iOS消息中心与传感

/*    NSNotification.h
    Copyright (c) 1994-2012, Apple Inc. All rights reserved.
*/

#import <Foundation/NSObject.h>

@class NSString, NSDictionary, NSOperationQueue;

/****************    Notifications    ****************/

@interface NSNotification : NSObject <NSCopying, NSCoding>

- (NSString *)name;
- (id)object;
- (NSDictionary *)userInfo;

@end

@interface NSNotification (NSNotificationCreation)

+ (id)notificationWithName:(NSString *)aName object:(id)anObject;
+ (id)notificationWithName:(NSString *)aName object:(id)anObject userInfo:(NSDictionary *)aUserInfo;

@end

/****************    Notification Center    ****************/

@interface NSNotificationCenter : NSObject {
    @package
    void * __strong _impl;
    void * __strong _callback;
    void *_pad[11];
}

+ (id)defaultCenter;
    
- (void)addObserver:(id)observer selector:(SEL)aSelector name:(NSString *)aName object:(id)anObject;

- (void)postNotification:(NSNotification *)notification;
- (void)postNotificationName:(NSString *)aName object:(id)anObject;
- (void)postNotificationName:(NSString *)aName object:(id)anObject userInfo:(NSDictionary *)aUserInfo;

- (void)removeObserver:(id)observer;
- (void)removeObserver:(id)observer name:(NSString *)aName object:(id)anObject;

#if NS_BLOCKS_AVAILABLE
- (id)addObserverForName:(NSString *)name object:(id)obj queue:(NSOperationQueue *)queue usingBlock:(void (^)(NSNotification *note))block NS_AVAILABLE(10_6, 4_0);
    // The return value is retained by the system, and should be held onto by the caller in
    // order to remove the observer with removeObserver: later, to stop observation.
#endif

@end

1. 观察者observer在消息中心中关注某一事件notification，当事件发生时消息中心给“对该事件感兴趣的观察者”发送消息，即为观察者模式。

2. 要注意的是，注册事件与移除事件是对应的，不能只注册不移除，那样会造成资源泄露。因此一般在已注册的观察者类的dealloc方法中移除本类关心的事件。

3. 给消息中心发送消息是同步的，这意味着在发送消息post函数返回前，消息中心会先把消息分发给各个观察者，最后才返回到post函数。因此一般在观察者收到相应的事件响应后，如果要做非常复杂的操作，那么最好延迟调用复杂操作以使post函数可以尽快返回。

设备传感也会给消息中心发送消息

UIKIT_EXTERN NSString *const UIDeviceOrientationDidChangeNotification; // 设备旋转
UIKIT_EXTERN NSString *const UIDeviceBatteryStateDidChangeNotification   NS_AVAILABLE_IOS(3_0);　　// 电池状态
UIKIT_EXTERN NSString *const UIDeviceBatteryLevelDidChangeNotification   NS_AVAILABLE_IOS(3_0);　　// 电池电量
UIKIT_EXTERN NSString *const UIDeviceProximityStateDidChangeNotification NS_AVAILABLE_IOS(3_0);　　// 近距离传感器