我可以在 Observer 中添加 Observer 吗？我实际上尝试过，但内部观察者无法正常工作。有时其中一个工作，有时没有人工作

Question

我使用了 Android 架构组件（ViewModel、LiveData）并使用 Room 使用 Repository 从本地 Sqlite 数据库中获取数据。最外层的观察者就好了，只有内层的观察者有问题。

查看代码：

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        BranchDetailsViewModel branchDetailsViewModel = new BranchDetailsViewModel(application);

        Observer<Bank> bankObserver = new Observer<Bank>() {
            @Override
            public void onChanged(Bank bank) {
                if(bank!= null)
                    binding.tvBank.setText(bank.getBank());
            }
        };


        branchDetailsViewModel.getBranchById(branchIid).observe(this, new Observer<Branch>() {
            @Override
            public void onChanged(Branch branch) {

                branchDetailsViewModel.getBankById(branch.getBankId()).observe(getViewLifecycleOwner(), bankObserver);

                branchDetailsViewModel.getCityById(branch.getCityId()).observe(requireActivity(), new Observer<City>() {
                    @Override
                    public void onChanged(City city) {
                        if(city != null)
                        binding.tvCity.setText(city.getCity() + " cty");
                    }
                });

                branchDetailsViewModel.getDistrictById(branch.getDistrictId()).observe(requireActivity(), new Observer<District>() {
                    @Override
                    public void onChanged(District district) {
                        if(district != null)
                        binding.tvDistrict.setText(district.getDistrict());
                    }
                });

                branchDetailsViewModel.getStateById(branch.getStateId()).observe(requireActivity(), new Observer<State>() {
                    @Override
                    public void onChanged(State state) {
                        if(state!= null)
                        binding.tvState.setText(state.getState());
                    }
                });
            }
        });
    }

我尝试过使用命名和匿名观察者。我不明白这里有什么问题。

有时会抛出 NullPointerExceptions。有时更改语言环境失败。

04-09 15:48:45.182 2096-2113/com.appsbharti.bharatbankdetails E/SQLiteDatabase: Failed to open database '/data/data/com.appsbharti.bharatbankdetails/databases/bank_details.db'.
    android.database.sqlite.SQLiteException: Failed to change locale for db '/data/data/com.appsbharti.bharatbankdetails/databases/bank_details.db' to 'en_IN'.
        at android.database.sqlite.SQLiteConnection.setLocaleFromConfiguration(SQLiteConnection.java:386)
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:218)
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:193)
        at android.database.sqlite.SQLiteConnectionPool.openConnectionLocked(SQLiteConnectionPool.java:463)
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:185)
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:177)
        at android.database.sqlite.SQLiteDatabase.openInner(SQLiteDatabase.java:804)
        at android.database.sqlite.SQLiteDatabase.open(SQLiteDatabase.java:789)
        at android.database.sqlite.SQLiteDatabase.openDatabase(SQLiteDatabase.java:694)
        at android.app.ContextImpl.openOrCreateDatabase(ContextImpl.java:804)
        at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:221)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
        at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
        at androidx.sqlite.db.framework.FrameworkSQLiteOpenHelper$OpenHelper.getWritableSupportDatabase(FrameworkSQLiteOpenHelper.java:92)
        at androidx.sqlite.db.framework.FrameworkSQLiteOpenHelper.getWritableDatabase(FrameworkSQLiteOpenHelper.java:53)
        at androidx.room.SQLiteCopyOpenHelper.getWritableDatabase(SQLiteCopyOpenHelper.java:90)
        at androidx.room.RoomDatabase.inTransaction(RoomDatabase.java:476)
        at androidx.room.RoomDatabase.assertNotSuspendingTransaction(RoomDatabase.java:281)
        at com.appsbharti.bharatbankdetails.Daos.HistoryDao_Impl.insert(HistoryDao_Impl.java:45)
        at com.appsbharti.bharatbankdetails.Repositories.HistoryRepository.run(HistoryRepository.java:37)
        at androidx.room.TransactionExecutor.run(TransactionExecutor.java:45)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
        at java.lang.Thread.run(Thread.java:856)
     Caused by: android.database.sqlite.SQLiteDiskIOException: disk I/O error (code 1802)
        at android.database.sqlite.SQLiteConnection.nativeExecute(Native Method)
        at android.database.sqlite.SQLiteConnection.execute(SQLiteConnection.java:548)
        at android.database.sqlite.SQLiteConnection.setLocaleFromConfiguration(SQLiteConnection.java:364)
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:218) 
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:193) 
        at android.database.sqlite.SQLiteConnectionPool.openConnectionLocked(SQLiteConnectionPool.java:463) 
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:185) 
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:177) 
        at android.database.sqlite.SQLiteDatabase.openInner(SQLiteDatabase.java:804) 
        at android.database.sqlite.SQLiteDatabase.open(SQLiteDatabase.java:789) 
        at android.database.sqlite.SQLiteDatabase.openDatabase(SQLiteDatabase.java:694) 
        at android.app.ContextImpl.openOrCreateDatabase(ContextImpl.java:804) 
        at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:221) 
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224) 
        at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164) 
        at androidx.sqlite.db.framework.FrameworkSQLiteOpenHelper$OpenHelper.getWritableSupportDatabase(FrameworkSQLiteOpenHelper.java:92) 
        at androidx.sqlite.db.framework.FrameworkSQLiteOpenHelper.getWritableDatabase(FrameworkSQLiteOpenHelper.java:53) 
        at androidx.room.SQLiteCopyOpenHelper.getWritableDatabase(SQLiteCopyOpenHelper.java:90) 
        at androidx.room.RoomDatabase.inTransaction(RoomDatabase.java:476) 
        at androidx.room.RoomDatabase.assertNotSuspendingTransaction(RoomDatabase.java:281) 
        at com.appsbharti.bharatbankdetails.Daos.HistoryDao_Impl.insert(HistoryDao_Impl.java:45) 
        at com.appsbharti.bharatbankdetails.Repositories.HistoryRepository.run(HistoryRepository.java:37) 
        at androidx.room.TransactionExecutor.run(TransactionExecutor.java:45) 
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076) 
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569) 
        at java.lang.Thread.run(Thread.java:856)

还有一个问题是我不确定我应该在 Fragment 中使用哪个作为 Observer 的所有者，requireActivity() 或 getActivity()（我收到警告说这可能 return 为 null，所以我没用过）或者getViewLifecycleOwner()（获取fragment的生命周期所有者对象）。

Answer 1

如果你在一个Fragment里面，你不能通过this来观察，你应该通过viewlifecycleOwner。在您的情况下，您正在设置观察以从房间获取分支的值，然后根据分支，您的其他观察者被调用。

这意味着您的内部观察者将在您的 branch 观察者被调用后观察。

当您将观察者绑定到片段的生命周期时，观察者会根据片段本身的生命周期工作。

将它与 lifecycleowner 绑定应该可以，主要是因为 this 在 fragment 的情况下不起作用。

Answer 2

实际上，viewlifecycleOwner 和 this 被接受。对于内部，您必须调用 YourFragmnent.this

我看到您没有调用任何函数来更新内部 LiveData 值。并且你不应该包裹嵌套的观察者，因为内部观察者将被调用几次对应于外部观察者被调用的次数。

只需要调用更新LiveData值的函数即可触发内部观察者的onChanged回调

请阅读本文以避免多次消耗数据https://medium.com/androiddevelopers/livedata-with-snackbar-navigation-and-other-events-the-singleliveevent-case-ac2622673150

Answer 3

请使用YouFragmentClassName.this或viewLifecycleOwner
添加日志以检查您的 onChange 回调是否收到。
如果您仍然得到空值，请检查您的 viewModel 代码。

Answer 4

好的问题是我没有在 synchronized 块中获取 AppDatabase。

在 AppDatabse class 中，我更改了 newInsance 方法：

public static AppDatabase getInstance(final Context context) {
    dbInstance = buildDatabaseInstance(context);
    return dbInstance;
}

到

public static AppDatabase getInstance(final Context context) {
    if (dbInstance == null)
        synchronized (AppDatabase.class) {
            if (dbInstance == null)
                dbInstance = buildDatabaseInstance(context);
         }
//  dbInstance = buildDatabaseInstance(context);
    return dbInstance;
}

现在所有观察者运行顺利

我可以在 Observer 中添加 Observer 吗？我实际上尝试过，但内部观察者无法正常工作。有时其中一个工作，有时没有人工作

Can I add Observer inside Observer? I actually tried but inner observers not working properly. Sometimes one of them work, sometimes no one work

android

observable

observer-pattern

android-livedata