Swift应用中Firestore使用count聚合获取未读通知数的实现疑问

原本使用以下fetch()函数从Firestore获取当前用户的未读通知（read字段值为"n"），并映射为Notification对象列表：

func fetch() async throws {
    guard let uid = Auth.auth().currentUser?.uid else { return }
    let snapshot = try await  Firestore.firestore().collection("notifications").document(uid).collection("user-notifications")
        .whereField("read", isEqualTo: "n")
        .limit(to: 10)
        .getDocuments()
    self.notifications = try snapshot.documents.compactMap({ try $0.data(as: Notification.self) })
}

现在想要切换到fetch2()函数，使用Firestore的count聚合仅获取未读通知的数量，但不清楚如何替换原fetch()中self.notifications = try snapshot.documents.compactMap({ try $0.data(as: Notification.self) })这部分代码：

func fetch2() async throws {
    guard let uid = Auth.auth().currentUser?.uid else { return }
    let query = Firestore.firestore().collection("notifications").document(uid).collection("user-notifications")
        .whereField("read", isEqualTo: "n")
        .limit(to: 10)
    let countQuery = query.count
    let snapshot = try await countQuery.getAggregation(source: .server)
}

请问该如何实现这部分代码的替换？

Firestore聚合查询返回的AggregationSnapshot对象直接包含计数结果，你可以通过snapshot.count属性获取未读通知数量，赋值给对应的存储属性即可：

完整的fetch2()实现 #

// 先在类中定义存储数量的属性
var unreadNotificationCount: Int64 = 0

func fetch2() async throws {
    guard let uid = Auth.auth().currentUser?.uid else { return }
    let query = Firestore.firestore().collection("notifications").document(uid).collection("user-notifications")
        .whereField("read", isEqualTo: "n")
        .limit(to: 10)
    let countQuery = query.count
    let snapshot = try await countQuery.getAggregation(source: .server)
    
    // 替换原赋值逻辑，直接获取count值
    self.unreadNotificationCount = snapshot.count
}

补充说明 #

• snapshot.count的类型是Int64，所以存储属性建议定义为Int64；如果需要转为Int，可以用Int(snapshot.count)，注意数值溢出风险。
• 原查询中的limit(to:10)会限制计数上限为10，即未读通知超过10条时，返回的count值为10，和原逻辑保持一致。

内容的提问来源于stack exchange，提问作者Eren Sketch