Room 数据库中的事务返回 LiveData

Question

我想从旧记录中删除一个 table，然后计算其中的记录数。

@Transaction
private fun getOrdersCount(): LiveData<Int>
{
    val now = LocalDateTime.now()
    val firstOfMonth = now.withDayOfMonth(1)
    subop_pruning(firstOfMonth) // query that deletes older orders
    return subop_select_count() // query that returns the number of orders
}

不幸的是，这个查询 return 是一个错误

error: Method annotated with @Transaction must not return deferred/async return type androidx.lifecycle.LiveData. Since transactions are thread confined and Room cannot guarantee that all queries in the method implementation are performed on the same thread, only synchronous @Transaction implemented methods are allowed. If a transaction is started and a change of thread is done and waited upon then a database deadlock can occur if the additional thread attempts to perform a query. This restrictions prevents such situation from occurring.

现在，我想在一个事务中按顺序执行 -delete- 和 -select count- 操作序列，return 来自 -select 的 LiveData计数操作。这在我看来是合法的。问题：

事务中的查询不是顺序执行的吗？
为什么会出错？
如果无法从交易中获取 LiveData，如何获得相同的行为，例如使用 MutableLiveData 和 InvalidationTracker.Observer#onInvalidated 通知？

Answer 1

第 1 点和第 2 点仍未得到解答，无论如何我是如何绕过这个问题的（根据第 3 点）。希望对某人有所帮助：

fun getOrderCount(): LiveData<Int> = object : LiveData<Int>(0) {
    val observer = object : InvalidationTracker.Observer("order_table") {
        override fun onInvalidated(tables: MutableSet<String>) {
            requestOrderCount()
        }
    }

    override fun onActive() {
        super.onActive()
        val tracker =
            MyRoomDatabase.getInstance(applicationContext).invalidationTracker;
        tracker.addObserver(observer)
        requestOrderCount()
    }

    override fun onInactive() {
        super.onInactive()
        val tracker =
            MyRoomDatabase.getInstance(applicationContext).invalidationTracker;
        tracker.removeObserver(observer)
    }

    private fun requestOrderCount() {
        // better to use coroutines than executors
        Executors.newSingleThreadExecutor().execute {
            postValue(
                MyRoomDatabase.getInstance(applicationContext).OrderDao()
                    .getOrderCount_()
            )
        }
    }
}


@Transaction
private fun getOrdersCount_(): LiveData<Int>
{
    val now = LocalDateTime.now()
    val firstOfMonth = now.withDayOfMonth(1)
    subop_pruning(firstOfMonth) // query that deletes older orders
    return subop_select_count() // query that returns the number of orders
}

Answer 2

使用@Transaction 注解时将LiveData 更改为MutableLiveData。我知道已经很晚了。但是，仍然有一些人可以从中受益。

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