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iOS 如何自动移除KVO观察者

百家作者：iOS开发 2017-12-18 11:54:16

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关键时刻，第一时间送达！

问题

我们都知道, 使用KVO模式, 对某个属性进行监听时, Observer 需要在必要的时刻进行移除, 否则 App 必然会 Crash. 这个问题有点烦人, 因为偶尔会忘记写移除 Observer 的代码...

我一直想要这样一个效果:

只管监听, 并处理监听方法. 不去分心, 管何时移除 Observer , 让其能够适时自动处理.

所幸, 它能够实现, 先预览一下:

@interface NSObject (SJObserverHelper)
- (void)sj_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;
@end
@interface SJObserverHelper : NSObject
@property (nonatomic, unsafe_unretained) id target;
@property (nonatomic, unsafe_unretained) id observer;
@property (nonatomic, strong) NSString *keyPath;
@property (nonatomic, weak) SJObserverHelper *factor;
@end
@implementation SJObserverHelper
- (void)dealloc {
if ( _factor ) {
[_target removeObserver:_observer forKeyPath:_keyPath];
}
}
@end
@implementation NSObject (ObserverHelper)
- (void)sj_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath {
[self addObserver:observer forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:nil];
SJObserverHelper *helper = [SJObserverHelper new];
SJObserverHelper *sub = [SJObserverHelper 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

项目源码

下面来说说一步一步的实现吧:

初步思路实现:

我们都知道, 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.

我就这样想, 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, 我借助这个时机, 在临时对象的dealloc方法中, 移除Observer就行了.

想法很好, 可总不能每个类里都加一个临时对象的属性吧. 那如何在不改变原有类的情况下, 为其关联一个临时对象呢?

关联属性

不改变原有类, 这时候肯定是要用Category了, 系统框架里面有很多的分类, 并且有很多的关联属性, 如下图 UIView 头文件第180行:

ex.png

依照上图, 我们先看一个示例, 为NSObject的添加一个Category, 并添加了一个property, 在.m中实现了它的setter和getter方法.

#import
@interface NSObject (Associate)
@property (nonatomic, strong) id tmpObj;
@end
@implementation NSObject (Associate)
static const char *testKey = "TestKey";
- (void)setTmpObj:(id)tmpObj {
// objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
objc_setAssociatedObject(self, testKey, tmpObj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (id)tmpObj {
// objc_getAssociatedObject(id object, const void *key)
return objc_getAssociatedObject(self, testKey);
}
@end

很明确, objc_setAssociatedObject 便是关联属性的setter方法, 而objc_getAssociatedObject便是关联属性的getter方法. 最需要关注的就是setter方法, 因为我们要用来添加关联属性对象.

初步思路探索

初步尝试:

既然属性可以随时使用objc_setAssociatedObject关联了, 那我就尝试先为self关联一个临时对象, 在其dealloc中, 将Observer移除.

@interface SJObserverHelper : NSObject
@property (nonatomic, weak) id target;
@property (nonatomic, weak) id observer;
@property (nonatomic, strong) NSString *keyPath;
@end

@implementation SJObserverHelper
- (void)dealloc {
[_target removeObserver:_observer forKeyPath:_keyPath];
}
@end

- (void)addObserver {
NSString *keyPath = @"name";
[_xiaoM addObserver:_observer forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:nil];

SJObserverHelper *helper_obj = [SJObserverHelper new];
helper_obj.target = _xiaoM;
helper_obj.observer = _observer;
helper_obj.keyPath = keyPath;

const char *helpeKey = [NSString stringWithFormat:@"%zd", [_observer hash]].UTF8String;
// 关联
objc_setAssociatedObject(_xiaoM, helpeKey, helper_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

于是, 美滋滋的运行了一下程序, 当将_xiaoM 置为 nil 时, 砰 App Crash......

reason: 'An instance 0x12cd1c370 of class Person was deallocated while key value observers were still registered with it.

分析: 临时对象的dealloc, 确确实实的跑了. 为什么会还有registered? 于是我尝试在临时对象的dealloc中, 打印实例变量target, 发现其为nil. 好吧, 这就是Crash问题原因!

尝试 unsafe_unretained

通过上面操作, 我们知道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. 同时self还是有效的, 那如何保持不为nil呢?

我们看看OC中的两个修饰符weak与unsafe_unretained:

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)

由上, unsafe_unretained很好的解决了我们的问题. 于是我做了如下修改:

@interface SJObserverHelper : NSObject
@property (nonatomic, unsafe_unretained) id target;
@property (nonatomic, unsafe_unretained) id observer;
@property (nonatomic, strong) NSString *keyPath;
@end

再次运行程序, 还行, 观察者移除了.

最终实现

还存在的问题

目前, 我们只是实现了, 如何在self释放的时候, 移除自己身上的Observer.

但如果Observer提前释放了呢?

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.

好吧, 既然一个不行, 那就各自关联一个:

- (void)addObserver {
.....

SJObserverHelper *helper_obj = [SJObserverHelper new];
SJObserverHelper *sub_obj = [SJObserverHelper new];

sub_obj.target = helper_obj.target = _xiaoM;
sub_obj.observer = helper_obj.observer = _observer;
sub_obj.keyPath = helper_obj.keyPath = keyPath;

const char *helpeKey = [NSString stringWithFormat:@"%zd", [_observer hash]].UTF8String;
// 关联
objc_setAssociatedObject(_xiaoM, helpeKey, helper_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
// 关联
objc_setAssociatedObject(_observer, helpeKey, sub_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

如上, 仔细想想, 存在一个很明显的问题, 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, 我又做了如下修改:

@interface SJObserverHelper : NSObject
@property (nonatomic, unsafe_unretained) id target;
@property (nonatomic, unsafe_unretained) id observer;
@property (nonatomic, strong) NSString *keyPath;
@property (nonatomic, weak) SJObserverHelper *factor; // 1. 新增一个 weak 变量
@end

@implementation SJObserverHelper
- (void)dealloc {
if ( _factor ) {
[_target removeObserver:_observer forKeyPath:_keyPath];
}
}
@end

- (void)addObserver {
.....

SJObserverHelper *helper_obj = [SJObserverHelper new];
SJObserverHelper *sub_obj = [SJObserverHelper new];

sub_obj.target = helper_obj.target = _xiaoM;
sub_obj.observer = helper_obj.observer = _observer;
sub_obj.keyPath = helper_obj.keyPath = keyPath;
// 2. 互相 weak 引用
helper_obj.factor = sub_obj;
sub_obj.factor = helper_obj;

const char *helpeKey = [NSString stringWithFormat:@"%zd", [_observer hash]].UTF8String;
// 关联
objc_setAssociatedObject(_xiaoM, helpeKey, helper_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
// 关联
objc_setAssociatedObject(_observer, helpeKey, sub_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}