无法在自定义上下文包装器注入中获取共享首选项

Question

我试图将用户所需的语言存储在共享首选项中,并使用它来通过此答案设置应用程序的语言.

我有相同的MyConxtextWrapper文件作为答案,但已将包装器注入更改为:

    @Override
    protected void attachBaseContext(Context newBase) {
        SharedPreferences sharedPref = getSharedPreferences("userLang", Context.MODE_PRIVATE);
        String lang = sharedPref.getString("lang", "");


        super.attachBaseContext(MyContextWrapper.wrap(newBase,lang));
    }
}

这给了我错误:

04-01 18:35:25.648 25032-25032/com.example.c1629177.cymruniapp E/AndroidRuntime: FATAL EXCEPTION: main
 Process: com.example.c1629177.cymruniapp, PID: 25032
 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.c1629177.cymruniapp/com.example.c1629177.cymruniapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
     at android.app.ActivityThread.-wrap11(ActivityThread.java)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
     at android.os.Handler.dispatchMessage(Handler.java:102)
     at android.os.Looper.loop(Looper.java:148)
     at android.app.ActivityThread.main(ActivityThread.java:5417)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference
     at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:171)
     at com.example.c1629177.cymruniapp.MainActivity.attachBaseContext(MainActivity.java:194)
     at android.app.Activity.attach(Activity.java:6175)
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2350)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:148) 
     at android.app.ActivityThread.main(ActivityThread.java:5417) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

如何将共享首选项值"lang"传递给该行super.attachBaseContext(MyContextWrapper.wrap(newBase,lang));？

Answer 1

您将覆盖attachBaseContext()用于将基本上下文附加到包装器的内容.在您调用getSharedPreferences()基本上下文的那一刻尚未设置,因此您正在尝试获取NullPointerException.

因此,直接在newBase上下文中调用此方法:

@Override
protected void attachBaseContext(Context newBase) {
    SharedPreferences sharedPref = newBase.getSharedPreferences("userLang", Context.MODE_PRIVATE);
    String lang = sharedPref.getString("lang", "");
    super.attachBaseContext(MyContextWrapper.wrap(newBase, lang));
}

并且可能使用该语言的有用默认值; 而不是"".