在 iOS 上使用 ARC 4，当使用 unsafe_unretained 而不是 weak 时，我是否需要 nil 我的 IBOutlet 属性？

Question

When using ARC with iOS 5, a weak IBOutlet creates a zeroing reference, avoiding the need to

self.< IBOutlet property > = nil;

in -(void)viewDidUnload

If I'm using iOS 4 ( and using ARC ) and forced to use unsafe_unretained instead, does it mean I have to override viewDidUnload and set the property to nil manually?

EDIT: This relates to my case: Should IBOutlets be strong or weak under ARC? The exception being: I can't use the 'weak' keyword which creates the zeroing reference.

Hope my question is clearer.

Thanks

Answer 1

When using ARC, as I am sure you have realized, the weak attribute cannot be used pre-iOS5. The other side of that coin would be to use unsafe_unretained. Weak attributes will automatically set your properties to nil. Unsafe_retained (aka "assign" in pre-iOS 5), will not, and you need to do this yourself.

Answer 2

Without a property (in iOS) the IBOutlet will be ivar is set and retained by KVC. With a @property the ivar is set by setting the property.

On an ARC project, if one creates a nib and drags an item (say UILabel ) to the.h file a strong @property will be added as well as in the.m file a line setting the property to nil will be added to the viewDidUnload method and a @synthesize statement for the property.

There are other ways to handle the retaining of nib IBOutlets that work and may even be better by some metric.

From the Apple document Resource Programming Guide - Managing the Lifetimes of Objects from Nib Files :

Because the behavior of outlets depends on the platform, the actual declaration differs:

For iOS, you should use:
@property (nonatomic, retain) IBOutlet UserInterfaceElementClass *anOutlet;

For OS X, you should use:
@property (assign) IBOutlet UserInterfaceElementClass *anOutlet;

My belief is to not fight the way Apple does things, doing so tends to make things harder. Also consider that Apple has inside information of the future of the platform. :-)