监听键盘是否在界面上

写一个单例来管理键盘的状态。
这个单例在初始化方法init种监听2个事件，分别是
UIKeyboardDidShowNotification（键盘弹出通知）和
UIKeyboardWillHideNotification （键盘消失通知 然后在相应的方法中设置一个属性就行了。

大致的实现如下：

-（id）init
{
self = [super init];
if (self)
{
NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
[center addObserver:self selector:@selector(keyboardDidShow) name:UIKeyboardDidShowNotification object:nil];
[center addObserver:self selector:@selector(keyboardDidHide) name:UIKeyboardWillHideNotification object:nil];
_keyboardIsVisible = NO;
}
return self;
}

- (void)keyboardDidShow
{
_keyboardIsVisible = YES;
}

- (void)keyboardDidHide
{
_keyboardIsVisible = NO;
}

- (BOOL)keyboardIsVisible
{
return _keyboardIsVisible;
}