得到键盘高度的问题

使用如下代码来调用键盘的高度。
每次重新下载APP之后的第一次，会出现yOffset被调用3次的情况，yOffset分别为  -184  -67.5 -67.5，以后就正常了，这个是怎么回事啊？效果如下图

– (void)regNotification
{
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillChangeFrame:) name:UIKeyboardWillChangeFrameNotification object:nil];
}
– (void)unregNotification
{
[[NSNotificationCenter defaultCenter] removeObserver:self name:UIKeyboardWillChangeFrameNotification object:nil];
}
#pragma mark – notification handler
– (void)keyboardWillChangeFrame:(NSNotification *)notification
{
NSDictionary *info = [notification userInfo];
CGRect beginKeyboardRect = [[info objectForKey:UIKeyboardFrameBeginUserInfoKey] CGRectValue];
CGRect endKeyboardRect = [[info objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];

CGFloat yOffset = endKeyboardRect.origin.y – beginKeyboardRect.origin.y;

NSLog(@”yOffset:%f”,yOffset);

self.view.center = CGPointMake(self.view.center.x, self.view.center.y + yOffset);

}