iOS消息中心與傳感

/*    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. 觀察者observer在消息中心中關注某一事件notification，當事件發生時消息中心給“對該事件感興趣的觀察者”發送消息，即為觀察者模式。

2. 要注意的是，注冊事件與移除事件是對應的，不能只注冊不移除，那樣會造成資源泄露。因此一般在已注冊的觀察者類的dealloc方法中移除本類關心的事件。

3. 給消息中心發送消息是同步的，這意味着在發送消息post函數返回前，消息中心會先把消息分發給各個觀察者，最后才返回到post函數。因此一般在觀察者收到相應的事件響應后，如果要做非常復雜的操作，那么最好延遲調用復雜操作以使post函數可以盡快返回。

 

設備傳感也會給消息中心發送消息

UIKIT_EXTERN NSString *const UIDeviceOrientationDidChangeNotification; // 設備旋轉
UIKIT_EXTERN NSString *const UIDeviceBatteryStateDidChangeNotification   NS_AVAILABLE_IOS(3_0);　　// 電池狀態
UIKIT_EXTERN NSString *const UIDeviceBatteryLevelDidChangeNotification   NS_AVAILABLE_IOS(3_0);　　// 電池電量
UIKIT_EXTERN NSString *const UIDeviceProximityStateDidChangeNotification NS_AVAILABLE_IOS(3_0);　　// 近距離傳感器