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android - Android Room 查询不返回 LiveData

问题描述

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

这是摘自Dao

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

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

调用无异常运行，getAllTransactions()但不返回结果。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;
      }
  }

标签： androidandroid-roomandroid-lifecycle

解决方案

LiveData 在不观察的情况下无法检索。这就是为什么下面的代码根本不起作用的原因。

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

要使其工作，请先观察 LiveData，然后监听数据更改：

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

            }
        });

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