java - 插入Room持久库后获取rowId

Question

我所要做的就是从插入中获取长整型返回值。我明白了，但它不起作用，我从返回值中得到 0。我正在使用 DAO、Repository 和 ViewModel，如 Google CodeLabs 中所述。 。我关注了这篇文章Rowid after Insert in Room .

玩家等级

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

DAO

@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

问题在于，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，这里是使用线程安全 CountDownLatch 的建议解决方案使用.await()方法暂停后续代码的执行，直到CountDownLatch计数达到0；每次调用 .countDown() 时它都会减 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;
        }
    }

}