layout中控件属性中的问号的意思
Notice the use of the at-symbol (@) and the question-mark (?) to reference resources. The at-symbol indicates that we're referencing a resource previously defined elsewhere (which may be from this project or from the Android framework). The question-mark indicates that we're referencing a resource value in the currently loaded theme.
大概意思是：@和？都是用来引用其他已经定义好的资源的。@引用的是之前定义好的资源当前项目或者android的framework里。而？则是引用的当前加载的样式文件里。意思就是说你在xml里某行定义了一个资源，在下面某行需要引用这个资源时用？即可
http://lhdst-163-com.iteye.com/blog/1175279
