NSArray 对 ARC 下对象的弱引用 (__unsafe_unretained)

使用 __unsafe_unretained 对象的弱引用在 ARC 下是一种常见的技术，它可以用于解决循环引用的问题。在本文中，我们将探讨 __unsafe_unretained 引用的概念、使用场景以及如何在代码中实现。

objective-c
@interface Person : NSObject
@property (nonatomic, strong) Car *car;
@end
@interface Car : NSObject
@property (nonatomic, __unsafe_unretained) Person *owner;
@end
@implementation Person
@end
@implementation Car
@end
int main() {
    Person *person = [[Person alloc] init];
    Car *car = [[Car alloc] init];
    
    person.car = car;
    car.owner = person;
    
    // 打破循环引用
    person.car = nil;
    car.owner = nil;
    
    return 0;
}

objective-c
@interface Person : NSObject
@property (nonatomic, strong) NSString *name;
@end
@implementation Person
- (void)sayHello {
    void (^block)(void) = ^{
        // 使用 __unsafe_unretained 引用外部对象
        __unsafe_unretained Person *unsafePerson = self;
        NSLog(@"Hello, %@", unsafePerson.name);
    };
    
    block();
}
@end
int main() {
    Person *person = [[Person alloc] init];
    person.name = @"John";
    
    [person sayHello];
    
    return 0;
}