Firebase & Firestore

Events for local changes metadata.hasPendingWrites

新增一個 doc 之後，會先出發 listener 才會送到後端，這個 metadata 可以判斷是不是這樣的情況 (local change)
https://firebase.google.com/docs/firestore/query-data/listen#events-local-changes

Add data to Cloud Firestore

https://firebase.google.com/docs/firestore/manage-data/add-data

Detach a listener

https://firebase.google.com/docs/firestore/query-data/listen#detach_a_listener

Firebase stop listening onAuthStateChanged

https://stackoverflow.com/questions/37370224/firebase-stop-listening-onauthstatechanged