Question

我正在尝试从Android Room返回LiveData。我无法将结果作为LiveData返回。

以下是Dao的摘录

  @Query("SELECT * FROM transaction_table")
  LiveData<List<MyTransaction>> getAllTransactions();

  @Query("SElECT * FROM transaction_table")
  List<MyTransaction> getMyTransaction();

getAllTransactions()调用无例外运行，但是不返回结果。另一方面，getMyTransactions()返回选择查询结果。

SDK Version: 23
Room Version: 2.1.0-beta01

编辑：

我正在初始化时填充数据库以进行测试。这是代码

  private static class PopulateDbAsync extends AsyncTask<Void, Void, Void> {
      private static final String TAG = "PopulateDbAsync";

      private final TransactionDao mDao;

      PopulateDbAsync(TransactionDatabase db) {
          mDao = db.transactionDao();
      }

      @Override
      protected Void doInBackground(Void... voids) {
          mDao.deleteAll();

          MyTransaction transaction = new MyTransaction();
          transaction.setCardNumber("0123023403450456");
          long result = mDao.insert(transaction);
          Log.d(TAG, "doInBackground: " + String.format(Locale.US, "Result %d", result));

          transaction = new MyTransaction();
          transaction.setCardNumber("0123023403450457");
          result = mDao.insert(transaction);
          Log.d(TAG, "doInBackground: " + String.format(Locale.US, "Result %d", result));

          LiveData<List<MyTransaction>> currentTransactions = mDao.getAllTransactions();
          List<MyTransaction> txns = currentTransactions.getValue();

          List<MyTransaction> transactions = mDao.getMyTransaction();
          if(transactions != null) {
              Log.d(TAG, "doInBackground: " + String.format(Locale.US, "Size = %d", transactions.size()));
          }

          return null;
      }
  }

Answer 1

如果不观察LiveData，就无法检索它。这就是以下代码根本不起作用的原因。

LiveData<List<MyTransaction>> currentTransactions = mDao.getAllTransactions();

要使其正常工作，请先观察LiveData，然后侦听数据更改：

 mDao.getAllTransactions().observe(this, new Observer<List<MyTransaction>>() {
            @Override
            public void onChanged(List<MyTransaction> myTransactionList) {

            }
        });

Android Room查询未返回LiveData

1 个答案: