许多UI开发的同学会对 android:id="@+id/my_button"的含义有所疑问，细心的同学会发现官方SDK是这样解释的，
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The at-symbol (@) at the beginning of the string indicates that the XML parser should parse and expand the rest of the ID string and identify it as an ID resource. The plus-symbol (+) means that this is a new resource name that must be created and added
to our resources (in the R.java file). There are a number of other ID resources that are offered by the Android framework. When referencing an Android resource ID, you do not need the plus-symbol, but
must add the android package namespace, like so:
android:id="@android:id/empty"
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意思也就是说 @符号要通知xml要转换@后面的字符串为ID资源，而+号表示这是要创建一个新的资源名字，并且添加到R.java文件中。
当你要引用已有的资源的时候 可以直接写成 @android:id/empty