iOS - 关联对象

详细总结 Objective-C 的关联对象功能：

1. 基本使用

// 1. 设置关联对象
objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy);// 2. 获取关联对象
id objc_getAssociatedObject(id object, const void *key);// 3. 移除关联对象
objc_removeAssociatedObjects(id object);

2. 关联策略

typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {OBJC_ASSOCIATION_ASSIGN = 0,           // 弱引用OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, // 强引用，非原子OBJC_ASSOCIATION_COPY_NONATOMIC = 3,   // 复制，非原子OBJC_ASSOCIATION_RETAIN = 01401,       // 强引用，原子OBJC_ASSOCIATION_COPY = 01403          // 复制，原子
};

3. 实现示例

3.1 分类中添加属性

// UIView+Extension.h
@interface UIView (Extension)
@property (nonatomic, strong) NSString *identifier;
@end// UIView+Extension.m
@implementation UIView (Extension)static void *IdentifierKey = &IdentifierKey;- (void)setIdentifier:(NSString *)identifier {objc_setAssociatedObject(self, IdentifierKey, identifier, OBJC_ASSOCIATION_COPY_NONATOMIC);
}- (NSString *)identifier {return objc_getAssociatedObject(self, IdentifierKey);
}@end

3.2 使用宏简化

// 定义关联属性的宏
#define OBJC_ASSOCIATION_PROPERTY_STRONG(_getter_, _setter_, _association_) \
- (void)_setter_:(id)object { \objc_setAssociatedObject(self, @selector(_getter_), object, _association_); \
} \
- (id)_getter_ { \return objc_getAssociatedObject(self, @selector(_getter_)); \
}// 使用宏
@implementation UIView (Extension)
OBJC_ASSOCIATION_PROPERTY_STRONG(identifier, setIdentifier, OBJC_ASSOCIATION_COPY_NONATOMIC)
@end

4. 关键点

4.1 Key 的选择

// 1. 静态变量地址
static char kAssociatedObjectKey;
objc_setAssociatedObject(obj, &kAssociatedObjectKey, value, policy);// 2. 选择器
objc_setAssociatedObject(obj, @selector(propertyName), value, policy);// 3. 字符串常量
static NSString *const kAssociatedKey = @"kAssociatedKey";
objc_setAssociatedObject(obj, (__bridge void *)kAssociatedKey, value, policy);

4.2 内存管理

// 强引用示例
objc_setAssociatedObject(self, @selector(strongProperty), object, OBJC_ASSOCIATION_RETAIN_NONATOMIC);// 弱引用示例
objc_setAssociatedObject(self, @selector(weakProperty), object, OBJC_ASSOCIATION_ASSIGN);

5. 线程安全

// 原子操作
objc_setAssociatedObject(self, key, value, OBJC_ASSOCIATION_RETAIN); // 使用带原子性的关联策略// 手动加锁
@synchronized(self) {objc_setAssociatedObject(self, key, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

6. 最佳实践

6.1 属性封装

@interface NSObject (Associated)@property (nonatomic, strong) id associatedObject;
@property (nonatomic, weak) id weakAssociatedObject;
@property (nonatomic, copy) NSString *copyAssociatedObject;@end

6.2 清理处理

- (void)dealloc {// 移除特定关联对象objc_setAssociatedObject(self, @selector(associatedObject), nil, OBJC_ASSOCIATION_RETAIN_NONATOMIC);// 或移除所有关联对象objc_removeAssociatedObjects(self);
}

7. 注意事项

// 1. 避免循环引用
@implementation MyClass
- (void)setupBlock {__weak typeof(self) weakSelf = self;self.associatedBlock = ^{__strong typeof(weakSelf) strongSelf = weakSelf;[strongSelf doSomething];};
}// 2. 合理选择关联策略
- (void)setDelegate:(id<MyDelegate>)delegate {objc_setAssociatedObject(self, @selector(delegate), delegate, OBJC_ASSOCIATION_ASSIGN); // 代理使用 ASSIGN
}

关联对象的优点：