关于iOS10动画

Question

我正在编写登录视图，当键盘显示时，背景将与动画移动相同的距离，但我发现一个我无法理解的问题，当动画开始时，在顶部背景将显示一个白色区域，白色区域只是动画结束时消失的区域。

这是我的代码

- (void)keyboardWillShow:(NSNotification *)notification{
     CGRect keyboardBounds;
     [[notification.userInfo valueForKey:UIKeyboardFrameEndUserInfoKey] getValue:&keyboardBounds];
     NSNumber * duration = [notification.userInfo objectForKey:UIKeyboardAnimationDurationUserInfoKey];
     NSNumber * curve = [notification.userInfo objectForKey:UIKeyboardAnimationCurveUserInfoKey];
     keyboardBounds = [self.view convertRect:keyboardBounds toView:nil];
     CGRect viewFrame = self.view.frame;
     viewFrame.origin.y = 0 - keyboardBounds.size.height;
     [UIView beginAnimations:nil context:nil];
     [UIView setAnimationBeginsFromCurrentState:YES];
     [UIView setAnimationDuration:[duration doubleValue]];
     [UIView setAnimationCurve:[curve intValue]];
     [UIView setAnimationDelegate:self];
     self.bgView.frame = viewFrame;
     [UIView commitAnimations];
}

提前致谢！