Firebase实时数据库：完成监听器是否离线持久化？离线写入同步疑问

Great questions—let's break down each one clearly based on how Firebase Realtime Database handles offline operations:

1. Do completion listeners work with offline persistence? #

Short answer: No, not immediately. Here's the full breakdown:

• Completion listeners are tied directly to the server's acknowledgment of a write operation. When your device is offline, Firebase caches the write locally (if persistence is enabled) but can't get confirmation from the server.
• The listener will only trigger once the device comes back online, the write is successfully synced to Firebase's servers, and the server sends back a confirmation.
• If the write fails to sync (e.g., due to security rules blocking it), the listener will trigger with an error at that point too.

In short: Offline persistence stores the write locally, but the completion listener waits for server confirmation before firing.

2. What happens to the 2 offline writes to the Items node when back online? #

Let's walk through this scenario step by step:

• First, remember that Firebase generates push IDs client-side, even when offline. So each of your two button clicks will create a unique, non-overlapping push ID for the Items node entry—no duplicates here.
• When offline, Firebase caches each Items write to disk (since you have persistence enabled). But since you're waiting for the Items write's completion listener to trigger before writing to Items_2, those Items_2 writes won't be cached yet—because the listener never fires offline.
• Once your device comes back online:
  1. Firebase will process the cached writes in the order they were triggered (first click's Items write, then the second).
  2. For each Items write:
     • It gets synced to the server successfully.
     • The completion listener fires, which then triggers the corresponding Items_2 write (which will sync to the server immediately, since you're online).
• End result: Your Items node will have two distinct entries (each with their own unique push ID), added in the order you clicked the button. The Items_2 node will have the corresponding two writes, each executed right after their linked Items entry is confirmed by the server.

If your code was written to execute both writes without waiting for the completion listener (not what you described, but just in case), Firebase would cache both writes per click, and sync all four operations in order once online—still resulting in two entries in Items and two in Items_2.

内容的提问来源于stack exchange，提问作者Hasan Bou Taam