android - 如何根据另一行的更改更新一行？

Question

假设我有两个表来跟踪银行账户:

Account:
------------------------------
AccountID| Account Value
------------------------------
| 0      | 1000

Transactions:
------------------------------------------------
TransactionID| Transaction Value| Paying Account
-------------------------------------------------    
| 23         | -50.0            | 0
--------------------------------------------------
| 24         | -5               | 0

TransactionID=24 有一个更新，新值是-10。更新账户值(value)以反射(reflect)这一变化的最佳方式是什么？

我在代码中所做的是

1. 在编辑对话框中点击确定后
2. 通过添加负值来反转原始交易:

3. 然后将新的交易值(value)应用到账户

   AccountDAO acc = mDatabase.AccountDAO().getAccount(0);
   acc.account_value += transaction_id_24.value * -1;
   acc.account_value += transaction_id_24_updated.value;
   mDatabase.AccountDAO().updateAccount(acc);
   

   上述方法可行，但我觉得有比上述方法更好的自动化方法。

有没有办法让数据库自动检测到某笔交易的金额发生变化，然后更新sqlite/android-room中对应的账户？

Answer 1

Room 内置了您需要的所有工具:

Use LiveData with Room

The Room persistence library supports observable queries, which return LiveData objects. Observable queries are written as part of a Database Access Object (DAO).

Room generates all the necessary code to update the LiveData object when a database is updated. The generated code runs the query asynchronously on a background thread when needed. This pattern is useful for keeping the data displayed in a UI in sync with the data stored in a database. You can read more about Room and DAOs in the Room persistent library guide.

只需在 Room 中观察从您 @Query 返回的 LiveData，您可以使用返回的结果执行其他逻辑。