Android Library 中的资源 id 不是 final

【流程】
ButterKnife 的说明中提到使用 R2 而非 R1
StackOverflow： 必须使用 R2 的情况仅限于 library，application 仍可以使用 R
理由：library 中的 R.id 不是 final，会变化
引申：library 中不能使用 switch-case 处理 资源 id
上面的解决办法：使用 if-else
根源：Google 在 ADT14开始移除 library R 中的 final 标记
why？有可能造成 id 碰撞，例如：在一个项目中使用了多个 library 的时候

贴上官方解释：
http://tools.android.com/tips/non-constant-fields

In a regular Android project, constants in the resource R class are declared like this:
public static final int main=0x7f030004;

However, as of ADT 14, in a library project, they will be declared like this:
public static int main=0x7f030004;

In other words, the constants are not final in a library project. The reason for this is simple: When multiple library projects are combined, the actual values of the fields (which must be unique) could collide. Before ADT 14, all fields were final, so as a result, all libraries had to 【have all their resources and associated Java code recompiled along with the main project whenever they were used. This was bad for performance, since it made builds very slow. It also prevented distributing library projects that didn't include the source code, limiting the usage scope of library projects】.

The reason the fields are no longer final is that it means that the library jars can be compiled once and reused directly in other projects. As well as allowing distributing binary version of library projects (coming in r15), this makes for much faster builds.