本篇内容介绍了“Objective-C如何使用KVO观察属性值变化”的有关知识,在实际案例的操作过程中,不少人都会遇到这样的困境,接下来就让小编带领大家学习一下如何处理这些情况吧!希望大家仔细阅读,能够学有所成!
KVO
是苹果为我们提供的一套强大的机制,用于观察属性值的变化,但是大家在日常开发中想必多少也感受到了使用上的一些不便利,比如:
添加观察者和移除观察者的次数需要一一对应,否则会 Crash
。
添加观察者和接受到属性变更通知的位置是分开的,不利于判断上下文。
多次对同一个属性值进行观察,会触发多次回调,影响业务逻辑。
为了解决上述三个问题,业界提出了一些方便开发者的开源方案,我们一起来看一下。
KVOController
建立在 Cocoa
久经考验的 KVO
实现之上。它提供了一个简单、现代的 API
,也是线程安全的。好处包括:
使用 blocks
、custom actions
或 NSKeyValueObserving
回调。
观察者移除没有异常。
控制器 dealloc
时隐式移除观察者。
具有防止观察者复活的特殊保护的线程安全。
其使用方式也很简单:
// create KVO controller with observer FBKVOController *KVOController = [FBKVOController controllerWithObserver:self]; self.KVOController = KVOController; // observe clock date property [self.KVOController observe:clock keyPath:@"date" options:NSKeyValueObservingOptionInitial|NSKeyValueObservingOptionNew block:^(ClockView *clockView, Clock *clock, NSDictionary *change) { // update clock view with new value clockView.date = change[NSKeyValueChangeNewKey]; }];
同时,KVOController
还提供了分类,通过关联引用自动帮你创建了 KVOController
框架,方便我们使用:
[self.KVOController observe:clock keyPath:@"date" options:NSKeyValueObservingOptionInitial|NSKeyValueObservingOptionNew action:@selector(updateClockWithDateChange:)];
我们来简单看一下 KVOController
是怎么做的:
- (instancetype)initWithObserver:(nullable id)observer retainObserved:(BOOL)retainObserved { self = [super init]; if (nil != self) { _observer = observer; NSPointerFunctionsOptions keyOptions = retainObserved ? NSPointerFunctionsStrongMemory|NSPointerFunctionsObjectPointerPersonality : NSPointerFunctionsWeakMemory|NSPointerFunctionsObjectPointerPersonality; _objectInfosMap = [[NSMapTable alloc] initWithKeyOptions:keyOptions valueOptions:NSPointerFunctionsStrongMemory|NSPointerFunctionsObjectPersonality capacity:0]; pthread_mutex_init(&_lock, NULL); } return self; }
KVOController
分为两种:强引用和弱引用,其中强引用会在使用时持有被观察的对象,反之弱引用则不会。所以在初始化的时候,会创建一个 objectInfosMap
,这个是 NSMapTable
,支持弱引用容器。同时会创建一个锁。
注册观察者的时候的代码如下:
- (void)observe:(nullable id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block { NSAssert(0 != keyPath.length && NULL != block, @"missing required parameters observe:%@ keyPath:%@ block:%p", object, keyPath, block); if (nil == object || 0 == keyPath.length || NULL == block) { return; } // create info _FBKVOInfo *info = [[_FBKVOInfo alloc] initWithController:self keyPath:keyPath options:options block:block]; // observe object with info [self _observe:object info:info]; }
通过创建 _FBKVOInfo
对象,来实现对观察者信息的封装,算是一个模型类,这个内部类的初始化方法如下:
- (instancetype)initWithController:(FBKVOController *)controller keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(FBKVONotificationBlock)block { return [self initWithController:controller keyPath:keyPath options:options block:block action:NULL context:NULL]; } - (instancetype)initWithController:(FBKVOController *)controller keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(nullable FBKVONotificationBlock)block action:(nullable SEL)action context:(nullable void *)context { self = [super init]; if (nil != self) { _controller = controller; _block = [block copy]; _keyPath = [keyPath copy]; _options = options; _action = action; _context = context; } return self; }
接下来会将观察者的信息存储到 KVOController
创建时初始化的 NSMapTable
中:
- (void)_observe:(id)object info:(_FBKVOInfo *)info { // lock pthread_mutex_lock(&_lock); NSMutableSet *infos = [_objectInfosMap objectForKey:object]; // check for info existence _FBKVOInfo *existingInfo = [infos member:info]; if (nil != existingInfo) { // observation info already exists; do not observe it again // unlock and return pthread_mutex_unlock(&_lock); return; } // lazilly create set of infos if (nil == infos) { infos = [NSMutableSet set]; [_objectInfosMap setObject:infos forKey:object]; } // add info and oberve [infos addObject:info]; // unlock prior to callout pthread_mutex_unlock(&_lock); [[_FBKVOSharedController sharedController] observe:object info:info]; }
objectInfosMap
是一个 NSMapTable
对象,使用被观察的对象 object
作为 key
, NSMutableSet
作为 value
,如果已经有 info
存在了,不会进行二次观察。集合存储自定义对象需要判断其 hash
值,_FBKVOInfo
的 hash
方法实现如下:
- (NSUInteger)hash { return [_keyPath hash]; } - (BOOL)isEqual:(id)object { if (nil == object) { return NO; } if (self == object) { return YES; } if (![object isKindOfClass:[self class]]) { return NO; } return [_keyPath isEqualToString:((_FBKVOInfo *)object)->_keyPath]; }
也就是说,观察者、被观察者和 keyPath
构成了观察的唯一性。
接下来来看 _FBKVOSharedController
如何进行的观察:
- (void)observe:(id)object info:(nullable _FBKVOInfo *)info { if (nil == info) { return; } // register info pthread_mutex_lock(&_mutex); [_infos addObject:info]; pthread_mutex_unlock(&_mutex); // add observer [object addObserver:self forKeyPath:info->_keyPath options:info->_options context:(void *)info]; if (info->_state == _FBKVOInfoStateInitial) { info->_state = _FBKVOInfoStateObserving; } else if (info->_state == _FBKVOInfoStateNotObserving) { // this could happen when `NSKeyValueObservingOptionInitial` is one of the NSKeyValueObservingOptions, // and the observer is unregistered within the callback block. // at this time the object has been registered as an observer (in Foundation KVO), // so we can safely unobserve it. [object removeObserver:self forKeyPath:info->_keyPath context:(void *)info]; } }
_FBKVOSharedController
会将 _FBKVOInfo
存储到一个 NSHashTable
对象中,并对其进行 KVO
。
在接受到回调时的处理如下所示:
- (void)observeValueForKeyPath:(nullable NSString *)keyPath ofObject:(nullable id)object change:(nullable NSDictionary<NSString *, id> *)change context:(nullable void *)context { NSAssert(context, @"missing context keyPath:%@ object:%@ change:%@", keyPath, object, change); _FBKVOInfo *info; { // lookup context in registered infos, taking out a strong reference only if it exists pthread_mutex_lock(&_mutex); info = [_infos member:(__bridge id)context]; pthread_mutex_unlock(&_mutex); } if (nil != info) { // take strong reference to controller FBKVOController *controller = info->_controller; if (nil != controller) { // take strong reference to observer id observer = controller.observer; if (nil != observer) { // dispatch custom block or action, fall back to default action if (info->_block) { NSDictionary<NSString *, id> *changeWithKeyPath = change; // add the keyPath to the change dictionary for clarity when mulitple keyPaths are being observed if (keyPath) { NSMutableDictionary<NSString *, id> *mChange = [NSMutableDictionary dictionaryWithObject:keyPath forKey:FBKVONotificationKeyPathKey]; [mChange addEntriesFromDictionary:change]; changeWithKeyPath = [mChange copy]; } info->_block(observer, object, changeWithKeyPath); } else if (info->_action) { #pragma clang diagnostic push #pragma clang diagnostic ignored "-Warc-performSelector-leaks" [observer performSelector:info->_action withObject:change withObject:object]; #pragma clang diagnostic pop } else { [observer observeValueForKeyPath:keyPath ofObject:object change:change context:info->_context]; } } } } }
就是根据在 _FBKVOInfo
中存储的信息,进行相应的回调。
在持有 KVOController
的对象被销毁的时候,KVOController
也会相应的取消对所有观察对象的 KVO
防止出现 Crash
:
- (void)dealloc { [self unobserveAll]; pthread_mutex_destroy(&_lock); } - (void)unobserveAll { [self _unobserveAll]; } - (void)_unobserveAll { // lock pthread_mutex_lock(&_lock); NSMapTable *objectInfoMaps = [_objectInfosMap copy]; // clear table and map [_objectInfosMap removeAllObjects]; // unlock pthread_mutex_unlock(&_lock); _FBKVOSharedController *shareController = [_FBKVOSharedController sharedController]; for (id object in objectInfoMaps) { // unobserve each registered object and infos NSSet *infos = [objectInfoMaps objectForKey:object]; [shareController unobserve:object infos:infos]; } }
需要注意的是,使用 KVOController
观察自身属性的时候,会出现内存泄露的情况,这种情况下请记得使用 KVOControllerNonRetaining
来进行观察,同时在观察者 dealloc 的时候,调用 unobserveAll
方法。
很多时候是否引入一个第三方库不是我们业务开发能决定的,而你又想在开发时安全方便的使用 KVO
,你可以参考 YYCategories
里提供的方案来做,使用方法如下:
[self.person addObserverBlockForKeyPath:@"age" block:^(id _Nonnull obj, id _Nonnull oldVal, id _Nonnull newVal) { NSLog(@"oldVal: %@, newVal: %@", oldVal, newVal); }];
其实现原理也很简单,通过关联对象设置一个 NSMutableDictionary
,这个字典以 keyPath
为 key
,与这个 key
有关的所有 block
组成的可变数组为 value
。
// 添加 `KVO` - (void)addObserverBlockForKeyPath:(NSString *)keyPath block:(void (^)(__weak id obj, id oldVal, id newVal))block { if (!keyPath || !block) return; _YYNSObjectKVOBlockTarget *target = [[_YYNSObjectKVOBlockTarget alloc] initWithBlock:block]; NSMutableDictionary *dic = [self _yy_allNSObjectObserverBlocks]; NSMutableArray *arr = dic[keyPath]; if (!arr) { arr = [NSMutableArray new]; dic[keyPath] = arr; } [arr addObject:target]; [self addObserver:target forKeyPath:keyPath options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL]; } // 根据 `keyPath` 移除 `KVO` - (void)removeObserverBlocksForKeyPath:(NSString *)keyPath { if (!keyPath) return; NSMutableDictionary *dic = [self _yy_allNSObjectObserverBlocks]; NSMutableArray *arr = dic[keyPath]; [arr enumerateObjectsUsingBlock: ^(id obj, NSUInteger idx, BOOL *stop) { [self removeObserver:obj forKeyPath:keyPath]; }]; [dic removeObjectForKey:keyPath]; } // 移除 `KVO` - (void)removeObserverBlocks { NSMutableDictionary *dic = [self _yy_allNSObjectObserverBlocks]; [dic enumerateKeysAndObjectsUsingBlock: ^(NSString *key, NSArray *arr, BOOL *stop) { [arr enumerateObjectsUsingBlock: ^(id obj, NSUInteger idx, BOOL *stop) { [self removeObserver:obj forKeyPath:key]; }]; }]; [dic removeAllObjects]; } // 获取当前注册的所有 `KVO` `Block` - (NSMutableDictionary *)_yy_allNSObjectObserverBlocks { NSMutableDictionary *targets = objc_getAssociatedObject(self, &block_key); if (!targets) { targets = [NSMutableDictionary new]; objc_setAssociatedObject(self, &block_key, targets, OBJC_ASSOCIATION_RETAIN_NONATOMIC); } return targets; }
而通知的回调则是放在 _YYNSObjectKVOBlockTarget
中的:
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { if (!self.block) return; BOOL isPrior = [[change objectForKey:NSKeyValueChangeNotificationIsPriorKey] boolValue]; if (isPrior) return; NSKeyValueChange changeKind = [[change objectForKey:NSKeyValueChangeKindKey] integerValue]; if (changeKind != NSKeyValueChangeSetting) return; id oldVal = [change objectForKey:NSKeyValueChangeOldKey]; if (oldVal == [NSNull null]) oldVal = nil; id newVal = [change objectForKey:NSKeyValueChangeNewKey]; if (newVal == [NSNull null]) newVal = nil; self.block(object, oldVal, newVal); }
不过从源码上看,还是需要自己在 dealloc
的时候移除观察者的,不过这种方案的好处是可以多次监听同一个 keyPath
,实现真正的一对多(虽然好像没啥荷包蛋用)。
“Objective-C如何使用KVO观察属性值变化”的内容就介绍到这里了,感谢大家的阅读。如果想了解更多行业相关的知识可以关注亿速云网站,小编将为大家输出更多高质量的实用文章!
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