我正在尝试确定我的数据库中是否存在具有特定 ID 的项目，但即使我知道它在数据库中，我仍然得到 null。我很确定我错误地定义了我的查询，但我不确定问题是什么。我一直在查看房间查询示例，但未能解决问题。当我运行代码时，除了在尝试打印查询日志时出现空指针异常外，我没有看到任何错误。

我尝试在 DAO 中执行的查询如下所示:

    @Query("select filmID from FilmDataModel where filmID = :currentId")
    LiveData<String> inDatabase(String currentId);

我的 View 模型有这个方法作为 UI 和 DAO 之间的过渡:

public LiveData<String> inDatabase(String currentID) {
        LiveData<String> filmID = filmDatabase.filmDao().inDatabase(currentID);
        return filmID;
    }

我的 FilmDataModel 对象定义如下:

@Entity public class FilmDataModel {
    @PrimaryKey(autoGenerate = true)
    public int id;
    @ColumnInfo(name = "posterURL")
    private String posterURL;
    private String releaseDate;
    @ColumnInfo(name = "voterAvg")
    private String voterAvg;
    @ColumnInfo(name = "description")
    private String description;
    @ColumnInfo(name = "title")
    private String title;
    @ColumnInfo(name = "filmID")
    private String filmId;

    public FilmDataModel(String posterURL, String releaseDate, String voterAvg, String description,
                         String title, String filmId) {
        this.posterURL = posterURL;
        this.releaseDate = releaseDate;
        this.voterAvg = voterAvg;
        this.description = description;
        this.title = title;
        this.filmId = filmId;
    }

    public String getPosterURL() {
        return this.posterURL;
    }

    public String getReleaseDate() {
        return releaseDate;
    }

    public String getDescription() {
        return description;
    }

    public String getFilmId() {
        return filmId;
    }

    public String getTitle() {
        return title;
    }

    public String getVoterAvg() {
        return voterAvg;
    } 
}

然后在我的 Activity 中，我在单击按钮时设置了以下代码:

favoriteButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        Log.d(TAG, "Favorite button pressed.");
        FilmDataModel filmDataModel = new FilmDataModel(posterURL, releaseDate, voterAvg,
                overview, title, id);
        if (favoriteSelected) {
            Log.d(TAG, "Delete Item.");
            favoriteSelected = false;
            favoriteButton.setImageResource(R.drawable.heart_icon_off);
            viewModel.deleteItem(filmDataModel);
        } else {
            Log.d(TAG, "Insert Item.");
            favoriteSelected = true;
            favoriteButton.setImageResource(R.drawable.heart_icon_on);
            viewModel.insertItem(filmDataModel);
            Log.d(TAG, viewModel.inDatabase(id).getValue());
        }
    }
});

根据 Activity 和 View 模型的打印语句，看起来数据已插入数据库并从数据库中删除，但当我尝试运行查询并打印 filmID 时，我得到一个空值。我可以不做我想从 Activity 中做的事情吗？我的查询语句是否由于某种原因不正确？

最佳答案

来自 Android Developers documentation :

LiveData is a data holder class that can be observed within a given lifecycle

这意味着您必须先注册一个Observer 才能接收数据更新。

同样，来自 Android Developers documentation :

Generally, LiveData delivers updates only when data changes, and only to active observers. An exception to this behavior is that observers also receive an update when they change from an inactive to an active state. Furthermore, if the observer changes from inactive to active a second time, it only receives an update if the value has changed since the last time it became active.

这是一个可能的解决方案:

favoriteButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        Log.d(TAG, "Favorite button pressed.");
        FilmDataModel filmDataModel = new FilmDataModel(posterURL, releaseDate, voterAvg, overview, title, id);
        if (favoriteSelected) {
            Log.d(TAG, "Delete Item.");                
        } else {
            Log.d(TAG, "Insert Item.");
            favoriteSelected = true;
            favoriteButton.setImageResource(R.drawable.heart_icon_on);
            viewModel.insertItem(filmDataModel);  
            viewModel.inDatabase(id).observe(this, new Observer<String>() {
                @Override
                public void onChanged(@Nullable final String name) {
                   // your code here
                   Log.d(TAG, name.getValue());
                }
            });
        }
    }
});

有关详细信息，请参阅 Work with LiveData objects

关于android - 房间查询返回null，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50844063/