也许有不少同学对ViewStub还比较陌生，首先来看看ViewStub在官方文档里是怎么介绍的：
A ViewStub is an invisible, zero-sized View that can be used to lazily inflate layout resources at runtime. When a ViewStub is made visible, or when inflate() is invoked, the layout resource is inflated. The ViewStub then replaces itself in its parent with the inflated View or Views. Therefore, the ViewStub exists in the view hierarchy until setVisibility(int) or inflate() is invoked. The inflated View is added to the ViewStub’s parent with the ViewStub’s layout parameters.
大致意思是：ViewStub是一个不可见的，能在运行期间延迟加载的大小为0的View，它直接继承于View。当对一个ViewStub调用 inflate()方法或设置它可见时，系统会加载在ViewStub标签中引入的我们自己定义的View，然后填充在父布局当中。也就是说，在对 ViewStub调用inflate()方法或设置visible之前，它是不占用布局空间和系统资源的。