在ARC下，iboutlet应该是强的还是弱的?

我想在这里指出一件事，那就是，尽管苹果工程师在他们自己的2015年全球开发者大会视频中说过:

https://developer.apple.com/videos/play/wwdc2015/407/

苹果公司在这个问题上一直在改变主意，这告诉我们这个问题没有唯一的正确答案。为了证明就连苹果的工程师在这个问题上也存在分歧，看看苹果最近的一份报告就知道了示例代码，你会看到有些人使用weak，有些人不用。

这个Apple Pay的例子使用了weak: https://developer.apple.com/library/ios/samplecode/Emporium/Listings/Emporium_ProductTableViewController_swift.html#//apple_ref/doc/uid/TP40016175-Emporium_ProductTableViewController_swift-DontLinkElementID_8

下面这个图中图的例子也是如此: https://developer.apple.com/library/ios/samplecode/AVFoundationPiPPlayer/Listings/AVFoundationPiPPlayer_PlayerViewController_swift.html#//apple_ref/doc/uid/TP40016166-AVFoundationPiPPlayer_PlayerViewController_swift-DontLinkElementID_4

就像Lister的例子一样: https://developer.apple.com/library/ios/samplecode/Lister/Listings/Lister_ListCell_swift.html#//apple_ref/doc/uid/TP40014701-Lister_ListCell_swift-DontLinkElementID_57

核心位置的例子也是如此: https://developer.apple.com/library/ios/samplecode/PotLoc/Listings/Potloc_PotlocViewController_swift.html#//apple_ref/doc/uid/TP40016176-Potloc_PotlocViewController_swift-DontLinkElementID_6

视图控制器预览示例如下: 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

HomeKit的例子也是如此: 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

所有这些都是针对iOS 9完全更新的，并且都使用弱outlet。这个问题并不像有些人想的那么简单。B.苹果一再改变主意，C.你可以用任何让你高兴的东西:)

特别感谢Paul Hudson (www.hackingwithsift.com的作者)，他给了我澄清，以及这个答案的参考资料。

我希望这能更好地阐明主题!

当心

2016-05-05 21:25:41

目前苹果推荐的最佳实践是iboutlet是强的，除非特别需要弱的以避免保留循环。就像Johannes上面提到的，在2015年WWDC的“在界面生成器中实现UI设计”会议上，一位苹果工程师说:

我想指出的最后一个选项是存储类型，它可以要么坚强，要么软弱。一般来说，你应该做出口强，特别是当你连接一个输出到子视图或约束并不总是会被视图保留层次结构。唯一真正需要使outlet弱的时候是if 您有一个自定义视图，它引用视图中的备份内容等级制度，一般来说不建议这样做。

我在Twitter上向IB团队的一位工程师询问了这个问题，他确认strong应该是默认的，并且开发人员文档正在更新。

https://twitter.com/_danielhall/status/620716996326350848 https://twitter.com/_danielhall/status/620717252216623104

2015-07-14 00:59:00

我看不出有什么问题。在arc之前，我总是让IBOutlets赋值，因为它们已经被它们的父视图保留了。如果你把它们设为弱的，你不应该在viewDidUnload中清空它们，正如你指出的那样。

警告:你可以支持iOS 4。x在ARC项目中，但如果你这样做，你不能使用weak，所以你必须让它们赋值，在这种情况下，你仍然想在viewDidUnload中nil引用以避免悬浮指针。下面是我经历过的一个悬浮指针错误的例子:

UIViewController有一个UITextField表示邮政编码。它使用CLLocationManager反向对用户的位置进行地理编码并设置邮政编码。这是委托回调:

-(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];
}

我发现，如果我在正确的时间解散这个视图，没有nil self.zip在viewDidUnload，委托回调可以抛出一个糟糕的访问异常self.zip.text。

2011-10-07 22:34:34

注意，IBOutletCollection应该是@property(强，非原子)。

2014-03-25 13:02:53

出于性能考虑，IBOutlet应该是强大的。参见iOS 9中的故事板参考，Strong IBOutlet, Scene Dock

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.

在Xcode 7中，它是强的

如果你观看了WWDC 2015会议407在界面生成器中实现UI设计，它建议(来自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.

2015-11-04 04:27:16