在Android library中不能使用switch-case语句访问资源ID的原因分析及解决方案

系统管理员 2022-07-26 04:30 205阅读 0赞

[**原文链接：http://www.jianshu.com/p/89687f618837**][http_www.jianshu.com_p_89687f618837]

# 原因分析： #

当我们在Android依赖库中使用switch-case语句访问资源ID时会报如下图所示的错误，报的错误是case分支后面跟的参数必须是常数，换句话说出现这个问题的原因是Android library中生成的R.java中的资源ID不是常数：

![library error][]

library error

打开library中的R.java，发现确实如此，每一个资源ID都没有被声明为final：

![library R.java][]

library R.java

但是当你打开你的主工程，在onClick、onItemClick等各种回调方法中是可以通过switch-case语句来访问资源ID的，因为在主工程的R.java中资源ID都被声明为了final常量。

project中能够通过switch-case语句正常引用资源ID：

![project right][]

project right

project中的R.java：

![project R.java][]

project R.java

# 解决方案： #

既然是由于library的R.java中的资源ID不是常量引起的，我们可以在library中通过if-else-if条件语句来引用资源ID，这样就避免了这个错误：

![library use][]

library use

# 参考资料： #

为了进一步了解问题的具体原因，在万能的StackOverflow上还真搜到了这个问题：

> 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. Therefore your code would no longer compile.
> 
> The solution for this is simple: Convert the switch statement into an if-else statement.
> 
>     public void onClick(View src) { int id = src.getId(); if (id == R.id.playbtn){ checkwificonnection(); } else if (id == R.id.stopbtn){ Log.d(TAG, "onClick: stopping srvice"); Playbutton.setImageResource(R.drawable.playbtn1); Playbutton.setVisibility(0); //visible Stopbutton.setVisibility(4); //invisible stopService(new Intent(RakistaRadio.this,myservice.class)); clearstatusbar(); timer.cancel(); Title.setText(" "); Artist.setText(" "); } else if (id == R.id.btnmenu){ openOptionsMenu(); } }
> 
> [http://tools.android.com/tips/non-constant-fields][http_tools.android.com_tips_non-constant-fields]
> 
> Tip  
> You can quickly convert a switch statement to an if-else statement using Eclipse's quick fix.
> 
> Click on the switch keyword and press Ctrl + 1 then select
> 
> Convert 'switch' to 'if-else'.

问题详见：[switch case statement error: case expressions must be constant expression][switch case statement error_ case expressions must be constant expression]

[http_www.jianshu.com_p_89687f618837]: http://www.jianshu.com/p/89687f618837
[library error]: https://img-my.csdn.net/uploads/201412/26/1419564510_7687.jpg
[library R.java]: https://img-my.csdn.net/uploads/201412/26/1419564512_3649.jpg
[project right]: https://img-my.csdn.net/uploads/201412/26/1419564512_5840.jpg
[project R.java]: https://img-my.csdn.net/uploads/201412/26/1419564513_5678.jpg
[library use]: https://img-my.csdn.net/uploads/201412/26/1419564512_5822.jpg
[http_tools.android.com_tips_non-constant-fields]: http://tools.android.com/tips/non-constant-fields
[switch case statement error_ case expressions must be constant expression]: http://stackoverflow.com/questions/9092712/switch-case-statement-error-case-expressions-must-be-constant-expression