当没有更改触发Android视图模型中的onChange方法时，如何通过视图模型中的LiveData <>从dao检索对象列表

Question

我正在使用 android studios 来构建一个字典应用程序。在 SO 社区的帮助下，一切进展顺利，现在我有另一个问题。

我已经设置了几个自定义词典，以便用户可以存储和查看不同词典中的单词。 （例如文学、编程、一般等）

这些将是单词实体中的 FK id，并在用户添加新单词时填充。

在 MainActivity 中，我有一个“更改字典”的选项菜单项。这会打开一个 AlertDialog，用户可以在其中更改字典并从理论上看到一组新单词。

这就是问题所在。当用户选择特定字典时，数据库不会发生任何更改，因此附加到 WordViewModel 的 getWordsByDictionaryId() 方法的 onChange 不会触发。因此，LiveData> 不会重新填充。

onCreate() 方法中的代码：

    // get all words from WordDao
    mWordViewModel = ViewModelProviders.of(MainActivity.this).get(WordViewModel.class);
    mWordViewModel.getWordByDictionaryId(dictionaryId).observe(MainActivity.this, new Observer<List<Word>>() {
        @Override
        public void onChanged(@Nullable List<Word> words) {
            mainAdapter.setWords(words);
            mAllWords = words;
            mainAdapter.notifyDataSetChanged();
        }
    });

OnOptionsItemSelected() 方法中的代码：

        AlertDialog.Builder changeDictionaryDialog = new AlertDialog.Builder(MainActivity.this);
        changeDictionaryDialog.setTitle("Select Dictionary:");
        LayoutInflater inflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View view = inflater.inflate(R.layout.alert_dialog_spinner_view, null);

        mDictionaryStringList = new ArrayList<>();
        mDictionaryStringList = convertToList(mDictionaryList);

        final Spinner dictionarySpinner = (Spinner) view.findViewById(R.id.alert_dialog_spinner);

        ArrayAdapter<String> spinnerAdapter = new ArrayAdapter(MainActivity.this, android.R.layout.simple_spinner_dropdown_item, mDictionaryStringList);
        spinnerAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        dictionarySpinner.setAdapter(spinnerAdapter);

        dictionarySpinner.setSelection(0);

        changeDictionaryDialog.setView(view);
        changeDictionaryDialog.setPositiveButton("OK", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                final String dictionaryValue = dictionarySpinner.getSelectedItem().toString();

                for (Dictionary dictionary : mDictionaryList) {
                    if (dictionary.getDictionaryName().trim().equals(dictionaryValue)) {
                        dictionaryId = dictionary.getDid();
                       LiveData<List<Word>> myNewList = mWordViewModel.getWordByDictionaryId(dictionaryId);
                        List<Word> testList = myNewList.
                    }
                }
            }
        });

        changeDictionaryDialog.setNegativeButton("Cancel", null);
        changeDictionaryDialog.create();
        changeDictionaryDialog.show();

WordViewModel 中的代码：

public LiveData<List<Word>> getWordByDictionaryId(long dictionaryId) {
    return mWordRepository.getWordByDictionaryId(dictionaryId);
}

WordDao 中的代码：

@Query("SELECT * FROM word_table WHERE dictionary_id =:dictionaryId")
public LiveData<List<Word>> getWordsByDictionaryID(long dictionaryId);

最后是Word实体：

@Entity(tableName = "word_table",
indices = { @Index(value = "word", unique = true),
@Index("dictionary_id") })
public class Word {
@PrimaryKey(autoGenerate = true)
private long id;
private String word;
@ColumnInfo(name = "dictionary_id")
private long dictionaryId;

@Ignore
public Word(String word, long dictionaryId) {
    this.word = word;
    this.dictionaryId = dictionaryId;
}

public Word(long id, String word, long dictionaryId) {
    this.id = id;
    this.word = word;
    this.dictionaryId = dictionaryId;
}

public long getId() {
    return id;
}

public void setId(long id) {
    this.id = id;
}

public String getWord() {
    return word;
}

public void setWord(String word) {
    this.word = word;
}

public long getDictionaryId() {
    return dictionaryId;
}

public void setDictionaryId(long dictionaryId) {
    this.dictionaryId = dictionaryId;
}
}

我曾尝试设置一个后台线程 (AsyncTask)，但有一次我这样做了，结果是在我需要它们之后出现的，因此返回值为 null。一些 SO 开发人员说不要使用 Asynctask，但我认为问题仍然是时间......

我还阅读了一些关于生成回调的内容。这在这种情况下真的有效吗？在我看来，它可能会让我与后台线程处于同一条船上。

当涉及到通过房间持久性通过视图模型通过 LiveData 通过 ???whatever 从 SQLite 数据库中获取数据时，我一直在努力学习如何从这种“非常简单的创建数据库的方法”中获取数据接下来呢？？？有没有“谷歌”的方式来做到这一点？或者这就是 RxJava 和其他库发挥作用的地方？

所以我的问题就是“如果没有通过实时数据的 onChange 事件，我究竟如何从数据库中获取实体对象列表？”在没有 LiveData 包装器的情况下调用查询是否有最佳实践或正确方法？还是一种直接在 LiveData 包装器中访问查询而不需要更改数据来完成任务的方法？

Answer 1

LiveData 解决方案

@Zohaib Amir 的评论是正确的。只要您记得清除观察者，您就可以在任何您想要的地方拨打LiveData#observe()。这种方法的一个缺点是您必须保留对这些观察者的引用，而LiveData 已经拥有Transformations.switchMap()，这样您就可以避免这种情况。

视图模型

// Instance variable that stores the current dictionaryId that user wants to see.
private final MutableLiveData<Long> currentDictionaryId = new MutableLiveData<>();

// Instance variable that stores the current list of words. This will automatically change when currentDictionaryId value changes.
private final LiveData<List<Word>> words = Transformations.switchMap(currentDictionaryId, dictionaryId -> 
    mWordRepository.getWordByDictionaryId(dictionaryId));



// Set new dictionaryId
public void setDictionaryId(long dictionaryId) {
    currentDictionaryId.postValue(dictionaryId);
}

// Get LiveData to listen to
public LiveData<List<Word>> getWords() {
    return words;
}

活动

// onCreate()
    // get all words from WordDao
    mWordViewModel = ViewModelProviders.of(MainActivity.this).get(WordViewModel.class);
    mWordViewModel.setDictionaryId(dictionaryId);
    mWordViewModel.getWords().observe(MainActivity.this, new Observer<List<Word>>() {
        @Override
        public void onChanged(@Nullable List<Word> words) {
            mainAdapter.setWords(words);
            mAllWords = words;
            mainAdapter.notifyDataSetChanged();
        }
    });

    // note that the order of calling setDictionaryId and getWords doesn't matter. setDictionaryId is supposed to call at anytime.

// OnOptionsItemSelected
    ...
    changeDictionaryDialog.setView(view);
        changeDictionaryDialog.setPositiveButton("OK", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                final String dictionaryValue = dictionarySpinner.getSelectedItem().toString();

                for (Dictionary dictionary : mDictionaryList) {
                    if (dictionary.getDictionaryName().trim().equals(dictionaryValue)) {
                        dictionaryId = dictionary.getDid();
                        mWordViewModel.setDictionaryId(dictionaryId);
                        break;
                    }
                }
            }
        });
    ...

这次我准备了其他解决方案，以便您进行比较。其他解决方案（几乎）做同样的事情。

---

非反应性解决方案

您可以基于回调实现上述功能，这意味着您必须手动处理生命周期更改、通知和线程。还有一个功能上的区别是LiveData会在db发生变化时自动通知观察者，而这种回调设计只是一次查找而已。

在本例中，我们将使用Executor在后台线程中执行任务。

WordDao

@Query("SELECT * FROM word_table WHERE dictionary_id =:dictionaryId")
public List<Word> getWordsByDictionaryID(long dictionaryId); // notice that livedata is gone.

WordRepository 数据库访问必须在后台线程中完成。所以当我们访问db时，我们需要在某个时候切换到后台线程。在这个例子中，我们将切换到后台 线程在这一层。

// This executor is needed to run Runnables on a background thread. In real application, you may
// create this executor outside of this repository and later inject it to this repository.
private final Executor ioExecutor = Executors.newSingleThreadExecutor();


private void getWordByDictionaryId(long dictionaryId, Consumer<List<Word>> callback) {
    ioExecutor.execute(() -> {
        List<Word> wordList = wordDao.getWordsByDictionaryId(dictionaryId);
        callback.accept(wordList);
    });
}

WordViewModel 在这个例子中，ViewModel 没有做太多的事情。只需将参数传递给存储库。

public void getWordByDictionaryId(long dictionaryId, Consumer<List<Word>> callback) {
    mWordRepository.getWordByDictionaryId(dictionaryId, callback);
}

活动 请注意，Consumer#accept 将在后台线程上运行。因此，在对 ui 进行任何操作之前，您需要切换回 ui 线程。

// onCreate
mWordViewModel = ViewModelProviders.of(MainActivity.this).get(WordViewModel.class);
mWordViewModel.getWordByDictionaryId(dictionaryId, words -> {
    runOnUiThread(() -> {
        if (isFinishing() || isDestroyed()) return; // if the activity is finishing, don't do anything.

        mainAdapter.setWords(words);
        mAllWords = words;
        mainAdapter.notifyDataSetChanged();
    });
});


// onOptionsItemSelected
changeDictionaryDialog.setPositiveButton("OK", new DialogInterface.OnClickListener() {
    @Override
    public void onClick(DialogInterface dialog, int which) {
        final String dictionaryValue = dictionarySpinner.getSelectedItem().toString();

        for (Dictionary dictionary : mDictionaryList) {
            if (dictionary.getDictionaryName().trim().equals(dictionaryValue)) {
                dictionaryId = dictionary.getDid();
                mWordViewModel.getWordByDictionaryId(dictionaryId, words -> {
                    runOnUiThread(() -> {
                        if (isFinishing() || isDestroyed()) return; // if the activity is finishing, don't do anything.

                        mainAdapter.setWords(words);
                        mAllWords = words;
                        mainAdapter.notifyDataSetChanged();
                    });
                });
                break;
            }
        }
    }
});

---

RxJava

RxJava 解决方案看起来很像LiveData 解决方案。与LiveData 相比，它有很多优点：它有很多可观察对象和操作符可供使用。这些优势在更复杂的应用程序中更加明显，您需要执行条件或延迟异步任务、周期性任务、一个接一个地链接的多个请求、错误处理等。

WordDao

@Query("SELECT * FROM word_table WHERE dictionary_id =:dictionaryId")
public Flowable<List<Word>> getWordsByDictionaryID(long dictionaryId);

视图模型

private final BehaviorProcessor<Long> currentDictionaryId = BehaviorProcessor.create();

// Set new dictionaryId
public void setDictionaryId(long dictionaryId) {
    currentDictionaryId.onNext(dictionaryId);
}

// Get Flowable to listen to
public Flowable<List<Word>> getWords() {
    return currentDictionaryId.switchMap(dictionaryId -> mWordRepository
        .getWordByDictionaryId(dictionaryId)
        // add ".take(1)" if you need one-time look up.
        .subscribeOn(Schedulers.io())); 
}

活动

private fianl CompositeDisposable disposables = new CompositeDisposable();

// onCreate()
mWordViewModel = ViewModelProviders.of(MainActivity.this).get(WordViewModel.class);
mWordViewModel.setDictionaryId(dictionaryId);
disposables.add(mWordViewModel.getWords()
    .observeOn(AndroidSchedulers.mainThread()))
    .subscribe(words -> {
        mainAdapter.setWords(words);
        mAllWords = words;
        mainAdapter.notifyDataSetChanged();
    });


// onOptionsItemSelected()
 changeDictionaryDialog.setPositiveButton("OK", new DialogInterface.OnClickListener() {
    @Override
    public void onClick(DialogInterface dialog, int which) {
        final String dictionaryValue = dictionarySpinner.getSelectedItem().toString();

        for (Dictionary dictionary : mDictionaryList) {
            if (dictionary.getDictionaryName().trim().equals(dictionaryValue)) {
                dictionaryId = dictionary.getDid();
                mWordViewModel.setDictionaryId(dictionaryId);
                break;
            }
        }
    }
});

// onDestroy()
disposables.clear();