如何在iOS和Swift中使用带查询启动属性的Firebase服务器时间戳和queryStartingAtValue来跟踪用户的当前操作

Question

I am developing an iOS app in Swift which has a like feature which is the same concept as liking a Facebook post, a Twitter Tweet etc.

I want to create a listener that only listens to the user liking posts with a timestamp that starts from now. ie once they open the app. I wanted to use a firebase server timestamp value to get the current timestamp

I have the following structure in my database

    "userLikes": {
        "$uid":{
            "$messageId": {
                "timestamp": 1212121212121 // timestamp created using fb server
            }
        }
    }

This was my attempted solution but it doesn't work and the problem I have is to do with -> FIRServerValue.timestamp()

    let queryRef = ref.child("userLikes").child(uid).queryOrdered(byChild: "timestamp").queryStarting(atValue: FIRServerValue.timestamp())
    queryRef.observe(.childAdded, with: { (snapshot) in

        let utterId = snapshot.key
        self.newsFeedModel.uttersInfo[utterId]?[Constants.UttersInfoKeys.isLikedByCurrentUser] = true as AnyObject

        DispatchQueue.main.async {
            self.collectionView.reloadData()
        }

    }, withCancel: nil)

Any ideas how I Can implement this? Perhaps I should just use NSDate to get the current time for comparison but thought that using a firebase server timestamp would be the optimum way for comparison purposes.

Answer 1

You can estimate the ServerTime by taking the local time and correcting for the clock-skew and latency. The Firebase documentation has a section on Clock Skew .

Clock Skew

While firebase.database.ServerValue.TIMESTAMP is much more accurate, and preferable for most read/write ops, it can occasionally be useful to estimate the clients clock skew with respect to the Firebase Realtime Database's servers. We can attach a callback to the location /.info/serverTimeOffset to obtain the value, in milliseconds, that Firebase Realtime Database clients will add to the local reported time (epoch time in milliseconds) to estimate the server time. Note that this offset's accuracy can be affected by networking latency, and so is useful primarily for discovering large (> 1 second) discrepancies in clock time.

 let offsetRef = FIRDatabase.database().referenceWithPath(".info/serverTimeOffset") offsetRef.observeEventType(.Value, withBlock: { snapshot in if let offset = snapshot.value as? Double { let estimatedServerTimeMs = NSDate().timeIntervalSince1970 * 1000.0 + offset } }) 

This will likely work better than purely using a client-side timestamp, since that is likely to be different between clients.

Answer 2

Update for Swift 3/4:

public func getCurrentTime(completionHandler:@escaping (Double) -> ()){
    Database.database().reference(withPath: ".info/serverTimeOffset").observeSingleEvent(of: .value) { (snapshot) in
        if let time = snapshot.value as? Double{
           completionHandler(Date().timeIntervalSince1970 + time)
        }else{
            completionHandler(0)

        }
    }
}

Usage:

    getCurrentTime(){ (date) in
        print(date)
    }