一般我们在xml文件里面，会经常性用到 @ 与 ？ 这个两个符号，比如
android:id = "@+id/id1" sytle="?android:attr/progressBarStyleHorizontal"
下面是 对于这两个符号的一点解析
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. This is done by referring to a specific