ViewModel LiveData 在 Android 中屏幕旋转后发出空列表

Question

我遇到了有关屏幕旋转期间 ViewModel 中 LiveData 行为的问题。

我的应用程序围绕 MVVM 模式构建，包括一个 Activity，它从 ViewModel 观察 LiveData，而 ViewModel 又从链接到 Room 数据库的存储库中获取数据。

以下是涉及的代码片段：

Create 上的活动：

ChatViewModel2.getInstance(this, topic).getTopicChatLiveData().observe(this, chatMessageDBS -> {
    chatAdapter.submitList(chatMessageDBS.stream().distinct().map(ChatMessageViewModel::createInstance).collect(Collectors.toList()));
});

视图模型：

public class ChatViewModel2 extends ViewModel {
    private final ChatMassagesRepository repository;
    private String topic;
    private static ChatViewModel2 instance;

    public ChatViewModel2(@NonNull String topic) {
        repository = new ChatMassagesRepository();
        this.topic = topic;
    }

    public static synchronized ChatViewModel2 getInstance(Context context, String topic) {
        if (instance == null) {
            instance = new ViewModelProvider((ViewModelStoreOwner) context, new ChatMassagesViewModelFactory(topic)).get(ChatViewModel2.class);
        }
        return instance;
    }

    public LiveData<List<ChatMessageDB>> getTopicChatLiveData() {
        LiveData<List<ChatMessageDB>> topicChatLiveData = repository.getAllRepliesByTopic(topic);
        return topicChatLiveData;
    }
}

视图模型工厂：

public class ChatMassagesViewModelFactory implements ViewModelProvider.Factory {
    private String topic;

    public ChatMassagesViewModelFactory(String topic) {
        this.topic = topic;
    }

    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        if (modelClass.isAssignableFrom(ChatViewModel2.class)) {
            return (T) new ChatViewModel2(topic);
        }
        throw new IllegalArgumentException("Unknown ViewModel class");
    }
}

存储库：

public class ChatMassagesRepository {
    ChatMessageDAO chatMessageDao;
    private static ChatMassagesRepository INSTANCE;

    public static ChatMassagesRepository getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new ChatMassagesRepository();
        }
        return INSTANCE;
    }

    public ChatMassagesRepository() {
        chatMessageDao= ChatDatabase.getInstance().chatMessageDao();
    }

    public LiveData<List<ChatMessageDB>> getAllRepliesByTopic(String topic){
        LiveData<List<ChatMessageDB>> replies = chatMessageDao.getTopicChat(topic);
        return  replies;
    }
}

这是来自数据库类的 getInstance：

@NonNull
public static ChatDatabase getInstance(final Context context){
    synchronized (ChatDatabase.class) {
        if (INSTANCE == null) {
            INSTANCE = Room.databaseBuilder(context.getApplicationContext(), ChatDatabase.class, "teleconsole_chat_database").fallbackToDestructiveMigration().build();
        }
    }
    return INSTANCE;
}

public static ChatDatabase getInstance(){
    return getInstance(AppController.getInstance());
}

一切正常，直到屏幕旋转。旋转后，活动中的观察者收到一个空列表。如果您能深入了解为什么会发生这种情况以及如何纠正它，我们将不胜感激。

我也尝试过

        new ViewModelProvider(this, new ChatMassagesViewModelFactory(topic)).get(ChatViewModel2.class).getTopicChatLiveData().observe(this, chatMessageDBS ->
                chatAdapter.submitList(chatMessageDBS.stream().distinct().map(ChatMessageViewModel::createInstance).collect(Collectors.toList())));

但没有帮助

Answer 1

您可以尝试在清单文件中使用 `android:"configChanges:orientation" 来忽略屏幕旋转。但这不是最好的方法。 文档链接：https://developer.android.com/guide/topics/resources/runtime-changes#restrict-activity

如果解决方案有帮助，请告诉我。