同步、异步扩散和不一致性检测错误

Question

suspend fun updateDataset(newDataset: List<Item>) = withContext(Dispatchers.Default) {
        val diff = DiffUtil.calculateDiff(object : DiffUtil.Callback()
        {
            override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean
                    = dataset[oldItemPosition].conversation.id == newDataset[newItemPosition].conversation.id

            override fun getOldListSize(): Int = dataset.size
            override fun getNewListSize(): Int = newDataset.size

            override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean
                    = dataset[oldItemPosition] == newDataset[newItemPosition]
        })

        withContext(Dispatchers.Main) {
            dataset = newDataset // <-- dataset is the Adapter's dataset
            diff.dispatchUpdatesTo(this@ConversationsAdapter)
        }
    }

private fun updateConversationsList(conversations: List<ConversationsAdapter.Item>)
{
    viewLifecycleOwner.lifecycleScope.launch {
        (listConversations.adapter as ConversationsAdapter).updateDataset(conversations)
    }
}

UpdateConversationsList()在很短的时间内被调用多次，因为Kotlin的Flows像Flow 一样调用了这个函数。

现在，我有时会得到一个java.lang.IndexOutOfBoundsException：Inconsistency Detection。无效视图保持器适配器PositionViewHolder错误。通过阅读这个线程，我了解到这是一个线程问题，我读过很多类似于这个的建议，都说：更新适配器的数据集的线程和向RecyclerView分派更新的线程必须是相同的。

正如你所看到的，我已经尊重这一点，我做了：

withContext(Dispatchers.Main) {
    dataset = newDataset
    diff.dispatchUpdatesTo(this@ConversationsAdapter)
}

既然主线程，而且只有它，做这两个操作，我怎么可能得到这个错误？

Answer 1

你的diff是赛车。如果您的更新在短时间内出现两次,则可能会出现这种情况：

Adapter has dataset 1 @Main
Dataset 2 comes
calculateDiff between 1 & 2 @Async
Dataset 3 comes
calculateDiff between 1 & 3 @Async
finished calculating diff between 1 & 2 @ Async
finished calculating diff between 1 & 3 @ Async
Dispatcher main starts handling messages
replace dataset 1 with dataset 2 using 1-2 diff @Main
replace dataset 2 with dataset 3 using 1-3 diff @Main - inconsistency

备选方案是1-3之间的差异，可以在1-2之前完成，但问题不变。当新的计算出现时，您必须取消正在进行的计算，并防止部署无效的diff（例如，在片段中存储作业引用：

var updateJob : Job? = null

private fun updateConversationsList(conversations: List<ConversationsAdapter.Item>)
{
    updateJob?.cancel()
    updateJob = viewLifecycleOwner.lifecycleScope.launch {
        (listConversations.adapter as ConversationsAdapter).updateDataset(conversations)
    }
}

如果取消，则withContext(dispatchers.main)将在内部检查继续状态，并且不会运行。