UI控件用weak還是用strong修飾

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，對象釋放的最終根據還是根據引用計數為0時去釋放。而weak與strong的根本差別是在set方法中，weak的set方法和strong的set方法都是釋放舊值保留新值，但是weak的set方法會對其autorelease，即延遲release一次，而strong的set方法也是釋放舊值保留新值，但是其不會延遲release。最終效果是strong會+1，weak不會+1.

要注意，用_去指派的時候是不調用set方法的，也就是說無論weak還是strong，隻要用_指派都不會對引用計數加1，差別在于self.文法會調用set方法，strong的self.會調用set方法+1.而weak的_和self.文法都不會+1.

是以，建議用weak，用weak時無論用_和.文法都不會導緻+1。用strong時用self.文法會導緻+1.建議用weak。同時weak在對象回收以後可以将對象指針置成nil。