安卓运行时权限

Question

我正在尝试添加运行时权限以使应用与 API 23+ 兼容。我浏览了 documentation但是当我只包含以下行时

private final int hasWriteFilePermission = ContextCompat.checkSelfPermission(getActivity(), Manifest.permission.WRITE_EXTERNAL_STORAGE);,

我的应用程序崩溃并出现以下错误:

07-10 20:54:15.571 31479-31479/com.ibm.watson.developer_cloud.android.examples E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                             Process: com.ibm.watson.developer_cloud.android.examples, PID: 31479
                                                                                             java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ibm.watson.developer_cloud.android.examples/com.ibm.watson.developer_cloud.android.examples.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                                                                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3254)
                                                                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3350)
                                                                                                 at android.app.ActivityThread.access$1100(ActivityThread.java:222)
                                                                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795)
                                                                                                 at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                                 at android.os.Looper.loop(Looper.java:158)
                                                                                                 at android.app.ActivityThread.main(ActivityThread.java:7229)
                                                                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
                                                                                              Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                                                                                                 at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:337)
                                                                                                 at android.widget.AbsSpinner.setAdapter(AbsSpinner.java:115)
                                                                                                 at android.widget.Spinner.setAdapter(Spinner.java:554)
                                                                                                 at com.ibm.watson.developer_cloud.android.examples.MainActivity$FragmentTabSTT.addItemsOnSpinnerModels(MainActivity.java:566)
                                                                                                 at com.ibm.watson.developer_cloud.android.examples.MainActivity$FragmentTabSTT.onCreateView(MainActivity.java:190)
                                                                                                 at android.app.Fragment.performCreateView(Fragment.java:2281)
                                                                                                 at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:983)
                                                                                                 at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1163)
                                                                                                 at android.app.BackStackRecord.run(BackStackRecord.java:793)
                                                                                                 at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1552)
                                                                                                 at android.app.FragmentController.execPendingActions(FragmentController.java:325)
                                                                                                 at android.app.Activity.performStart(Activity.java:6914)
                                                                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3217)
                                                                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3350) 
                                                                                                 at android.app.ActivityThread.access$1100(ActivityThread.java:222) 
                                                                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795) 
                                                                                                 at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                                 at android.os.Looper.loop(Looper.java:158) 
                                                                                                 at android.app.ActivityThread.main(ActivityThread.java:7229) 
                                                                                                 at java.lang.reflect.Method.invoke(Native Method) 
                                                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                                                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

我的代码如下:

public static class FragmentTabSTT extends Fragment implements ISpeechToTextDelegate {
    .
    .
    .
@Override
     public void onAttach(Context context) {
         super.onAttach(context);

         Activity a;

         if (context instanceof Activity){
             a = (Activity) context;
         }

     }



     public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                  Bundle savedInstanceState) {
     .
     .
     .
     }
     .
     .
     .
        // One method that requires permission
public ArrayList<String> GetFiles(String DirectoryPath) {

         int hasReadFilesPermission = ContextCompat.checkSelfPermission(getActivity(), Manifest.permission.READ_EXTERNAL_STORAGE);

         if (hasReadFilesPermission != PackageManager.PERMISSION_GRANTED) {
             if (!shouldShowRequestPermissionRationale(permission.READ_EXTERNAL_STORAGE)) {
                 showMessageOKCancel("You must give permission to read from storage.",
                         new DialogInterface.OnClickListener() {
                             @Override
                             public void onClick(DialogInterface dialog, int which) {
                                 requestPermissions(new String[] {permission.READ_EXTERNAL_STORAGE},
                                         REQUEST_CODE_ASK_PERMISSIONS);
                             }
                         });
                 return null;
             }

             requestPermissions(new String[] {permission.READ_EXTERNAL_STORAGE},
                     REQUEST_CODE_ASK_PERMISSIONS);
             return null;
         }
         ArrayList<String> MyFiles = new ArrayList<String>();
         File f = new File(DirectoryPath);

         f.mkdirs();
         File[] files = f.listFiles();
         if (files.length == 0)
             return null;
         else {
             for (int i=0; i<files.length; i++)
                 MyFiles.add(files[i].getName());
         }

         return MyFiles;
     }
}

我应该如何检查 fragment 中的权限？是什么导致了这个错误？我该如何解决这个问题？谢谢!

Answer 1

getActivity() 在首次创建 Fragment 时返回 null(即，在调用 onAttach() 之前)。

您应该在需要之前直接检查权限，而不是将它们保存在最终类变量中。