Block循环引用问题解析

场景如下：

{
     Person *person = [[Person alloc] init];
          person.age = 10;
          person.block = [^{
          NSLog(@"age is %d", person.age);       
     }]; 
}

• 当离开作用域时，person对象销毁，所以“1”号线消失；

• “2”、“3”因为是强引用，所以相互持有，对方都不会得到释放。

  
  

---

解决：

RAC 环境下：

• 1. __weak：不会产生强引用，指向的对象销毁时，会自动让指针置为nil;

Person *person = [[Person alloc] init];
 __weak typeof(person) weakPerson = person;
 person.block = ^{
      NSLog(@"age is %d", weakPerson.age);
 };

__weak typeof(self) weakSelf = self;
self.block = ^{
    NSlog("%p",weakSelf);
}

• 2. __unsafe_unretained：不会产生强引用，不安全，指向的对象销毁时，指针存储的地址值不变.

Person *person = [[Person alloc] init];    
 __unsafe_unretained typeof(person) weakPerson = person;
 person.block = ^{
      NSLog(@"age is %d", weakPerson.age);
 };

__unsafe_unretained id weakSelf = self;
self.block = ^ {
    NSlog("%p",weakSelf);
}

• 3. __block：必须添加对象 = ni 和调用block()

__block Person *person = [[Person alloc] init];    
person.block = ^ {
      NSLog(@"age is %d", weakPerson.age);
      person = nil;
 };
person.block();

__block id weakSelf = self;
self.block = ^ {
      NSLog(@"age is %d", weakSelf);
      weakSelf = nil;
}
self.block();

首选：

• __weak

 __weak typeof(self) weakSelf = self;

---

MRC 环境下：

• MRC没有 __weak
• 用__unsafe_unretained解决;
• 用__block解决.

__block id weakSelf = self;
self.block = ^ {
      NSLog(@"age is %d", weakSelf);
}

用__block,为什么MRC环境下不需要再调用block() ?

• 因为在MRC环境下，__block修饰的对象被copy到堆上时，copy内部的_Block_object_assign函数不会对对象进行retain操作，即：弱引用。