Library Project里面使用Case语句判断R.id值报错。case expressions must be constant expressions

原文地址：http://blog.csdn.net/wchinaw/article/details/7325641

在一般的Android项目里R里面的资源声明看起来是这样的：

public static final int ...

但是在ADT14之后，声明是这样的

public static int ..

所有case语句换成if 就可以了

Non-constant Fields in Case Labels

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.

However, it has one impact on the source code of the library. Code of the following form will no longer compile:

 

int id = view.getId();

switch (id) {

case R.id.button1:

        action1();

        break;

    case R.id.button2:

        action2();

        break;

    case R.id.button3:

        action3();

        break;

}

That's because the switch statement requires all the case labels, such as
R.id.button1, to be constant at compile time (such that the values can be directly
copied into the .class files).

The solution for this is simple: Convert the switch statement into
an if-else statement. Fortunately, this is very easy in Eclipse. Just
place the caret on the switch keyword, and press Ctrl-1 (or Cmd-1 on
Mac):

In the above scenario, it will turn the switch statement into this:

 

int id = view.getId();

if (id == R.id.button1) {

    action1();

} else if (id == R.id.button2) {

    action2();

} else if (id == R.id.button3) {

    action3();

}

This is typically in UI code and the performance impact is negligible.

We have a detector which finds these errors (non-constant case
labels referencing an R field) and provides a brief explanation of the
problem (and points to this page for more information.)

More information about the automatic detection.

P.S. If your switch statement looks like this:

then you end up with an inefficient if/else chain where each
if check repeats the view.getId() call. Just extract this
expression first (using the "Extract Local Variable" refactoring
keystroke), then convert the switch statement.