ARC下IBOutlet用weak还是strong

原文来自这里。

今天用Xcode5的时候，发现默认的IBoutlet的属性设置为weak——因为Xcode5建立的工程都是ARC的了。但是当时还有点不明白，因为项目的原因，一直没有正式使用过ARC。于是，为了搞清楚为什么，google了一下，有很多答案。试着从Apple文档寻找线索，在这里找到了说明：

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 should therefore typically be weak, because:

• Outlets that you create to subviews of a view controller’s view or a window controller’s window, for example, 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’swindow outlet).

简单的说，如果IBOutlet对象是nib/sb scene的拥有者（File’s owner）所持有的对象，那么很显然拥有者必须“拥有”对象的指针，因此属性应设置为strong。而其他的IBOutlet对象的属性需要设置为weak，因为拥有者并不需要“拥有”他们的指针。举例来说，UIViewController的view属性是strong，因为controller要直接拥有view。而添加到view上的subviews，作为IBOutlet只需要设置为weak就可以了，因为他们不是controller直接拥有的。直接拥有subviews的是controller的view，ARC会帮助管理内存。

紧接着，文档里又提到：

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.

第一种情形前面已经解释过了，对于第二种，通俗点将，就是controller需要直接控制某一个subview并且将subview添加到其他的view tree上去。

单纯从ARC的角度思考，用weak也是很显然的：因为subview添加到view上时，view会“拥有”subview。当然，给IBOutlet属性设置为strong也没有错，“纠结谁对谁错“的问题可能需要上升到模式或者编码习惯的问题，已经超出本文的范围。

最后附上stakoverflow上的答案：

http://stackoverflow.com/questions/7678469/should-iboutlets-be-strong-or-weak-under-arc/7729141#7729141

除了选中的答案之外，其他人的回答也非常有意思，大家可以看看。