key-value observing,观察者模式
观察者,观察对象属性的变化,当被观察者该属性发生变化时,观察者会接收到通知,可以在回调函数中做相应的处理
变化处理操作可以在同一个函数中进行,先前本人都会在每次修改属性值的地方调用后续操作,比较繁琐,修改的地方也比较多,现在只要在同一个函数中操作就可以
用kvo只要做监控就行,更加方便易用,减少代码逻辑
当一个控件某个属性变化需要做别的相应操作时,比较适合用kvo,只要当该属性发生变化时,会发消息给观察者,在回调函数中做相应的操作
typedef NS_OPTIONS(NSUInteger, NSKeyValueObservingOptions) { NSKeyValueObservingOptionNew = 0x01,//改变后的值 NSKeyValueObservingOptionOld = 0x02,//改变前的值 NSKeyValueObservingOptionInitial NS_ENUM_AVAILABLE(10_5, 2_0) = 0x04, //addobserving之后会马上调用observeValueForKeyPath,不会等到值改变 NSKeyValueObservingOptionPrior NS_ENUM_AVAILABLE(10_5, 2_0) = 0x08 //分2次调用。在值改变之前和值改变之后 };
NSKeyValueObservingOptionNew = 0x01,//改变后的值
NSKeyValueObservingOptionOld = 0x02,//改变前的值
这两个用到的比较多
NSObject(NSKeyValueObserving) //一旦被观察者属性发生改变,就会调用此方法后续操作在这个方法中进行 - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context;
keyPath:是被观察对象的属性,字符串表示
object:被观察对象
change:属性改变的值,字典,通过 objectForKey (key为
FOUNDATION_EXPORT NSString *const NSKeyValueChangeKindKey;
FOUNDATION_EXPORT NSString *const NSKeyValueChangeNewKey;
FOUNDATION_EXPORT NSString *const NSKeyValueChangeOldKey;
FOUNDATION_EXPORT NSString *const NSKeyValueChangeIndexesKey;
FOUNDATION_EXPORT NSString *const NSKeyValueChangeNotificationIsPriorKey NS_AVAILABLE(10_5, 2_0);
对应addobserving指定的NSKeyValueObservingOptions
)
context:需要传输的数据(void *:任意指针类型),一般传(__bridgevoid*)self 或者 nil,用户也能传别的
for example:
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { if (context == (__bridge void*)self) { if ([keyPath isEqualToString:kKeyPathForNavigationItemRightBarButtonItems]) { //取值 NSArray *rightBarButtonItems = [change objectForKey:NSKeyValueChangeNewKey]; //需要做操作 self.navigationItem.rightBarButtonItems = rightBarButtonItems; } } else { [super observeValueForKeyPath:keyPath ofObject:objectchange:changecontext:context]; } }
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NSObject(NSKeyValueObserverRegistration) - (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context; - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context NS_AVAILABLE(10_7, 5_0); - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath; NSArray(NSKeyValueObserverRegistration) - (void)addObserver:(NSObject *)observer toObjectsAtIndexes:(NSIndexSet *)indexes forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context; - (void)removeObserver:(NSObject *)observer fromObjectsAtIndexes:(NSIndexSet *)indexes forKeyPath:(NSString *)keyPath context:(void *)contextNS_AVAILABLE(10_7,5_0); - (void)removeObserver:(NSObject *)observer fromObjectsAtIndexes:(NSIndexSet *)indexes forKeyPath:(NSString *)keyPath; - (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context; - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context NS_AVAILABLE(10_7, 5_0); - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath; NSOrderedSet(NSKeyValueObserverRegistration) - (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context; - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context NS_AVAILABLE(10_7, 5_0); - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath; NSSet(NSKeyValueObserverRegistration) - (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context; - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(void *)context NS_AVAILABLE(10_7, 5_0); - (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath; NSObject(NSKeyValueObserverNotification) //这些方法都为了手动通知用到 - (void)willChangeValueForKey:(NSString *)key; - (void)didChangeValueForKey:(NSString *)key; - (void)willChange:(NSKeyValueChange)changeKind valuesAtIndexes:(NSIndexSet *)indexes forKey:(NSString *)key; - (void)didChange:(NSKeyValueChange)changeKind valuesAtIndexes:(NSIndexSet *)indexes forKey:(NSString *)key; - (void)willChangeValueForKey:(NSString *)key withSetMutation:(NSKeyValueSetMutationKind)mutationKind usingObjects:(NSSet *)objects; - (void)didChangeValueForKey:(NSString *)key withSetMutation:(NSKeyValueSetMutationKind)mutationKind usingObjects:(NSSet *)objects; NSObject(NSKeyValueObservingCustomization) + (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)keyNS_AVAILABLE(10_5,2_0); + (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key;** addObserver和removeObserver要成对出现
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**手动通知: 有两种通知观察者的方式,自动通知和手动通知。顾名思义,手动通知需要在值变化时调用 willChangeValueForKey:和didChangeValueForKey: 方法通知调用者。为求简便,我们一般使用自动通知。要使用手动通知,需要在 automaticallyNotifiesObserversForKey方法中明确告诉cocoa,哪些键值要使用手动通知:
forExample:
[self willChangeValueForKey:@"frame"]; self.frame = CGRectMake(0,0,320,100); [self didChangeValueForKey:@"frame"];
这时候就会调用
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context;
//重新实现NSObject类中的automaticallyNotifiesObserversForKey:方法,返回yes表示自动通知。 + (BOOL)automaticallyNotifiesObserversForKey:(NSString*)key { //当这两个值改变时,使用自动通知已注册过的观察者,观察者需要实现observeValueForKeyPath:ofObject:change:context:方法 if ([key isEqualToString:@"frame"]) { return NO; } return [super automaticallyNotifiesObserversForKey:key]; } 这时候frame就必须要手动通知
*手动通知一般不用,为了方便,都自动通知,所以这部分知道就可以了
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上面一些接口方法说明NSObject,NSArray,NSSet均实现了以上方法,因此我们不仅可以观察普通对象,还可以观察数组或结合类对象。
一般用的都是观察NSObject的某个属性 对NSArray进行观察是观察NSArray中每个model的属性 NSSet和NSArray差不多,只不过NSSet是无序集合