Catagory 模拟添加 Weak 属性

1、关联对象模拟weak的实现，自动置空

• NSObject+weak.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface NSObject (weak)

@property(nonatomic,weak)id weakObjc;

@end

NS_ASSUME_NONNULL_END

• NSObject+weak.m

#import "NSObject+weak.h"
#import <objc/runtime.h>

typedef void (^DeallocBlock)(void);

@interface NSObject (weak)

@property(nonatomic,copy)DeallocBlock block;

@end

@implementation NSObject (weak)

- (id)weakObjc {
    return objc_getAssociatedObject(self, @selector(weakObjc));
}

- (void)setWeakObjc:(id)weakObjc {
    //⚠️注意 此处是 把block赋值给了weakObjc 把block当中间变量。
    [weakObjc setBlock:^{ //B关联了A中的weakObjc。当B对象释放的时候就会通过A去找weakObjc。所以会触发A的dealloc
        objc_setAssociatedObject(self, @selector(weakObjc), nil, OBJC_ASSOCIATION_ASSIGN);
    }];
    objc_setAssociatedObject(self,  @selector(weakObjc), weakObjc,     OBJC_ASSOCIATION_ASSIGN);
}

- (DeallocBlock)block {
    return objc_getAssociatedObject(self, @selector(block));
}

- (void)setBlock:(DeallocBlock)block {
    objc_setAssociatedObject(self, @selector(block), block,     OBJC_ASSOCIATION_COPY_NONATOMIC);
}

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-protocol-method-implementation" //未使用default
- (void)dealloc {
    self.block?self.block():nil;  //weakObjc置空
    objc_setAssociatedObject(self, @selector(block), nil,OBJC_ASSOCIATION_COPY_NONATOMIC);//block置空
}
#pragma clang diagnostic pop

@end

• 测试使用

   NSObject *obj1 = [NSObject new];
   NSObject *obj2 = [NSObject new];
   obj1.weakObjc = obj2;
   NSLog(@"Feng %@",obj1.weakObjc);
   obj2 = nil; //obj2持有obj1中的weakObjc。obj2为nil会调用obj2的dealloc，会通过obj1去找weakObjc，触发obj1的dealloc
   NSLog(@"Feng %@",obj1.weakObjc);
  

• 输出结果

   2022-06-01 08:29:39.132352+0800 Demo[2495:44993] Feng <NSObject: 0x600001ac1680>
   2022-06-01 08:29:39.132563+0800 Demo[2495:44993] Feng (null)
  

参考连接：https://www.jianshu.com/p/df5bd59f14c1

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