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

144
作者 张明云
2014.12.26 11:58 字数 527

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

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

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