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From: http://blog.163.com/fuxiaohui@126/blog/static/131745826201196104455203/

什么是Notification

    这个要求其实也很容易实现. 每个运行中的application都有一个NSNotificationCenter的成员变量,它的功能就类似公共栏. 对象注册关注某个确定的notification(如果有人捡到一只小狗,就去告诉我). 我们把这些注册对象叫做observer. 其它的一些对象会给center发送notifications(我捡到了一只小狗). center将该notifications转发给所有注册对该notification感兴趣的对象. 我们把这些发送notification的对象叫做 poster

很多的标准Cocoa类会发送notifications: 在改变size的时候,Window会发送notification; 选择table view中的一行时,table view会发送notification;我们可以在在线帮助文档中查看到标准cocoa对象发送的notification

在对象释放前,我们必须从notification center移除我们注册的observer. 一般我们在dealloc方法中做这件事

NSNotification类

提供给observer的信息包裹. notification对象有两个重要的成员变量: name 和 object.

- (NSString *)name;

- (id)object;

- (NSDictionary *)userInfo;我们想要notification对象传递更多的信息

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

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

NSNotificationCenter类

+ (id)defaultCenter;返回notification center [类方法,返回全局对象, 单件模式.cocoa的很多的全局对象都是通过类似方法实现]

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

 如果notificationName为nil. 那么notification center将anObject发送的所有notification转发给observer

. 如果anObject为nil.那么notification center将所有名字为notificationName的notification转发给observer

- (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;

NSNotificationCenter 的使用方法举例（虚拟键盘的显示隐藏）

1. 定义一个方法

update

2.订阅通知

 [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(update) name:@"update" object:nil]

3. 在要发出通知消息的地方

[[NSNotificationCenter defaultCenter] postNotificationName:@"update" object:nil];

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虚拟键盘显示和消失的通知

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWasShown:) name:UIKeyboardDidShowNotification

object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWasHidden:) name:UIKeyboardDidHideNotification

object:nil];

- (void)keyboardWasShown:(NSNotification *) aNotification{

if(keyboardShown)return;

NSDictionary *info = [aNotification userInfo];//获取通知信息

//get the size of the keyboard.

NSValue *aValue = [info objectForKey:UIKeyboardFrameBeginUserInfoKey];

CGSize keyboardSize = [aValue CGRectValue].size;

//Resize the scroll view

CGRect viewFrame = [scrollView frame];

viewFrame.size.height -= keyboardSize.height;

//Scroll the active text field into view

CGRect textFieldRect = [activeField frame];

[scrollView scrollRectToVisible:textFieldRect animated:YES];

keyboardShown = YES;

}

//Called when the UIKeyboardDidHideNotification is sent

- (void)keyboardWasHidden:(NSNotification *) aNotification{

NSDictionary *info = [aNotification userInfo];

//Get the size of the keyboard.

NSValue *aValue = [info objectForKey:UIKeyboardFrameEndUserInfoKey];

CGSize keyboardSize = [aValue CGRectValue].size;

//Reset the height of the scroll view to its original value

CGRect viewFrame = [scrollView Frame];

viewFrame.size.height += keyboardSize.height;

scrollView.frame = viewFrame;

keyboardShown = NO;

}