基本原理：通過獲取鍵盤消息的開始狀態、結束狀態，以及變化周期，可以計算出具體的Y偏移，從而在相同時間裡做相同偏移量。

代碼如下：

- (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];
    CGFloat duration = [[info objectForKey:UIKeyboardAnimationDurationUserInfoKey] floatValue];
    CGRect beginKeyboardRect = [[info objectForKey:UIKeyboardFrameBeginUserInfoKey] CGRectValue];
    CGRect endKeyboardRect = [[info objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
    
    CGFloat yOffset = endKeyboardRect.origin.y - beginKeyboardRect.origin.y;
    
    CGRect inputFieldRect = self.inputview.frame;
    
    inputFieldRect.origin.y += yOffset;
    
    [UIView animateWithDuration:duration animations:^{
        self.inputview.frame = inputFieldRect;
    }];
}