iOS自動(dòng)移除KVO觀察者的實(shí)現(xiàn)方法

2020-07-26 02:35:47

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供稿：網(wǎng)友

問(wèn)題

KVO即：Key-Value Observing，直譯為：基于鍵值的觀察者。它提供一種機(jī)制，當(dāng)指定的對(duì)象的屬性被修改后，則對(duì)象就會(huì)接受到通知。簡(jiǎn)單的說(shuō)就是每次指定的被觀察的對(duì)象的屬性被修改后，KVO就會(huì)自動(dòng)通知相應(yīng)的觀察者了。

KVO的優(yōu)點(diǎn):當(dāng)有屬性改變，KVO會(huì)提供自動(dòng)的消息通知。這樣開(kāi)發(fā)人員不需要自己去實(shí)現(xiàn)這樣的方案：每次屬性改變了就發(fā)送消息通知。這是KVO機(jī)制提供的最大的優(yōu)點(diǎn)。因?yàn)檫@個(gè)方案已經(jīng)被明確定義，獲得框架級(jí)支持，可以方便地采用。開(kāi)發(fā)人員不需要添加任何代碼，不需要設(shè)計(jì)自己的觀察者模型，直接可以在工程里使用。其次，KVO的架構(gòu)非常的強(qiáng)大，可以很容易的支持多個(gè)觀察者觀察同一個(gè)屬性，以及相關(guān)的值。

但我們都知道, 使用KVO模式, 對(duì)某個(gè)屬性進(jìn)行監(jiān)聽(tīng)時(shí), Observer 需要在必要的時(shí)刻進(jìn)行移除, 否則 App 必然會(huì) Crash. 這個(gè)問(wèn)題有點(diǎn)煩人, 因?yàn)榕紶枙?huì)忘記寫(xiě)移除 Observer 的代碼...

我一直想要這樣一個(gè)效果:

只管監(jiān)聽(tīng), 并處理監(jiān)聽(tīng)方法. 不去分心, 管何時(shí)移除 Observer , 讓其能夠適時(shí)自動(dòng)處理.

所幸, 它能夠?qū)崿F(xiàn), 先預(yù)覽一下:

@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

項(xiàng)目源碼

下面來(lái)說(shuō)說(shuō)一步一步的實(shí)現(xiàn)吧:

初步思路實(shí)現(xiàn):

我們都知道, 對(duì)象被釋放之前, 會(huì)調(diào)用dealloc方法, 其持有的實(shí)例變量也會(huì)被釋放.

我就這樣想, 在監(jiān)聽(tīng)注冊(cè)時(shí), 為self和Observer關(guān)聯(lián)個(gè)臨時(shí)對(duì)象, 當(dāng)兩者在釋放實(shí)例變量時(shí), 我借助這個(gè)時(shí)機(jī), 在臨時(shí)對(duì)象的dealloc方法中, 移除Observer就行了.

想法很好, 可總不能每個(gè)類里都加一個(gè)臨時(shí)對(duì)象的屬性吧. 那如何在不改變?cè)蓄惖那闆r下, 為其關(guān)聯(lián)一個(gè)臨時(shí)對(duì)象呢?

關(guān)聯(lián)屬性

不改變?cè)蓄? 這時(shí)候肯定是要用Category了, 系統(tǒng)框架里面有很多的分類, 并且有很多的關(guān)聯(lián)屬性, 如下圖 UIView 頭文件第180行:

依照上圖, 我們先看一個(gè)示例, 為NSObject的添加一個(gè)Category, 并添加了一個(gè)property, 在.m中實(shí)現(xiàn)了它的setter和getter方法.

#import <objc/message.h>@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

很明確, objc_setAssociatedObject 便是關(guān)聯(lián)屬性的setter方法, 而objc_getAssociatedObject便是關(guān)聯(lián)屬性的getter方法. 最需要關(guān)注的就是setter方法, 因?yàn)槲覀円脕?lái)添加關(guān)聯(lián)屬性對(duì)象.

初步思路探索

初步嘗試:

既然屬性可以隨時(shí)使用objc_setAssociatedObject關(guān)聯(lián)了, 那我就嘗試先為self關(guān)聯(lián)一個(gè)臨時(shí)對(duì)象, 在其dealloc中, 將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; // 關(guān)聯(lián) objc_setAssociatedObject(_xiaoM, helpeKey, helper_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}

于是, 美滋滋的運(yùn)行了一下程序, 當(dāng)將_xiaoM 置為 nil 時(shí), 砰 App Crash......

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

分析: 臨時(shí)對(duì)象的dealloc, 確確實(shí)實(shí)的跑了. 為什么會(huì)還有registered? 于是我嘗試在臨時(shí)對(duì)象的dealloc中, 打印實(shí)例變量target, 發(fā)現(xiàn)其為nil. 好吧, 這就是Crash問(wèn)題原因!

嘗試 unsafe_unretained

通過(guò)上面操作, 我們知道self在被釋放之前, 會(huì)先釋放其持有的關(guān)聯(lián)屬性, self并未完全釋放, 可在臨時(shí)對(duì)象中target卻成了nil. 同時(shí)self還是有效的, 那如何保持不為nil呢?

我們看看OC中的兩個(gè)修飾符weak與unsafe_unretained:

weak: 持有者不會(huì)對(duì)目標(biāo)進(jìn)行retain, 當(dāng)目標(biāo)銷毀時(shí), 持有者的實(shí)例變量會(huì)被置空
unsafe_unretained: 持有者不會(huì)對(duì)目標(biāo)進(jìn)行retain, 當(dāng)目標(biāo)釋放后, 持有者的實(shí)例變量還會(huì)依然指向之前的內(nèi)存空間(野指針)

由上, unsafe_unretained很好的解決了我們的問(wèn)題. 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

再次運(yùn)行程序, 還行, 觀察者移除了.

最終實(shí)現(xiàn)

還存在的問(wèn)題

目前, 我們只是實(shí)現(xiàn)了, 如何在self釋放的時(shí)候, 移除自己身上的Observer.

但如果Observer提前釋放了呢?

而添加關(guān)聯(lián)屬性, 兩者還不能同時(shí)持有臨時(shí)對(duì)象, 否則臨時(shí)對(duì)象也不會(huì)及時(shí)的釋放.

好吧, 既然一個(gè)不行, 那就各自關(guān)聯(lián)一個(gè):

- (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; // 關(guān)聯(lián) objc_setAssociatedObject(_xiaoM, helpeKey, helper_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC); // 關(guān)聯(lián) objc_setAssociatedObject(_observer, helpeKey, sub_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}

如上, 仔細(xì)想想, 存在一個(gè)很明顯的問(wèn)題, 兩個(gè)關(guān)聯(lián)屬性釋放的同時(shí), 進(jìn)行了兩次觀察移除的操作. 為避免這個(gè)問(wèn)題, 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. 新增一個(gè) weak 變量@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; // 關(guān)聯(lián) objc_setAssociatedObject(_xiaoM, helpeKey, helper_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC); // 關(guān)聯(lián) objc_setAssociatedObject(_observer, helpeKey, sub_obj, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}

在之前的操作中, 我們知道, weak 修飾的變量, 在目標(biāo)釋放時(shí),持有者的實(shí)例變量都會(huì)自動(dòng)置為nil, 因此如上dealloc方法中, 我們只需要判斷weak引用的實(shí)例變量factor是否為空即可.

抽取

以上操作, 我們就可以解決偶爾忘記寫(xiě)移除Observer的代碼了. 現(xiàn)在只需要把實(shí)現(xiàn)抽取出來(lái), 做成一個(gè)通用的工具方法:

我新建了一個(gè)NSObject的Category, 并添加了一個(gè)方法, 如下: