转：
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android:[email protected]，而@+id是新增加一个id
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在Dev Guide -> User interface -> Declaring Layout 里面看到的。
??? 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"