iOS的weakSelf与strongSelf

避免循环引用的标准做法:weakSelf+strongSelf

假设我们的类有一个属性叫做 model, 我们想要当 model 中的 data 变化的时候，有一个 label 的 text 会随之改变，为了达到这目的，我们设置 model :

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    model.dataChanged = ^(NSString *title) {
        self.titleLabel.text = title;                
    };
   
    self.model = model;
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    printf("retain count:%ld\n", CFGetRetainCount((__bridge CFTypeRef)(self)));
    
    [self setUpModel];
    
    printf("retain count:%ld\n", CFGetRetainCount((__bridge CFTypeRef)(self)));
}

果然，打印显示，引用计数+1了

image.png

为啥？我们的 ViewController 持有了 model , model 持有了一个 Block , 这个 Block 又持有了 ViewController ，导致了循环引用。so通过添加 _weak 和 _strong 修饰的变量来打破循环引用，代码如下：

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    __weak typeof(self) weakSelf = self;
    model.dataChanged = ^(NSString *title) {
        __strong typeof(self) strongSelf = weakSelf;
        strongSelf.titleLabel.text = title;   
    };
    
    self.model = model;
}

果然，效果很理想

image.png

为什么要用strongSelf

其实大家可以试一下，直接用weakSelf，你会发现引用计数不变，那为啥要加这玩意儿，难道为了装逼？！主要是有时候weakSelf在block里在执行doSomething还存在，但在执行doMorething前，可能会被释放了，故为了保证self在block执行过程里一直存在，对他强引用strongSelf，一句话的事情

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    __weak typeof(self) weakSelf = self;
    model.dataChanged = ^(NSString *title) {
        [weakSelf doSomething];        
        [weakSelf doMore];                
    };
    
    self.model = model;
}

如果出现双层block嵌套甚至更多怎么办😰

看下面的例子，出现了两个block

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    __weak typeof(self) weakSelf = self;
    model.dataChanged = ^(NSString *title) {
        __strong typeof(self) strongSelf = weakSelf;
        strongSelf.titleLabel.text = title;
                      
        strongSelf.model.dataChanged = ^(NSString *title2) {
            strongSelf.titleLabel.text = title2;
        };        
    };
    
    self.model = model;
}

结果引用计数+1了，shit，我们还需要对strongSelf再进行一次weakSelf-strongSelf
，如下

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    __weak typeof(self) weakSelf = self;
    model.dataChanged = ^(NSString *title) {
        __strong typeof(self) strongSelf = weakSelf;
        strongSelf.titleLabel.text = title;
        
        __weak typeof(self) weakSelf2 = strongSelf;
        strongSelf.model.dataChanged = ^(NSString *title2) {
            __strong typeof(self) strongSelf2 = weakSelf2;
            strongSelf2.titleLabel.text = title2;
        };
    };
    
    self.model = model;
}

这样，就避免的引用循环，总结一下，不管都多少个block嵌套，皆按此法

更装逼的写法weakify + strongify

weakify 和 strongify其实是这样：

#define weakify(var) __weak typeof(var) XYWeak_##var = var;
#define strongify(var) \
_Pragma("clang diagnostic push") \
_Pragma("clang diagnostic ignored \"-Wshadow\"") \
__strong typeof(var) var = XYWeak_##var; \
_Pragma("clang diagnostic pop")
\\define加入反斜杠\表示换行意思

具体按下面使用：

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    weakify(self);
    model.dataChanged = ^(NSString *title) {
        strongify(self);
        self.titleLabel.text = title;   
    };
    
    self.model = model;
}

以上代码翻译下：

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    __weak typeof(self) XYWeak_self = self;
    model.dataChanged = ^(NSString *title) {
        __strong typeof(self) self = XYWeak_self;
        self.titleLabel.text = title;   
    };
    
    self.model = model;
}