Issue 9656: Library projects don't support custom XML attributes for custom classes
Solution:
Upgrade to latest SDK & ADT version (fixed was released since r17) and use/apk/res-auto
Added support for custom views with custom attributes in libraries. Layouts using custom attributes must use the namespace URI
/apk/res-auto instead of the URI that includes the app package name. This URI is replaced with the app specific one at build time.
res与res-auto的区别
通常我们在布局文件中使用自定义属性的时候会这样写
xmlns:app="/apk/res/包路径"
但如果你当前工程是做为lib使用，那么你如上所写 ，会出现找不到自定义属性的错误 。
这时候你就可以 写成
xmlns:app="/apk/res-auto"