设置Firestore文档时如何检测缺少网络连接 [英] How to detect absent network connection when setting Firestore document
问题描述
我们正在使用 Firestore 构建实时聊天应用.我们需要处理没有 Internet 连接的情况.基本的消息发送代码如下
We are building a real-time chat app using Firestore. We need to handle a situation when Internet connection is absent. Basic message sending code looks like this
let newMsgRef = database.document("/users/(userId)/messages/(docId)")
newMsgRef.setData(payload) { err in
if let error = err {
// handle error
} else {
// handle OK
}
}
连接设备后，一切正常.当设备未连接时，不会调用回调，也不会得到错误状态.
When device is connected, everything is working OK. When device is not connected, the callback is not called, and we don't get the error status.
当设备重新上线时，记录出现在数据库和回调触发器中，但是这个解决方案对我们来说是不可接受的，因为在此期间应用程序可能已经被终止，然后我们将永远无法获得回调并能够设置已发送消息的状态.
When device goes back online, the record appears in the database and callback triggers, however this solution is not acceptable for us, because in the meantime application could have been terminated and then we will never get the callback and be able to set the status of the message as sent.
我们认为禁用离线持久性(默认情况下处于启用状态)会使其立即触发失败回调，但出乎意料的是 - 事实并非如此.
We thought that disabling offline persistence (which is on by default) would make it trigger the failure callback immediately, but unexpectedly - it does not.
我们还尝试添加超时，在此之后发送操作将被视为失败，但是当设备重新联机时无法取消消息传递，因为 Firestore 使用其队列，这会导致更多混乱，因为消息是在接收方交付，而我无法在发送方处理.
We also tried to add a timeout after which the send operation would be considered failed, but there is no way to cancel message delivery when the device is back online, as Firestore uses its queue, and that causes more confusion because message is delivered on receiver’s side, while I can’t handle that on sender’s side.
如果我们可以减少超时 - 这可能是一个很好的解决方案 - 我们将很快获得成功/失败状态，但 Firebase 不提供这样的设置.
If we could decrease the timeout - it could be a good solution - we would quickly get a success/failure state, but Firebase doesn’t provide such a setting.
内置离线缓存可能是另一种选择，我可以将所有写入视为成功并依赖 Firestore 同步机制，但如果应用程序在离线期间终止，则不会传递消息.
A built-in offline cache could be another option, I could treat all writes as successful and rely on Firestore sync mechanism, but if the application was terminated during the offline, message is not delivered.
最终，我们需要一个一致的反馈机制来触发回调，或者提供一种方法来监控队列中的消息等等——这样我们就可以确定消息已经或没有发送，以及何时发生.
Ultimately we need a consistent feedback mechanism which would trigger a callback, or provide a way to monitor the message in the queue etc. - so we know for sure that the message has or has not been sent, and when that happened.
推荐答案
Firestore 的完成回调仅在服务器上写入(或拒绝)数据时调用.没有网络连接时没有回调，因为这被认为是 Firestore SDK 的正常情况.
The completion callbacks for Firestore are only called when the data has been written (or rejected) on the server. There is no callback for when there is no network connection, as this is considered a normal condition for the Firestore SDK.
您最好的选择是以其他方式检测是否有网络连接，然后相应地更新您的用户界面.一些相关的搜索结果:
Your best option is to detect whether there is a network connection in another way, and then update your UI accordingly. Some relevant search results:
或者，您可以检查使用 Firestore 的内置元数据来确定消息是否已传递.如关于本地更改事件的文档:
As an alternatively, you can check use Firestore's built-in metadata to determine whether messages have been delivered. As shown in the documentation on events for local changes:
检索到的文档有一个 metadata.hasPendingWrites 属性，用于指示文档是否具有尚未写入后端的本地更改.您可以使用此属性来确定快照侦听器接收到的事件源:
Retrieved documents have a
metadata.hasPendingWritesproperty that indicates whether the document has local changes that haven't been written to the backend yet. You can use this property to determine the source of events received by your snapshot listener:
db.collection("cities").document("SF")
.addSnapshotListener { documentSnapshot, error in
guard let document = documentSnapshot else {
print("Error fetching document: (error!)")
return
}
let source = document.metadata.hasPendingWrites ? "Local" : "Server"
print("(source) data: (document.data() ?? [:])")
}
有了这个，您还可以在 UI 中正确显示消息
With this you can also show the message correctly in the UI
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