Android Room：处理与 LiveData 的关系

Question

所以我有两个模型（表），WordGroups和Words，它们之间的关系是一对多的。所以我的模型和 Dao 看起来像：

@Entity
public class WordGroup {

    @PrimaryKey
    private int id;

    @Ignore
    private List<Word> words;

    public int getId() {
        return id;
    }

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

    public List<Word> getWords() {
        return words;
    }

    public void setWords(List<Word> words) {
        this.words = words;
    }
}

@Entity
public class Word {

    @PrimaryKey
    private int id;

    @ForeignKey(entity = WordGroup.class, parentColumns = "id", childColumns = "wordGroupId")
    private int wordGroupId;

    public int getId() {
        return id;
    }

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

    public int getWordGroupId() {
        return wordGroupId;
    }

    public void setWordGroupId(int wordGroupId) {
        this.wordGroupId = wordGroupId;
    }
}

和

@Dao
public interface Dao {

    @Query("select * from WordGroup where id = :id")
    LiveData<WordGroup> selectWordGroupById(int id);

    @Query("select * from WordGroup")
    LiveData<List<WordGroup>> selectAllWordGroups();

    @Query("select * from Word where wordGroupId = :wordGroupId")
    LiveData<List<Word>> selectWordsByWordGroupId(int wordGroupId);
}

现在我有一个方法可以选择 WordGroup 并使用 Transformations:

设置它的 Words

public LiveData<WordGroup> getWordGroupWithWords(int id) {
    LiveData<WordGroup> wordGroupLiveData = dao.selectWordGroupById(id);

    LiveData<WordGroup> wordGroupWithWordsLiveData = Transformations.switchMap(wordGroupLiveData, inputWordGroup -> {
        LiveData<List<Word>> wordsLiveData = dao.selectWordsByWordGroupId(inputWordGroup.getId());

        LiveData<WordGroup> outputWordGroupsLiveData = Transformations.map(wordsLiveData, inputWords -> {
            inputWordGroup.setWords(inputWords);
            return inputWordGroup;
        });

        return outputWordGroupsLiveData;
    });

    return wordGroupWithWordsLiveData;
}

现在我想编写一个方法来选择所有 WordGroup，同时为每个 WordGroup 设置它们的 Word。该方法的签名应该是这样的：

public LiveData<List<WordGroup>> getAllWordGroupsWithWords();

Answer 1

好吧，我尝试了更多，并在 MediatorLiveData:

的帮助下想出了这个解决方案

public LiveData<List<WordGroup>> getAllWordGroupsWithWords() {
    LiveData<List<WordGroup>> wordGroupsLiveData = dao.selectAllWordGroups();
    LiveData<List<WordGroup>> outputWordGroups = Transformations.switchMap(wordGroupsLiveData, inputWordGroups -> {
        MediatorLiveData<List<WordGroup>> wordGroupsMediatorLiveData = new MediatorLiveData<>();
        for (WordGroup wordGroup : inputWordGroups) {
            wordGroupsMediatorLiveData.addSource(dao.selectWordsByWordGroupId(wordGroup.getId()), words -> {
                wordGroup.setWords(words);
                wordGroupsMediatorLiveData.postValue(inputWordGroups);
            });
        }

        return wordGroupsMediatorLiveData;
    });

    return outputWordGroups;
}

现在完全是我想要的方式，但是有一些大问题。

首先，如果您在 MediatorLiveData 上使用方法 setValue，它会多次调用观察者，我可以通过使用 [=14 来减少它调用观察者的次数=]
其次即使你只改变一个词，它也会为每个wordGroup再次调用观察者！

当然，它不是最优化的解决方案，但确实有效。我非常乐于接受建议和改进。

Android Room：处理与 LiveData 的关系

Android Room: Handling Relations with LiveData

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