UI控件用weak还是用strong修饰

1. #import "ViewController.h"  
2. @interface ViewController ()  
3. @property (nonatomic, weak) UIView *weakView;  
4. @property (nonatomic, weak) UIView *selfWeakView;  
5. @property (nonatomic, strong) UIView *strongView;  
6. @property (nonatomic, strong) UIView *selfStrongView;  
7. @end  
8. @implementation ViewController  
9. - (void)viewDidLoad  
10. {  
11.     [super viewDidLoad];  
12.     self.selfStrongView = [[UIView alloc]initWithFrame:CGRectMake(25, 25, 50, 50)];  
13.     printf("self.selfStrongViewretain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self.selfStrongView)));  
14.     [self.view addSubview:self.selfStrongView];  
15.     printf("self.selfStrongView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self.selfStrongView)));  
16.     _strongView = [[UIView alloc]initWithFrame:CGRectMake(100, 100, 50, 50)];  
17.     printf("_strongView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(_strongView)));  
18.     [self.view addSubview:self.strongView];  
19.     printf("_strongView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(_strongView)));  
20.     UIView *selfView = [[UIView alloc]initWithFrame:CGRectMake(150, 150, 50, 50)];  
21.     self.selfWeakView = selfView;  
22.     printf("self.selfWeakView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self.selfWeakView)));  
23.     [self.view addSubview:self.selfWeakView];  
24.     printf("self.selfWeakView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self.selfWeakView)));  
25.     UIView *weaksView = [[UIView alloc]initWithFrame:CGRectMake(200, 200, 50, 50)];  
26.     printf("weakView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weaksView)));  
27.     _weakView = weaksView;  
28.     printf("_weakView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(_weakView)));  
29.     [self.view addSubview:_weakView];  
30.     printf("_weakView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(_weakView)));  
31. }  
32. - (void)viewWillAppear:(BOOL)animated  
33. {  
34.     printf("self.selfStrongView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(_strongView)));  
35.     printf("_strongView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(_strongView)));  
36.     printf("self.selfWeakView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self.selfWeakView)));  
37.     printf("_weakView retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(_weakView)));  
38. }  
39. @end  

在ARC中，对象释放的最终根据还是根据引用计数为0时去释放。而weak与strong的根本区别是在set方法中，weak的set方法和strong的set方法都是释放旧值保留新值，但是weak的set方法会对其autorelease，即延迟release一次，而strong的set方法也是释放旧值保留新值，但是其不会延迟release。最终效果是strong会+1，weak不会+1.

要注意，用_去赋值的时候是不调用set方法的，也就是说无论weak还是strong，只要用_赋值都不会对引用计数加1，区别在于self.语法会调用set方法，strong的self.会调用set方法+1.而weak的_和self.语法都不会+1.

所以，建议用weak，用weak时无论用_和.语法都不会导致+1。用strong时用self.语法会导致+1.建议用weak。同时weak在对象回收以后可以将对象指针置成nil。