iOS block内部为什么要加__strong
2018-12-27 本文已影响0人
wp_Demo
block的循环引用这里就不说了,现在是有一种场景,比如在当前的VC中,延迟执行了block方法,但是当block执行的时候,当前的VC已经不存在了,获取不到到VC的属性了,我们该怎么解决
定义一个block还有全局的字符串属性
@property (nonatomic,copy)Block block;
@property (nonatomic,strong)NSString *str;
然后我们点击,vc返回了
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
__weak typeof(self) weakSelf = self;
self.block = ^{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"-----%@",weakSelf.str);
});
};
self.block();
[self.navigationController popViewControllerAnimated:YES];
}
2018-12-27 15:42:47.773924+0800 TestDemo[33437:595246] dealloc
2018-12-27 15:42:49.267220+0800 TestDemo[33437:595246] -----(null)
可以从控制台看出,在block执行之前vc已经被销货了,所以再去向一个空的对象发送消息,肯定是发送失败的.
那么我们为了保证block的正常执行,可以尝试使用在block内部使用: __strong,防止self提前释放
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
__weak typeof(self) weakSelf = self;
self.block = ^{
//在block的执行过程中,使用强对象对弱对象进行引用
__strong TwoViewController *strongSelf = weakSelf;
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"-----%@",strongSelf.str);
});
};
self.block();
[self.navigationController popViewControllerAnimated:YES];
}
2018-12-27 15:45:20.427785+0800 TestDemo[33486:599182] -----123
2018-12-27 15:45:20.427937+0800 TestDemo[33486:599182] dealloc
可以发现,可以获取的到str属性,并且在打印完成之后VC才会被销毁
那么问题来了,也就是本文的主题,为什么加了__strong就可以获取的到了呢?
分析:既然获取的到,那就是没有销货,在iOS中控制一个视图是否被销货是由RetainCount来判断的,那么我们是否可以打印看下RetainCount来看下呢:
首先百度了一个在ARC中获取RetainCount的方法:
CFGetRetainCount((__bridge CFTypeRef)(weakSelf))
我们先看下在没有加__strong的时候:
printf("点击时 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self)));
__weak typeof(self) weakSelf = self;
self.block = ^{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
//此处不可以打印,因为weakSelf已经被销货了,否则会崩溃
printf("block 内部retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
NSLog(@"-----%@",weakSelf.str);
});
};
self.block();
[self.navigationController popViewControllerAnimated:YES];
点击时 retain count = 3
dealloc时 retain count = 1
2018-12-27 18:08:03.368435+0800 TestDemo[35790:805937] dealloc
2018-12-27 18:08:05.059696+0800 TestDemo[35790:805937] -----(null)
加了__strong:
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
printf("点击时 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(self)));
__weak typeof(self) weakSelf = self;
self.block = ^{
//在block的执行过程中,使用强对象对弱对象进行引用
printf("strongSelf赋值之前 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
__strong TwoViewController *strongSelf = weakSelf;
printf("strongSelf赋值之后 retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2*NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
printf("block 内部retain count = %ld\n",CFGetRetainCount((__bridge CFTypeRef)(weakSelf)));
NSLog(@"-----%@",strongSelf.str);
});
};
self.block();
[self.navigationController popViewControllerAnimated:YES];
}
点击时 retain count = 3
strongSelf赋值之前 retain count = 4
strongSelf赋值之后 retain count = 5
block 内部retain count = 2
2018-12-27 18:09:12.434576+0800 TestDemo[35823:808094] -----123
dealloc时 retain count = 1
2018-12-27 18:09:12.434785+0800 TestDemo[35823:808094] dealloc
从以上我们可以得出结论 :
