oc——类——strong weak __unsafe_unretained

最新推荐文章于 2022-10-21 01:29:28 发布

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#__strong #__weak #__unsafe_unretained #property

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本文深入探讨了iOS开发中强引用、弱引用及不安全未保留引用的概念与应用场景，并通过实例展示了不同引用类型如何影响对象的生命周期。

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概述

__strong：强引用，改变instance object RC，instance object指针默认为__strong
__weak：弱引用，不改变instance object RC，当RC = 0 dealloc时，__weak instance object指针置nil
__unsafe_unretained：顾名思义，unsafe和unretained，不改变instance object RC，当RC = 0，dealloc时，__unsafe_unretained instance object指针不置nil

注：__strong，__weak，__unsafe_unretained是ARC引入的关键字，因此其意义仅存于ARC下，在MRC下，__strong和____unsafe_unretained ignored，而__weak禁用

应用

@interface FBAnimal : NSObject

@end

@implementation FBAnimal

- (void)dealloc
{
    NSLog(@"FBAnimal dealloc");
}

@end

@interface FBDog : FBAnimal

@end

@implementation FBDog

- (void)dealloc
{
    NSLog(@"FBDog dealloc");
}

@end

- (void)instance_obj_ref
{
    {
        NSLog(@"-----RC change-----");
        __strong FBAnimal *animal = [[FBDog alloc] init];
        
        animal = [[FBDog alloc] init];
        animal = nil;
        animal = [[FBDog alloc] init];
    }
    
    {
        NSLog(@"-----strong-----");
        __strong FBAnimal *animal1 = [[FBDog alloc] init];
        __strong FBAnimal *animal2 = animal1;
        NSLog(@"animal1 = %p, animal2 = %p", animal1, animal2);
        animal1 = nil;
        NSLog(@"animal1 = %p, animal2 = %p", animal1, animal2);
    }
    
    {
        NSLog(@"-----weak-----");
        __strong FBAnimal *animal1 = [[FBDog alloc] init];
        __weak FBAnimal *animal2 = animal1;
        NSLog(@"animal1 = %p, animal2 = %p", animal1, animal2);
        animal1 = nil;
        NSLog(@"animal1 = %p, animal2 = %p", animal1, animal2);
    }
    
    {
        NSLog(@"-----unsafe_unretained-----");
        __strong FBAnimal *animal1 = [[FBDog alloc] init];
        __unsafe_unretained FBAnimal *animal2 = animal1;
        NSLog(@"animal1 = %p, animal2 = %p", animal1, animal2);
        animal1 = nil;
        NSLog(@"animal1 = %p, animal2 = %p", animal1, animal2);
    }
}

output：

-----RC change-----
FBDog dealloc
FBAnimal dealloc
FBDog dealloc
FBAnimal dealloc
FBDog dealloc
FBAnimal dealloc
-----strong-----
animal1 = 0x7faa835a4b50, animal2 = 0x7faa835a4b50
animal1 = 0x0, animal2 = 0x7faa835a4b50
FBDog dealloc
FBAnimal dealloc
-----weak-----
animal1 = 0x7faa835a4b50, animal2 = 0x7faa835a4b50
FBDog dealloc
FBAnimal dealloc
animal1 = 0x0, animal2 = 0x0
-----unsafe_unretained-----
animal1 = 0x7faa835a6500, animal2 = 0x7faa835a6500
FBDog dealloc
FBAnimal dealloc
animal1 = 0x0, animal2 = 0x7faa835a6500

总结：

__strong指向instance object，RC加1，__strong不再指向instance object，RC减1
__strong不再指向instance object包括__strong指向另一instance object，__strong置nil，__strong离开作用域
__weak不改变RC值，当RC = 0 dealloc时，__weak置nil
__unsafe_unretained，即unsafe+unretained，不改变RC值，当RC = 0 dealloc时，__unsafe_unretained不置nil

strong&weak&unsafe_unretained

strong，weak，unsafe_unretained修饰property，本质修饰property合成的对应数据成员(如果有合成数据成员)，分别对应__strong，__weak，__unsafe_unretained，因此strong，weak，unsafe_unretained只能修饰类类型property，显然在ARC下使用才有意义

@interface FBAnimal : NSObject

@end

@implementation FBAnimal

- (void)dealloc
{
    NSLog(@"FBAnimal dealloc");
}

@end

@interface FBDog : FBAnimal

@end

@implementation FBDog

- (void)dealloc
{
    NSLog(@"FBDog dealloc");
}

@end

@interface FBFarm : NSObject

//@property(strong) NSUInteger area;
@property(strong) FBAnimal* strongAnimal;
@property(weak) FBAnimal* weakAnimal;
@property(unsafe_unretained) FBAnimal* unsafe_unretainedAnimal;

@end

@implementation FBFarm

@end

- (void)property_objc_ref
{
    {
        NSLog(@"-----RC change-----");
        FBFarm *farm = [[FBFarm alloc] init];
        farm.strongAnimal = [[FBDog alloc] init];
        
        farm.strongAnimal = [[FBDog alloc] init];
        farm.strongAnimal = nil;
        farm.strongAnimal = [[FBDog alloc] init];
    }
    
    {
        NSLog(@"-----strong-----");
        FBFarm *farm1 = [[FBFarm alloc] init];
        FBFarm *farm2 = [[FBFarm alloc] init];
        farm1.strongAnimal = [[FBDog alloc] init];
        farm2.strongAnimal = farm1.strongAnimal;
        NSLog(@"farm1.strongAnimal = %p, farm2.strongAnimal = %p", farm1.strongAnimal, farm2.strongAnimal);
        farm1.strongAnimal = nil;
        NSLog(@"farm1.strongAnimal = %p, farm2.strongAnimal = %p", farm1.strongAnimal, farm2.strongAnimal);
    }
    
    {
        NSLog(@"-----weak-----");
        NSLog(@"-----strong-----");
        FBFarm *farm1 = [[FBFarm alloc] init];
        FBFarm *farm2 = [[FBFarm alloc] init];
        farm1.strongAnimal = [[FBDog alloc] init];
        farm2.weakAnimal = farm1.strongAnimal;
        NSLog(@"farm1.strongAnimal = %p, farm2.weakAnimal = %p", farm1.strongAnimal, farm2.weakAnimal);
        farm1.strongAnimal = nil;
        NSLog(@"farm1.strongAnimal = %p, farm2.strongAnimal = %p", farm1.strongAnimal, farm2.strongAnimal);
    }
    
    {
        NSLog(@"-----unsafe_unretained-----");
        NSLog(@"-----strong-----");
        FBFarm *farm1 = [[FBFarm alloc] init];
        FBFarm *farm2 = [[FBFarm alloc] init];
        farm1.strongAnimal = [[FBDog alloc] init];
        farm2.unsafe_unretainedAnimal = farm1.strongAnimal;
        NSLog(@"farm1.strongAnimal = %p, farm2.unsafe_unretainedAnimal = %p", farm1.strongAnimal, farm2.unsafe_unretainedAnimal);
        farm1.strongAnimal = nil;
        NSLog(@"farm1.strongAnimal = %p, farm2.unsafe_unretainedAnimal = %p", farm1.strongAnimal, farm2.unsafe_unretainedAnimal);
    }
}

output：

-----RC change-----
FBDog dealloc
FBAnimal dealloc
FBDog dealloc
FBAnimal dealloc
FBDog dealloc
FBAnimal dealloc
-----strong-----
farm1.strongAnimal = 0x7fd8b3f1b3a0, farm2.strongAnimal = 0x7fd8b3f1b3a0
farm1.strongAnimal = 0x0, farm2.strongAnimal = 0x7fd8b3f1b3a0
FBDog dealloc
FBAnimal dealloc
-----weak-----
-----strong-----
farm1.strongAnimal = 0x7fd8b3f0ba20, farm2.weakAnimal = 0x7fd8b3f0ba20
FBDog dealloc
FBAnimal dealloc
farm1.strongAnimal = 0x0, farm2.strongAnimal = 0x0
-----unsafe_unretained-----
-----strong-----
farm1.strongAnimal = 0x7fd8b3f15d00, farm2.unsafe_unretainedAnimal = 0x7fd8b3f15d00
FBDog dealloc
FBAnimal dealloc
farm1.strongAnimal = 0x0, farm2.unsafe_unretainedAnimal = 0x7fd8b3f15d00
(lldb)

总结：

strong，weak，unsafe_unretained修饰类类型property本质修饰property合成的对应数据成员(如果有合成数据成员)，分别对应__strong，__weak，__unsafe_unretained
如果unsafe_unretained property指向的instance object已经dealloc，则访问该unsafe_unretained property会crash