背景
因为 OC 中 无法直接为类新增属性(继承、私有 extension 除外),那么通过 category 结合 object association 是常用的为一个类添加属性的手段。主要是用到
OBJC_EXPORT void
objc_setAssociatedObject(id _Nonnull object, const void * _Nonnull key,
id _Nullable value, objc_AssociationPolicy policy)
OBJC_EXPORT id _Nullable
objc_getAssociatedObject(id _Nonnull object, const void * _Nonnull key)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);
为一个新的对象新增属性
#import
@interface Person (XAssociate)
@property (nonatomic, weak) id xProperty;
@end
@implementation Person (XAssociate)
- (void)setXProperty:(id)xProperty {
objc_setAssociatedObject(self, @selector(xProperty), xProperty, OBJC_ASSOCIATION_ASSIGN);
}
- (id)xProperty {
return objc_getAssociatedObject(self, _cmd);
}
@end
由于 objc association 提供的策略中,没有直接提供 weak 的策略,一般情况下折中使用 上文实例代码中的assign 的方式:
typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
OBJC_ASSOCIATION_ASSIGN = 0, /**< Specifies a weak reference to the associated object. */
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object.
* The association is not made atomically. */
OBJC_ASSOCIATION_COPY_NONATOMIC = 3, /**< Specifies that the associated object is copied.
* The association is not made atomically. */
OBJC_ASSOCIATION_RETAIN = 01401, /**< Specifies a strong reference to the associated object.
* The association is made atomically. */
OBJC_ASSOCIATION_COPY = 01403 /**< Specifies that the associated object is copied.
* The association is made atomically. */
};
使用assign策略本质上是保存了对象的地址而不是真正的弱引用,在一些情定的情况下 属性对象释放时再调用方法会出现野指针异常
解决方案
- 使用 strong + WeakContainer 的方式,实现对属性对象的 weak 引用。 思路是
声明一个 WeakContainer 类对真实的属性对象进行 weak 属性引用 - 通过
OBJC_ASSOCIATION_RETAIN_NONATOMIC
策略对WeakContainer
进行retain association
- 这样在 get 关联属性对象时由于 WeakContainer 对真是属性对象的 weak 引用,会返回 nil 而不是野指针
实现WeakContainer
@interface XWeakObjectContainer : NSObject
@property (nonatomic, readonly, weak) id weakObject;
- (instancetype)initWithWeakObject:(id)object;
@end
@implementation XWeakObjectContainer
- (instancetype)initWithWeakObject:(id)object {
self = [super init];
if (self) {
_weakObject = object;
}
return self;
}
@end
从而变相地实现 weak association 如下:
@implementation NSObject (XAssociate)
- (void)setXProperty:(id)xProperty {
XWeakObjectContainer *container = [[XWeakObjectContainer alloc] initWithWeakObject:xProperty];
objc_setAssociatedObject(self, @selector(xProperty), container, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (id)xProperty {
XWeakObjectContainer *container = objc_getAssociatedObject(self, _cmd);
return container.weakObject;
}
@end
备注
虽然 retain 了一个 WeakContainer,但是 WeakContainer 最终会随着属性的持有对象一起销毁,不存在泄露。