I wrote on Xcode6 the example of Objective-C delegation pattern on Wikipedia. Every time you set the delegate there is a Warning "Assigning retaining object to weak property; object will be released after assignment". After running the example, methods f and g of A class dont execute.
Changing the line
#property (weak, nonatomic) id i; // delegation
to
#property (strong, nonatomic) id i; // delegation
fix the problem. Why is that?
Generally, in the delegate pattern you want the reference to be weak, because it refers to a delegate which is the strong property of another object. Since these are generally not owned by the object that has the delegate property (i in your example), you don't want to keep a strong reference to them. In fact, it is quite common that you would have an object, such as a window controller (or navigation controller in iOS) that owns an object (such as a control of some kind) whose delegate you want to set back to the window controller (or navigation controller).
In this case, you need the delegate reference to be weak in order to prevent a retain loop (otherwise the window controller owns a reference to the control which owns a reference to the window controller as the delegate).
So, in your above example, you would be better off exploring this concept by using a more real-world example where the delegate itself is retained by another object, which more closely mirrors the way that delegates are used in the wild.
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WARNING, OUTDATED ANSWER: this answer is not up to date as per WWDC 2015, for the correct answer refer to the accepted answer (Daniel Hall) above.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
I have an iOS (Obj-C) project that uses ARC (Automatic Reference Counting). According to Apple Documentation, and pure experimentation, you cannot have statements such as:
[UIThingamabob release];
This was previously the way to release items after being allocated or retained. Now I do know that you have to manage how IB objects are created in the
#property (nonatomic, retain) IBOutlet ...
portion of your header file.
I've been using the above statement as it is (with the nonatomic and retain (or strong- what's the difference anyway, how are they used?) properties) for all of my IB items. When I test on an iOS Device, I'll randomly get a UIAlertView (that I created for debugging purposes) that only displays when the didRecieveMemoryWarning event is fired.
Some of my BETA testers are bombarded with these views nonstop until they manage to quit the app.
My question is, what do I put in the didRecieveMemoryWarning event since I can't release objects? If there isn't anything to put there, then are these errors occurring due to the way I create my objects with the #property function?
You should use #property (nonatomic, weak) IBOutlet... for all of your IBOutlets. If you use strong, the outlet is retained by the view controller and by it's superview. When the view disappears, the view controller still has a reference to that outlet which is no longer visible. You could set the outlet property to nil in -viewDidUnload or by using weak setting the pointer to nil is done automatically when the view disappears.
You should not use retain in the #property statement. The ARC options are assign, weak and strong. For more details, see here:
https://developer.apple.com/library/mac/#releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html
In particular, check the part on Property Attributes!
edit: Thanks for the comments, both retain and strong seem to be equivalent right now.
It is impossible to know what to do without seeing your project but you can do things like:
Clear out arrays of data that you can regenerate later. Clear buffers. Set currently unused objects to nil (which will release them if they are not retained elsewhere).
Though I'm sure they exists, I'm having difficulties finding or pinning down an official best practice for declaring outlets in a ViewController.
There are 3 options so far as I can see:
ivar only
property only
property backed with an ivar
Xcode currently crashes when I try and auto-generate a property by dragging into my ViewController from IB, but from what I remember, doing so creates a property without an ivar. It is also possible to drag into the ivar section and this will create an ivar without a property. This suggests that property-only and ivar only outlets are both OK with apple.
So in viewDidUnload we need to assign nil to any of our outlets, but what about dealloc. If we have used a property without an ivar, how can we release our outlet give that we are not supposed to use any accessors in an init or dealloc?
It seems to me that the only pattern which would allow us to release our outlet without an accessor is using a property backed with an ivar, so we can manually release our ivar in dealloc without using its accessor, however this is the one option which Apple's code-generation doesn't support.
As a rule of thumb, I usually create accessors for IBOutlets.
In ARC or non-ARC projects I usually do the following:
//.h (ARC)
#property (nonatomic, weak) IBOutlet UILabel* myLabel;
//.h (non-ARC)
#property (nonatomic, retain) IBOutlet UILabel* myLabel;
//.m
#synthesize myLabel;
In this manner you can let the compiler to create an instance variable for you. But you can also declare your instance variable and tell the compiler to use that.
Then you can use that accessors/instance variable wherever you want.
The Apple Memory Management guide says that you have to avoid accessors methods in init or dealloc methods when you have non-ARC projects. So, for example:
// (non-ARC)
- (void)dealloc
{
[myLabel release]; myLabel = nil; // I'm using the instance variable here!
[super dealloc];
}
This is very important in non-ARC projects. The reason is that, if there is no accessor, KVC will assign the nib object to the instance variable and will put a retain on it. If you forget to release it, you could have a memory leak. Using an accessor force you to release that object at the end.
I strongly suggest to read friday-qa-2012-04-13-nib-memory-management by Mike Ash. It's a very cool article on nib and memory management.
Hope it helps.
Here's my understanding
Use properties for variables that will be accessed by other classes, either read from (getters) or written to (setters). Both setters and getters are synthesized for properties.
Use ivars for variables that will be used internally by the owning class only, that is, other classes will not set or get their values.
Sure you can use properties in lieu of ivars, but they incur the function-call overhead whenever they're accessed. So if you have an internal variable that is accessed by your class a LOT, the function calls will affect the real-time performance, and this can be avoided by declaring them as ivars.
The template "MasterDetail" application writes an IBOutlet property in DetailViewController.h with strong qualifier.
#property (strong, nonatomic) IBOutlet UILabel *detailDescriptionLabel;
While in Standford CS193 lessons from iTunesU (lesson 2 time 14:30) they set an IBOutlet as weak saying the label already has a strong pointer to it created by window.
Now, my question is how we have to declare storage for IBOutlet like Label and Button ? is it correct to use weak ? and if i'm on iOS4 is it ok use unsafe_unretained ?
EDIT-----
I found answer about strong or weak ... the main rules are :
Use strong for top level elements of the xib
Use weak for subelements like label buttons etc..
You can check documentation
But i still can't find something about iOS 4
Well, it's basically the same case as in MRC.
Your IBOutlets are usually inside your controller's view hierarchy and implicitly retained by their parent views. You can use assign (MRC) or weak or unsafe_unretained (ARC). In all cases your pointer is invalidated when you remove the object from your view hierarchy.
The only difference is that on MRC and ARC 4.0 the pointer is not set automatically to nil. It still contains a memory address but the address is invalid.
In any case, you shouldn't use the pointer any more.
Whenever you want to use the object even if it's not a part of your view hierarchy (usually every top level element in xib or when you are removing views from your hierarchy dynamically and you do not want them to be invalidated), you use retain or strong specifiers.
Note that many examples use strong for every IBOutlets and it's not an error. It's just not necessary.