NSNotificationCenter

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(changeNavigationBarAlpha:) name:@"changeNavigationBarAlpha" object:nil];
[[NSNotificationCenter defaultCenter] postNotificationName:@"changeNavigationBarAlpha" object:[NSString stringWithFormat:@"%f",a]];




[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(callBack1:) name:@"PosterOne" object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(callBack2:) name:@"PosterTwo" object:nil];  

-(void)callBack1:(NSNotification*)notification{  
    NSString *nameString = [notification name];  
    NSString *objectString = [notification object];  
    NSLog(@"name = %@,object = %@",nameString,objectString);  
}  
  
-(void)callBack2:(NSNotification*)notification{  
    NSString *nameString = [notification name];  
    NSString *objectString = [notification object];  
    NSDictionary *dictionary = [notification userInfo];  
    NSLog(@"name = %@,object = %@,userInfo = %@",nameString,objectString,[dictionary objectForKey:@"key"]);  
}  


//1.下面两条语句等价  
    //二者的区别是第一条语句是直接发送消息内容，第二条语句先创建一个消息，然后再发送消息  
    [[NSNotificationCenter defaultCenter] postNotificationName:@"PosterOne" object:@"This is posterone!"];  
      
//    [[NSNotificationCenter defaultCenter] postNotification:[NSNotification notificationWithName:@"PosterOne" object:@"This is posterone"]];  
      
    //2.下面两条语句等价  
    //参数：userInfo  --- Information about the the notification.  
    [[NSNotificationCenter defaultCenter] postNotificationName:@"PosterTwo" object:@"This is postertwo" userInfo:[NSDictionary dictionaryWithObject:@"value" forKey:@"key"]];  
      
//    [[NSNotificationCenter defaultCenter] postNotification:[NSNotification notificationWithName:@"PosterTwo" object:@"This is postertwo" userInfo:[NSDictionary dictionaryWithObject:@"value" forKey:@"key"]]];  
}