java - 插入 Room Persistence Library 后获取 rowId

Question

我所要做的就是从 insert 中获取 long 的返回值。我明白了，但它不起作用，我从返回值中取回 0。我正在使用 DAO、存储库和 ViewModel，如 Google CodeLabs 中所述.我已经关注了这篇文章 Rowid after Insert in Room .

玩家类

@Entity(tableName = "player_table")
public class Player {
    @PrimaryKey(autoGenerate = true)
    private long id;
    @NonNull
    @ColumnInfo(name = "username")
    private String username;
}

道

@Insert
long insert(Player player);

存储库

public long insert(Player player) {
    new insertAsyncTask(mPlayerDao).execute(player);
    rowId = player.getId();
return rowId;
}

View 模型

public long insert(Player player){
    rowId = mRepository.insert(player);
    return rowId;
}

Activity

String playerString = editTextUsername.getText().toString();
Player player = new Player(playerString);
long rowId = mDreamViewModel.insert(player);

Answer 1

问题是您在 AsyncTask 完成其后台插入工作之前返回了 player.getId()；您必须等到它为您提供正确的插入结果，这是使用线程安全的建议解决方案 CountDownLatch它使用 .await() 方法暂停后续代码的执行，直到 CountDownLatch 的计数达到 0；每次调用 .countDown() 时它减 1。

public class Repository {

    private long rowId = -1;
    private CountDownLatch mLatch;

    public long insert(Player player) {
        mLatch = new CountDownLatch(1);
        new insertAsyncTask(mPlayerDao).execute(player);
        try {
            mLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Log.i("LOG_TAG", String.valueOf(rowId));

        return rowId;
    }

    class insertAsyncTask extends AsyncTask<Player, Void, Void> {

        @Override
        protected Void doInBackground(Player... players) {
            rowId = mDatabase.getContactDAO().addContact(players[0]);
            mLatch.countDown();
            return null;
        }
    }

}