在ViewModel中更改TextBlock的文本时运行动画

I am making an WPF application following MVVM pattern. I have one button and textblock. TextBlock is only shown when its text is not empty. On start of application text is empty to textblock is not shown. When i click button sample text is set and textblock is shown. And when i click again button text is set to empty and textblock hides.I am making an WPF application following MVVM p