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iOS 代理为啥要用weak修饰? (刨根问底一)

96
汉斯哈哈哈
2015.08.14 16:55* 字数 448

在开发中我们经常使用代理,或自己写个代理,而代理属性都用weak(assign)修饰,看过有些开发者用strong(retain),但并没发现有何不妥,也不清楚weak(assign)与strong(retain)修饰有何区别

功能实现就行了,考虑这么多干嘛~~~我只能哈哈哈

  • weak:指明该对象并不负责保持delegate这个对象,delegate这个对象的销毁由外部控制
@property (nonatomic, weak) id<HSDogDelegate>delegate;
  • strong该对象强引用delegate,外界不能销毁delegate对象,会导致循环引用(Retain Cycles)
@property (nonatomic, strong) id<HSDogDelegate>delegate;

可能你还不太理解,没关系,下面先举例,看结果,再分析!

举例

  • HSDog类

HSDog.h:

@protocol HSDogDelegate <NSObject>
@end

@interface HSDog : NSObject

@property (nonatomic, weak) id<HSDogDelegate>delegate;

@end

HSDog.m:

#import "HSDog.h"

@implementation HSDog

- (void)dealloc
{
    NSLog(@"HSDog----销毁");
}

@end
  • HSPerson类

HSPerson.h:

@interface HSPerson : NSObject

@end

HSPerson.m:

#import "HSPerson.h"
#import "HSDog.h"

@interface HSPerson()<HSDogDelegate>
/** 强引用dog*/
@property (nonatomic, strong) HSDog *dog;
@end

@implementation HSPerson

- (instancetype)init
{
    self = [super init];
    if (self) {
        // 实例化dog
        self.dog = [[HSDog alloc] init];
        // dog的delegate引用self,self的retainCount,取决于delegate修饰,weak:retainCount不变,strong:retainCount + 1
        self.dog.delegate = self;

    }
    return self;
}

- (void)dealloc
{
    NSLog(@"HSPerson----销毁");
}

@end
  • 在ViewController实现
#import "ViewController.h"
#import "HSPerson.h"

@interface ViewController ()
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    // 实例化person, self对person弱引用,person的retainCount不变
    HSPerson *person = [[HSPerson alloc] init];

}
@end

结果

  • weak修饰代理
@property (nonatomic, weak) id<HSDogDelegate>delegate;

运行->打印

HSPerson----销毁
HSDog----销毁
  • strong修饰代理
@property (nonatomic, strong) id<HSDogDelegate>delegate;

运行->打印
....并未打印,说明HSPerson、HSDog对象没调用dealloc方法,两个对象未销毁
这也是我们经常说的内存泄露,该释放的内存并未释放!

分析

  • 使用strong

person对dog强引用

@property (nonatomic, strong) HSDog *dog; person

self.dog.delegate又对person强引用,使person的retainCount + 1

@property (nonatomic, strong) id<HSDogDelegate>delegate;

当viewController不对person引用后,dog.delegate对person还强引用着,person的retainCount为1,所以person不会释放,dog固然也不会释放,这就是造成循环引用的导致内存泄露的原因!

  • 使用weak

person对dog强引用

@property (nonatomic, strong) HSDog *dog; person

self.dog.delegate只对person弱引用,并未使person的retainCount + 1

@property (nonatomic, weak) id<HSDogDelegate>delegate;

所以当viewController不对person引用后,person的retainCount为0,即person会被释放,那么dog也被释放

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