监听键盘

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillChangeFrame:) name:UIKeyboardWillChangeFrameNotification object:nil];

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    
    self.topic.top_cmt = self.savedTopCmt;
    self.topic.cellHeight = 0;
}

- (void)keyboardWillChangeFrame:(NSNotification *)note
{
    // 修改约束
    CGFloat keyboardY = [note.userInfo[UIKeyboardFrameEndUserInfoKey] CGRectValue].origin.y;
    CGFloat screenH = [UIScreen mainScreen].bounds.size.height;
    self.bottomMargin.constant = screenH - keyboardY;
    
    // 执行动画
    CGFloat duration = [note.userInfo[UIKeyboardAnimationDurationUserInfoKey] doubleValue];
    [UIView animateWithDuration:duration animations:^{
        [self.view layoutIfNeeded];
    }];
}