Should IBOutlets be strong or weak under ARC?

Question

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.

Answer 1

WARNING, OUTDATED ANSWER: this answer is not up to date as per WWDC 2015, for the correct answer refer to the accepted answer (Daniel Hall) above.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.

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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; 

Answer 2

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