layout布局动画不生效问题

动画不生效调用方式如下：

    
    [UIView animateWithDuration:1 animations:^{
        self.constraint.constant = height;
        [self.view layoutIfNeeded];
    }completion:^(BOOL finished) {
        
    }];

修改方式如下：

[UIView animateWithDuration:1 animations:^{
        self.constraint.constant = -height;
        [self.view layoutIfNeeded];
    }completion:^(BOOL finished) {
        
    }];