从不同的 LiveData 源填充适配器

Question

我正在使用 LiveData 并想了解它可以做什么。我想通过开关用来自不同来源的数据填充我的 RecyclerView（如果你愿意，可以使用过滤器）。

过滤适配器内的值不是一种选择。所以，我决定在我的视图模型中使用MediatorLiveData。

道：

@Query("SELECT * FROM tasks WHERE completed = 0")
LiveData<List<Task>> getActiveTasksLiveData();

@Query("SELECT * FROM tasks")
LiveData<List<Task>> getAllTasksLiveData();

@Query("SELECT * FROM tasks WHERE completed = 1")
LiveData<List<Task>> getClosedTasksLiveData();

回购：

    public LiveData<List<Task>> getActiveTasks() {
        return mTaskDao.getActiveTasksLiveData();
    }

    public LiveData<List<Task>> getAllTasks() {
        return mTaskDao.getAllTasksLiveData();
    }

    public LiveData<List<Task>> getClosedTasks() {
        return mTaskDao.getClosedTasksLiveData();
    }

视图模型

public class MainViewModel extends AndroidViewModel {

    private final String TAG = "MainViewModel";

    private final AppDataRepository mData;

    private MediatorLiveData<List<Task>> mMediatorTasks;

    public MainViewModel(@NonNull Application application) {
        super(application);

        mData = AppDataInjector.getDataRepository(application.getApplicationContext());

        mMediatorTasks = new MediatorLiveData<>();
        mMediatorTasks.setValue(null);
    }

    public LiveData<List<Task>> getTasks(){
        return mMediatorTasks;
    }

    public void changeTasksOption(int index){
        mMediatorTasks.removeSource(mData.getAllTasks());
        mMediatorTasks.removeSource(mData.getActiveTasks());
        mMediatorTasks.removeSource(mData.getClosedTasks());
        if (index == R.id.navigation_all){
            Log.i(TAG, "Add source: all");
            mMediatorTasks.addSource(mData.getAllTasks(), new Observer<List<Task>>() {
                @Override
                public void onChanged(List<Task> tasks) {
                    Log.i(TAG, "Add source: all - setValue");
                    mMediatorTasks.setValue(tasks);
                }
            });
        } else if (index == R.id.navigation_closed){
            Log.i(TAG, "Add source closed");
            mMediatorTasks.addSource(mData.getClosedTasks(), new Observer<List<Task>>() {
                @Override
                public void onChanged(List<Task> tasks) {
                    Log.i(TAG, "Add source: closed - setValue");
                    mMediatorTasks.setValue(tasks);
                }
            });
        } else {
            Log.i(TAG, "Add source active");
            mMediatorTasks.addSource(mData.getActiveTasks(), new Observer<List<Task>>() {
                @Override
                public void onChanged(List<Task> tasks) {
                    Log.i(TAG, "Add source: active - setValue");

                    mMediatorTasks.setValue(tasks);
                }
            });
        }
    }
}

分段

    public View onCreateView(@NonNull LayoutInflater inflater,
                             @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_main, container, false);

        mNavigationView = view.findViewById(R.id.navigation);
        mFab = view.findViewById(R.id.fabMain);
        mRecyclerView = view.findViewById(R.id.mainRecyclerView);

        tasksAdapterLive = new TasksAdapterLive(mAdapterCallback);
        RecyclerView.LayoutManager manager = new GridLayoutManager(getContext(), 1);
        mRecyclerView.setLayoutManager(manager);
        mRecyclerView.setAdapter(tasksAdapterLive);

        // set up bottom navigation listener
        mNavigationView.setOnNavigationItemSelectedListener(item -> {
            mViewModel.changeTasksOption(item.getItemId());
            return true;
        });

        return view;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mViewModel = ViewModelProviders.of(this).get(MainViewModel.class);

        mViewModel.getTasks().observe(this, tasks -> {
            if (tasks != null) {
                tasksAdapterLive.setTasks(tasks);
                tasksAdapterLive.notifyDataSetChanged();
            }
        });

        mViewModel.changeTasksOption(mNavigationView.getSelectedItemId());
    }

如您所见，我决定在我的视图模型中使用MediatorLiveData。我的主要目标 -从片段调用changeTasksOption()时更改适配器内的数据。

我使用removeSource()，因为我如何理解它LiveData从观察中删除源。但是，就我而言，它没有。

当我启动应用程序时，日志是：

MainViewModel: Add source active
MainViewModel: Add source: active - setValue

当我尝试切换到另一个来源时 - 日志是

MainViewModel: Add source: all
MainViewModel: Add source: all - setValue
MainViewModel: Add source: active - setValue
MainViewModel: Add source: all - setValue
MainViewModel: Add source: active - setValue
*** repeats about 100 times

RecyclerView 闪烁

所以，我请教。我究竟做错了什么？我误解了文档吗？removeSourse() 的真正作用是什么？因为就我而言，它不会删除源。

如果我的方法是错误的，你建议我怎么做？

谢谢！

编辑：

经过几个小时的试验，我找到了解决方案。是的，这很糟糕（或者可能不是？）。但显然这不是通用的，因为我们不使用 Romm + LiveData

创建返回 List 的普通 Room 函数

@Query("SELECT * FROM tasks WHERE completed = 0")
List<Task> getActiveTasks();

@Query("SELECT * FROM tasks")
List<Task> getAllTasks();

@Query("SELECT * FROM tasks WHERE completed = 1")
List<Task> getClosedTasks();

在 repo 中创建了 MutableLiveData

private MutableLiveData<List<Task>> mTasksTestActive, mTasksTestAll, mTasksTestClosed;

将这些函数添加到 repo

public LiveData<List<Task>> getActiveTasksTest() {
    Executors.newSingleThreadExecutor().execute(() -> {
        List<Task> taskList = mTaskDao.getActiveTasks();
        mTasksTestActive.postValue(taskList);
    });
    return mTasksTestActive;
}

public LiveData<List<Task>> getAllTasksTest() {
    Executors.newSingleThreadExecutor().execute(() -> {
        List<Task> taskList = mTaskDao.getAllTasks();
        mTasksTestAll.postValue(taskList);
    });
    return mTasksTestAll;
}

public LiveData<List<Task>> getClosedTasksTest() {
    Executors.newSingleThreadExecutor().execute(() -> {
        List<Task> taskList = mTaskDao.getClosedTasks();
        mTasksTestClosed.postValue(taskList);
    });
    return mTasksTestClosed;
}

视图模型更改：

public void changeTasksOption(int index) {
    mMediatorTasks.removeSource(mData.getAllTasksTest());
    mMediatorTasks.removeSource(mData.getActiveTasksTest());
    mMediatorTasks.removeSource(mData.getClosedTasksTest());
    if (index == R.id.navigation_all) {
        Log.i(TAG, "Add source: all");
        mMediatorTasks.addSource(mData.getAllTasksTest(), tasks -> {
            Log.i(TAG, "Add source: all - postValue");
            mMediatorTasks.postValue(tasks);
        });
    } else if (index == R.id.navigation_closed) {
        Log.i(TAG, "Add source closed");
        mMediatorTasks.addSource(mData.getClosedTasksTest(), tasks -> {
            Log.i(TAG, "Add source: closed - postValue");
            mMediatorTasks.postValue(tasks);
        });
    } else {
        Log.i(TAG, "Add source active");
        mMediatorTasks.addSource(mData.getActiveTasksTest(), tasks -> {
            Log.i(TAG, "Add source: active - postValue");

            mMediatorTasks.postValue(tasks);
        });
    }
}

现在，通过切换 UI，我得到了结果。没有更多的循环，一切似乎都很顺利。

但是还是！这是一个糟糕的解决方案。也许房间有问题？

Answer 1

public void changeTasksOption(int index){
    mMediatorTasks.removeSource(mData.getAllTasks());
    mMediatorTasks.removeSource(mData.getActiveTasks());
    mMediatorTasks.removeSource(mData.getClosedTasks());

不，这不应该是这样！

所选选项应位于 LiveData 中。然后您可以使用Transformations.switchMap {该 LiveData 来选择正确的LiveData<List<Task>>.

private MutableLiveData<Integer> mSelectedIndex = new MutableLiveData<>();

private final LiveData<List<Task>> mMediatorTasks = Transformations.switchMap(mSelectedIndex, (index) -> {
    if (index == R.id.navigation_all) {
        return mData.getAllTasksTest();
    } else if (index == R.id.navigation_closed) {
        return mData.getClosedTasksTest();
    } else {
        return mData.getActiveTasksTest();
    }
});

public void changeTasksOption(int index) {
    mSelectedIndex.setValue(index);
}

public LiveData<List<Task>> getTasks(){
    return mMediatorTasks;
}

此外，您应该再次从 DAOmData.get*()返回您的方法LiveData<List<Task>>，这是一个更好的解决方案。