, it will take care of making them weak for you.) Anyway, I'll quote from the Resource Programming Guide, which says:
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 will therefore
typically be weak by default, 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).
In general you don't have to use weak except to avoid a retain cycle. That is, as long as you don't have two objects with strong references to each other you will be fine.
I am developing exclusively for iOS 5 using ARC. Should IBOutlets to UIViews (and subclasses) be strong or weak?
The following:
#property (nonatomic, weak) IBOutlet UIButton *button;
Would get rid of all of this:
- (void)viewDidUnload
{
// ...
self.button = nil;
// ...
}
Are there any problems doing this? The templates are using strong as are the automatically generated properties created when connecting directly to the header from the 'Interface Builder' editor, but why? The UIViewController already has a strong reference to its view which retains its subviews.
WARNING, OUTDATED ANSWER: this answer is not up to date as per WWDC 2015, for the correct answer refer to the accepted answer (Daniel Hall) above. This answer will stay for record.
Summarized from the developer library:
From a practical perspective, in iOS and OS X outlets should be defined as declared properties. 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 will therefore typically be weak by default, 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).
#property (weak) IBOutlet MyView *viewContainerSubview;
#property (strong) IBOutlet MyOtherClass *topLevelObject;
The current recommended best practice from Apple is for IBOutlets to be strong unless weak is specifically needed to avoid a retain cycle. As Johannes mentioned above, this was commented on in the "Implementing UI Designs in Interface Builder" session from WWDC 2015 where an Apple Engineer said:
And the last option I want to point out is the storage type, which can
either be strong or weak. In general you should make your outlet
strong, especially if you are connecting an outlet to a subview or to
a constraint that's not always going to be retained by the view
hierarchy. The only time you really need to make an outlet weak is if
you have a custom view that references something back up the view
hierarchy and in general that's not recommended.
I asked about this on Twitter to an engineer on the IB team and he confirmed that strong should be the default and that the developer docs are being updated.
https://twitter.com/_danielhall/status/620716996326350848
https://twitter.com/_danielhall/status/620717252216623104
While the documentation recommends using weak on properties for subviews, since iOS 6 it seems to be fine to use strong (the default ownership qualifier) instead. That's caused by the change in UIViewController that views are not unloaded anymore.
Before iOS 6, if you kept strong links to subviews of the controller's view around, if the view controller's main view got unloaded, those would hold onto the subviews as long as the view controller is around.
Since iOS 6, views are not unloaded anymore, but loaded once and then stick around as long as their controller is there. So strong properties won't matter. They also won't create strong reference cycles, since they point down the strong reference graph.
That said, I am torn between using
#property (nonatomic, weak) IBOutlet UIButton *button;
and
#property (nonatomic) IBOutlet UIButton *button;
in iOS 6 and after:
Using weak clearly states that the controller doesn't want ownership of the button.
But omitting weak doesn't hurt in iOS 6 without view unloading, and is shorter. Some may point out that is also faster, but I have yet to encounter an app that is too slow because of weak IBOutlets.
Not using weak may be perceived as an error.
Bottom line: Since iOS 6 we can't get this wrong anymore as long as we don't use view unloading. Time to party. ;)
I don't see any problem with that. Pre-ARC, I've always made my IBOutlets assign, as they're already retained by their superviews. If you make them weak, you shouldn't have to nil them out in viewDidUnload, as you point out.
One caveat: You can support iOS 4.x in an ARC project, but if you do, you can't use weak, so you'd have to make them assign, in which case you'd still want to nil the reference in viewDidUnload to avoid a dangling pointer. Here's an example of a dangling pointer bug I've experienced:
A UIViewController has a UITextField for zip code. It uses CLLocationManager to reverse geocode the user's location and set the zip code. Here's the delegate callback:
-(void)locationManager:(CLLocationManager *)manager
didUpdateToLocation:(CLLocation *)newLocation
fromLocation:(CLLocation *)oldLocation {
Class geocoderClass = NSClassFromString(#"CLGeocoder");
if (geocoderClass && IsEmpty(self.zip.text)) {
id geocoder = [[geocoderClass alloc] init];
[geocoder reverseGeocodeLocation:newLocation completionHandler:^(NSArray *placemarks, NSError *error) {
if (self.zip && IsEmpty(self.zip.text)) {
self.zip.text = [[placemarks objectAtIndex:0] postalCode];
}
}];
}
[self.locationManager stopUpdatingLocation];
}
I found that if I dismissed this view at the right time and didn't nil self.zip in viewDidUnload, the delegate callback could throw a bad access exception on self.zip.text.
IBOutlet should be strong, for performance reason. See Storyboard Reference, Strong IBOutlet, Scene Dock in iOS 9
As explained in this paragraph, the outlets to subviews of the view
controller’s view can be weak, because these subviews are already
owned by the top-level object of the nib file. However, when an Outlet
is defined as a weak pointer and the pointer is set, ARC calls the
runtime function:
id objc_storeWeak(id *object, id value);
This adds the pointer
(object) to a table using the object value as a key. This table is
referred to as the weak table. ARC uses this table to store all the
weak pointers of your application. Now, when the object value is
deallocated, ARC will iterate over the weak table and set the weak
reference to nil. Alternatively, ARC can call:
void objc_destroyWeak(id * object)
Then, the object is
unregistered and objc_destroyWeak calls again:
objc_storeWeak(id *object, nil)
This book-keeping associated
with a weak reference can take 2–3 times longer over the release of a
strong reference. So, a weak reference introduces an overhead for the
runtime that you can avoid by simply defining outlets as strong.
As of Xcode 7, it suggests strong
If you watch WWDC 2015 session 407 Implementing UI Designs in Interface Builder, it suggests (transcript from http://asciiwwdc.com/2015/sessions/407)
And the last option I want to point out is the storage type, which can either be strong or weak.
In general you should make your outlet strong, especially if you are connecting an outlet to a sub view or to a constraint that's not always going to be retained by the view hierarchy.
The only time you really need to make an outlet weak is if you have a custom view that references something back up the view hierarchy and in general that's not recommended.
So I'm going to choose strong and I will click connect which will generate my outlet.
In iOS development NIB loading is a little bit different from Mac development.
In Mac development an IBOutlet is usually a weak reference: if you have a subclass of NSViewController only the top-level view will be retained and when you dealloc the controller all its subviews and outlets are freed automatically.
UiViewController use Key Value Coding to set the outlets using strong references. So when you dealloc your UIViewController, the top view will automatically deallocated, but you must also deallocate all its outlets in the dealloc method.
In this post from the Big Nerd Ranch, they cover this topic and also explain why using a strong reference in IBOutlet is not a good choice (even if it is recommended by Apple in this case).
One thing I wish to point out here, and that is, despite what the Apple engineers have stated in their own WWDC 2015 video here:
https://developer.apple.com/videos/play/wwdc2015/407/
Apple keeps changing their mind on the subject, which tells us that there is no single right answer to this question. To show that even Apple engineers are split on this subject, take a look at Apple's most recent
sample code, and you'll see some people use weak, and some don't.
This Apple Pay example uses weak:
https://developer.apple.com/library/ios/samplecode/Emporium/Listings/Emporium_ProductTableViewController_swift.html#//apple_ref/doc/uid/TP40016175-Emporium_ProductTableViewController_swift-DontLinkElementID_8
As does this picture-in-picture example:
https://developer.apple.com/library/ios/samplecode/AVFoundationPiPPlayer/Listings/AVFoundationPiPPlayer_PlayerViewController_swift.html#//apple_ref/doc/uid/TP40016166-AVFoundationPiPPlayer_PlayerViewController_swift-DontLinkElementID_4
As does the Lister example:
https://developer.apple.com/library/ios/samplecode/Lister/Listings/Lister_ListCell_swift.html#//apple_ref/doc/uid/TP40014701-Lister_ListCell_swift-DontLinkElementID_57
As does the Core Location example:
https://developer.apple.com/library/ios/samplecode/PotLoc/Listings/Potloc_PotlocViewController_swift.html#//apple_ref/doc/uid/TP40016176-Potloc_PotlocViewController_swift-DontLinkElementID_6
As does the view controller previewing example:
https://developer.apple.com/library/ios/samplecode/ViewControllerPreviews/Listings/Projects_PreviewUsingDelegate_PreviewUsingDelegate_DetailViewController_swift.html#//apple_ref/doc/uid/TP40016546-Projects_PreviewUsingDelegate_PreviewUsingDelegate_DetailViewController_swift-DontLinkElementID_5
As does the HomeKit example:
https://developer.apple.com/library/ios/samplecode/HomeKitCatalog/Listings/HMCatalog_Homes_Action_Sets_ActionSetViewController_swift.html#//apple_ref/doc/uid/TP40015048-HMCatalog_Homes_Action_Sets_ActionSetViewController_swift-DontLinkElementID_23
All those are fully updated for iOS 9, and all use weak outlets. From this we learn that A. The issue is not as simple as some people make it out to be. B. Apple has changed their mind repeatedly, and C. You can use whatever makes you happy :)
Special thanks to Paul Hudson (author of www.hackingwithsift.com) who gave me the clarification, and references for this answer.
I hope this clarifies the subject a bit better!
Take care.
From WWDC 2015 there is a session on Implementing UI Designs in Interface Builder. Around the 32min mark he says that you always want to make your #IBOutlet strong.
Be aware, IBOutletCollection should be #property (strong, nonatomic).
It looks like something has changed over the years and now Apple recommends to use strong in general. The evidence on their WWDC session is in session 407 - Implementing UI Designs in Interface Builder and starts at 32:30. My note from what he says is (almost, if not exactly, quoting him):
outlet connections in general should be strong especially if we connect a subview or constraint that is not always retained by the
view hierarchy
weak outlet connection might be needed when creating custom views that has some reference to something back up in the view hierarchy
and in general it is not recommended
In other wards it should be always strong now as long as some of our custom view doesn't create a retain cycle with some of the view up in the view hierarchy
EDIT :
Some may ask the question. Does keeping it with a strong reference doesn't create a retain cycle as the root view controller and the owning view keeps the reference to it? Or why that changed happened?
I think the answer is earlier in this talk when they describe how the nibs are created from the xib. There is a separate nib created for a VC and for the view. I think this might be the reason why they change the recommendations. Still it would be nice to get a deeper explanation from Apple.
I think that most important information is:
Elements in xib are automatically in subviews of view. Subviews is NSArray. NSArray owns it's elements. etc have strong pointers on them. So in most cases you don't want to create another strong pointer (IBOutlet)
And with ARC you don't need to do anything in viewDidUnload