example of listening firestore realtime update in golang

firestore-listen-query.go

package main

import (
	"firebase.google.com/go"
	"cloud.google.com/go/firestore"
	"context"
	"log"
	"fmt"
)

var client *firestore.Client
var ctx = context.Background()

func initDB(){
	colref := client.Collection("users")
	colref.Doc("user1").Set(ctx, map[string]interface{}{
		"id": "user1",
		"name": "username",
		"state": 0,
	})
	colref.Doc("user2").Set(ctx, map[string]interface{}{
		"id": "user2",
		"name": "username",
		"state": 1,
	})
	colref.Doc("user3").Set(ctx, map[string]interface{}{
		"id": "user3",
		"name": "username",
		"state": 3,
	})
	colref.Doc("user4").Set(ctx, map[string]interface{}{
		"id": "user4",
		"name": "username",
		"state": 1,
	})
	colref.Doc("user5").Set(ctx, map[string]interface{}{
		"id": "user5",
		"name": "username",
		"state": 5,
	})
}

func main(){
	conf := &firebase.Config{ProjectID: "PROJECT_ID"}
	app, err := firebase.NewApp(ctx, conf)
	if err != nil {
		log.Fatalln(err)
	}

	client, err = app.Firestore(ctx)
	if err != nil {
		log.Fatalln(err)
	}
	defer client.Close()
	fmt.Println("firebase app is initialized.")

	initDB()

	// Get user2 and user4
	snapIter := client.Collection("users").Where("state", "==", 1).Snapshots(ctx)
	defer snapIter.Stop()

	for{
		snap, err := snapIter.Next()
		if err != nil {
			log.Fatalln(err)
		}

		fmt.Printf("change size: %d\n", len(snap.Changes))
		for _, diff := range snap.Changes {
			fmt.Printf("diff: %+v\n", diff)
		}
		fmt.Println("")
	}
}