在ARC下，IBOutlets到底应该定义成strong 还是 weak ？

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在ARC下，IBOutlets到底应该定义成strong 还是 weak ？支持这个答案的人最多，答案仅是摘自官方文档的一个片段：

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;
```

大意是说，在ARC 中,一般outlet属性都推荐使用weak,应该使用 strong的 outlet是 File's Owner连接到 nib的顶层对象。

什么是 File's Owner连接到 nib 的顶层对象呢？说白话一点，就是自定义的view，不是直接作为main view里面一个sub view直接显示出来，而是需要通过实例化创建出来的。你自己实例化，当然需要strong了，不然谁还替你保留对象所有权呢？

以上的分析都没有错，但是总觉得少了点什么。对于到底是weak 还是 strong，归根结底，还是要刨到对对象所有权的问题上，但是不便于总结出浅显易懂的规律性使用法则。于是，就会有一个又一个的特例打破文档所总结的常规，不明白规则的根是什么，还是会碰到麻烦的。

我来举一个简单的例子，创建一个程序入口指向navigation controller的工程，导航栏上拖2个按钮:

右侧按钮用于控制相机按钮的显示与否，按照文档的指示，我们在程序中定义这两个按钮应为weak属性

#import <UIKit/UIKit.h>

@interface TestViewController : UIViewController
{
BOOL isShowing;
}
@property (nonatomic,weak)IBOutlet UIBarButtonItem *controlBtn;
@property (nonatomic,weak)IBOutlet UIBarButtonItem *cameraBtn;
-(IBAction)controlAction:(id)sender;
@end

#import <UIKit/UIKit.h>@interface TestViewController : UIViewController{    BOOL isShowing;}@property (nonatomic,weak)IBOutlet UIBarButtonItem *controlBtn;@property (nonatomic,weak)IBOutlet UIBarButtonItem *cameraBtn;-(IBAction)controlAction:(id)sender;@end

用右侧按钮，控制相机按钮的隐藏和显示：

#import "TestViewController.h"

@interface TestViewController ()
@end
@implementation TestViewController
@synthesize cameraBtn,controlBtn;
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
isShowing = YES;
}
- (void)viewDidUnload
{
[super viewDidUnload];
// Release any retained subviews of the main view.
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
}
-(IBAction)controlAction:(id)sender
{
if (isShowing) {
self.controlBtn.title = @"显示相机";
self.navigationItem.leftBarButtonItem = nil;
isShowing = NO;
}else {
self.controlBtn.title = @"隐藏相机";
self.navigationItem.leftBarButtonItem = cameraBtn;
isShowing = YES;
}
}
@end

#import "TestViewController.h"@interface TestViewController ()@end@implementation TestViewController@synthesize cameraBtn,controlBtn;- (void)viewDidLoad{    [super viewDidLoad];// Do any additional setup after loading the view, typically from a nib.    isShowing = YES;}- (void)viewDidUnload{    [super viewDidUnload];    // Release any retained subviews of the main view.}- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation{    return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);}-(IBAction)controlAction:(id)sender{    if (isShowing) {        self.controlBtn.title = @"显示相机";        self.navigationItem.leftBarButtonItem = nil;        isShowing = NO;    }else {        self.controlBtn.title = @"隐藏相机";        self.navigationItem.leftBarButtonItem = cameraBtn;        isShowing = YES;    }}@end

实验结果是，第一次隐藏了相机按钮后，就再也显示不出来了。原因很简单，cameraBtn指向了空，我们丢失了cameraBtn的对象所有权。

解决问题的办法有两个：

1.不在xib或者storyboard上拖相机按钮，而是用代码创建，自己控制对象所有权

2.将 cameraBtn 定义为strong

我想强调的当然是第二种方法，当然了，改成strong后，相应的也需要配合ARC做下工作：

- (void)viewDidUnload

{
[super viewDidUnload];
// Release any retained subviews of the main view.
self.cameraBtn = nil;
}

- (void)viewDidUnload{    [super viewDidUnload];    // Release any retained subviews of the main view.    self.cameraBtn = nil;}

顺便提一下ARC其他属性的规则：

strong:等同于"retain",属性成为对象的拥有者
weak:属性是 weak pointer,当对象释放时会自动设置为 nil
unsafe_unretained:等同于之前的"assign",只有 iOS 4 才应该使用
copy:和之前的 copy 一样,复制一个对象并创建 strong 关联
assign:对象不能使用 assign,但原始类型(BOOL、int、float)仍然可以使用