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：
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.