根据这个问题，我正在尝试将当前项目从 SharedPreferences 迁移到 dataStore 以存储用户选择的布局值，问题在于读取值，我得到了 NPE，首先这是我的代码
内部DataStoreRepository类
public class Utils {
/*
* Some unrelated codes
*/
public static class DataStoreRepository {
RxDataStore<Preferences> dataStore;
public DataStoreRepository(Context context) {
dataStore =
new RxPreferenceDataStoreBuilder(Objects.requireNonNull(context), /*name=*/ "settings").build();
}
public Preferences.Key<String> RECYCLER_VIEW_LAYOUT_KEY;
public void saveValue(String keyName, String value) {
RECYCLER_VIEW_LAYOUT_KEY = PreferencesKeys.stringKey(keyName);
dataStore.updateDataAsync(prefsIn -> {
MutablePreferences mutablePreferences = prefsIn.toMutablePreferences();
String currentKey = prefsIn.get(RECYCLER_VIEW_LAYOUT_KEY);
if (currentKey == null) {
saveValue(keyName, value);
}
mutablePreferences.set(RECYCLER_VIEW_LAYOUT_KEY,
currentKey != null ? value : "cardLayout");
return Single.just(mutablePreferences);
}).subscribe();
// The update is completed once updateResult is completed.
}
public Flowable<String> readLayoutFlow =
dataStore.data().map(prefs -> prefs.get(RECYCLER_VIEW_LAYOUT_KEY));
}
}
我在这样的片段中使用它
首先，我阅读了可流动的
private Utils.DataStoreRepository dataStoreRepository;
private String layout2;
public View onCreateView(@NonNull LayoutInflater inflater,
ViewGroup container, Bundle savedInstanceState) {
dataStoreRepository = new Utils.DataStoreRepository(requireContext());
dataStoreRepository.readLayoutFlow.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new FlowableSubscriber<String>() {
@Override
public void onSubscribe(@io.reactivex.rxjava3.annotations.NonNull Subscription s) {
}
@Override
public void onNext(String layout) {
layout2 = layout;
}
@Override
public void onError(Throwable t) {
Log.e(TAG, "onError: " + t.getMessage());
}
@Override
public void onComplete() {
}
});
然后在同一个类中，我将值存储在方法上changeAndSaveLayout()
private void changeAndSaveLayout() {
android.app.AlertDialog.Builder builder
= new android.app.AlertDialog.Builder(getContext());
builder.setTitle(getString(R.string.choose_layout));
String[] recyclerViewLayouts = getResources().getStringArray(R.array.RecyclerViewLayouts);
// SharedPreferences.Editor editor = sharedPreferences.edit();
builder.setItems(recyclerViewLayouts, (dialog, index) -> {
switch (index) {
case 0: // Card List Layout
adapter.setViewType(0);
binding.homeRecyclerView.setLayoutManager(layoutManager);
binding.homeRecyclerView.setAdapter(adapter);
// editor.putString("recyclerViewLayout", "cardLayout");
// editor.apply();
dataStoreRepository.saveValue("recyclerViewLayout","cardLayout");
break;
case 1: // Cards Magazine Layout
adapter.setViewType(1);
binding.homeRecyclerView.setLayoutManager(layoutManager);
binding.homeRecyclerView.setAdapter(adapter);
// editor.putString("recyclerViewLayout", "cardMagazineLayout");
// editor.apply();
dataStoreRepository.saveValue("recyclerViewLayout","cardMagazineLayout");
break;
case 2: // PostTitle Layout
adapter.setViewType(2);
binding.homeRecyclerView.setLayoutManager(titleLayoutManager);
binding.homeRecyclerView.setAdapter(adapter);
// editor.putString("recyclerViewLayout", "titleLayout");
// editor.apply();
dataStoreRepository.saveValue("recyclerViewLayout","titleLayout");
break;
case 3: //Grid Layout
adapter.setViewType(3);
binding.homeRecyclerView.setLayoutManager(gridLayoutManager);
binding.homeRecyclerView.setAdapter(adapter);
// editor.putString("recyclerViewLayout", "gridLayout");
// editor.apply();
dataStoreRepository.saveValue("recyclerViewLayout","gridLayout");
}
});
android.app.AlertDialog alertDialog = builder.create();
alertDialog.show();
}
当我运行时，我得到了这个 NPE，它是关于读取值readLayoutFlow的，我试图在这样的方法之外创建密钥
public Preferences.Key<String> RECYCLER_VIEW_LAYOUT_KEY = PreferencesKeys.stringKey("recyclerViewLayout");
并在 saveValue() 中像这样更改它的值，但它也不起作用
RECYCLER_VIEW_LAYOUT_KEY.to(keyName);
NPE的输出
Process: com.blogspot.abtallaldigital, PID: 9184
java.lang.NullPointerException: Attempt to invoke virtual method 'io.reactivex.rxjava3.core.Flowable androidx.datastore.rxjava3.RxDataStore.data()' on a null object reference
at com.blogspot.abtallaldigital.utils.Utils$DataStoreRepository.<init>(Utils.java:4)
at com.blogspot.abtallaldigital.ui.home.HomeFragment.onCreateView(HomeFragment.java:10)
at androidx.fragment.app.Fragment.performCreateView(Fragment.java:4)
at androidx.fragment.app.FragmentStateManager.f(FragmentStateManager.java:15)
at androidx.fragment.app.FragmentStateManager.l(FragmentStateManager.java:20)
at androidx.fragment.app.FragmentStore.s(FragmentStore.java:3)
at androidx.fragment.app.FragmentManager.h0(FragmentManager.java:6)
at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3)
at androidx.fragment.app.FragmentManager.G(FragmentManager.java:1)
at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:2)
at androidx.fragment.app.FragmentStateManager.f(FragmentStateManager.java:26)
at androidx.fragment.app.FragmentStateManager.l(FragmentStateManager.java:20)
at androidx.fragment.app.FragmentStore.s(FragmentStore.java:3)
at androidx.fragment.app.FragmentManager.h0(FragmentManager.java:6)
at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3)
at androidx.fragment.app.FragmentManager.m(FragmentManager.java:4)
at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:1)
at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:6)
at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:1)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1435)
at android.app.Activity.performStart(Activity.java:8018)
at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3475)
at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:221)
at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:201)
at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:173)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)