对NSObject写一个分类：
#import <Foundation/Foundation.h>
@interface NSObject (FMObserverHelper)
- (void)fm_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;
@end
// 对象被释放之前, 会调用dealloc方法, 其持有的实例变量也会被释放.
// 在监听注册时, 为self和Observer关联个临时对象, 当两者在释放实例变量时, 借助这个时机, 在临时对象的dealloc方法中, 移除Observer
// self在被释放之前, 会先释放其持有的关联属性, self并未完全释放, 可在临时对象中target却成了nil.
// weak: 持有者不会对目标进行retain, 当目标销毁时, 持有者的实例变量会被置空
// unsafe_unretained: 持有者不会对目标进行retain, 当目标释放后, 持有者的实例变量还会依然指向之前的内存空间(野指针)
// 如果Observer提前释放,而添加关联属性, 两者还不能同时持有临时对象, 否则临时对象也不会及时的释放,既然一个不行, 那就各自关联一个.
// 两个关联属性释放的同时, 进行了两次观察移除的操作. 为避免这个问题, 需要判断weak引用的实例变量factor是否为空即可
#import "NSObject+FMObserverHelper.h"
#import <objc/runtime.h>
@interface FMObserverHelper : NSObject
@property (nonatomic, unsafe_unretained) id target;
@property (nonatomic, unsafe_unretained) id observer;
@property (nonatomic, strong) NSString * keyPath;
@property (nonatomic, weak) FMObserverHelper * factor;
@end
@implementation FMObserverHelper
- (void)dealloc {
if ( _factor ) {
[_target removeObserver:_observer forKeyPath:_keyPath];
}
}
@end
@implementation NSObject (FMObserverHelper)
- (void)fm_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath {
[self addObserver:observer forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:nil];
FMObserverHelper * helper = [FMObserverHelper new];
FMObserverHelper * sub = [FMObserverHelper new];
sub.target = helper.target = self;
sub.observer = helper.observer = observer;
sub.keyPath = helper.keyPath = keyPath;
helper.factor = sub;
sub.factor = helper;
const char * helpeKey = [NSString stringWithFormat:@"%zd", [observer hash]].UTF8String;
objc_setAssociatedObject(self, helpeKey, helper, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
objc_setAssociatedObject(observer, helpeKey, sub, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end