I am using Firebase's Realtime database in my app. I am fetching data from the database and do some change and after that I am removing the observer which is not working fine.
I have some data in Realtime Database like this:
I am using firebase's observe(.value) function to get this value and after that I am updating an entry and then I am removing the observer. This is my code:
func updatePoints() {
let firebaseId = UserDefaults.standard.value(forKey: "firebaseId") as? String ?? ""
let reference = self.database.child("Points").child(firebaseId)
var handler : UInt = 0
handler = reference.observe(.value, with: { snapshot in
guard let userPoints = snapshot.value as? [String : Any] else {
print("no points data found")
return
}
let pointsLeft = userPoints["points_left"] as? Int ?? 0
reference.child("points_left").setValue(pointsLeft - 1)
reference.removeObserver(withHandle: handler)
})
}
The problem now is, this observer runs twice. For example, if "points_left" : 10, then after this function the points left will have 8 value but it should have 9 instead. It is running twice and I am not understanding why is it doing so as I am using removeObserver. Can someone help me with this?
The reason to the above unexpected behaviour is the setValue function you called to update the points is triggering another .value event in the database. Then it triggers the observer again. Therefore, by the time you remove the observer, it has already triggered twice. This leads to decrease of points by 2 instead of 1.
So if u interchange the last two lines, by the time you call the setValue function observer is removed. So it will not get triggered for the second time.
Related
I'm trying to make multiple updates to my firebase realtime database at the same time using transactions, but when I do this the calls aren't waiting for one another, and instead I only get one update instead of the seven I should be getting.
Below is a segment of my code that is called 7 times in a for loop:
global.newGame.runTransactionBlock({ (currentData: MutableData) -> TransactionResult in
if let post = currentData.value as? NSDictionary {
let player0Info = post["player0Info"] as? NSDictionary
var serverHand = player0Info!["Hand"] as? Array<Int>
serverHand?.append(2)
print("The value of serverHand is \(serverHand ?? [-1])")
global.newGame.child("player0Info/Hand").setValue(serverHand)
}
return TransactionResult.success(withValue: currentData)
}) { (error, committed, snapshot) in
if let error = error {
print(error.localizedDescription)
}
}
And the output it gives is
The value of serverHand is [2]
The value of serverHand is [2]
The value of serverHand is [2]
The value of serverHand is [2]
The value of serverHand is [2]
The value of serverHand is [2]
Is there a way to prevent this from happening? Why doesn't putting this inside of a transaction keep it from being simultaneously changed by multiple calls?
As a side note, everything works as expected when I make these calls in spread-out intervals, I only get this issue from overlapping transactions.
I'm not an iOS dev, so expect typos/syntax errors.
From what I can tell, the way you handle the transaction is incorrect. When using a transaction, you ask for the "latest" data (see note), mutate that data, then send it back. However, in your code you are getting the data, mutating it and then at the same time, both overriding it using setValue and sending the changed data back. Because you are using setValue here, you create a loose infinite loop - where sometimes the transaction finishes first (everything is okay) and sometimes the setValue finishes first (the transaction is tried again).
A transaction is not a one-time operation, if the data at the location you are using is modified, the transaction can be retried. In this case, the call to setValue can trigger one of these retries.
If serverHand.append() mutates the array (i.e. you don't need to also add player0Info!["Hand"] = serverHand.append()), then simply removing the setValue should fix your problem.
Lastly, as pointed out by #Jay in this answer's comments, you need to also set currentData.value with the new data to commit any changes.
global.newGame.runTransactionBlock({ (currentData: MutableData) -> TransactionResult in
if let post = currentData.value as? NSDictionary {
let player0Info = post["player0Info"] as? NSDictionary
var serverHand = player0Info!["Hand"] as? Array<Int>
serverHand?.append(2)
print("The value of serverHand is \(serverHand ?? [-1])")
// note that the setValue was removed
currentData.value = post; // link new data for updating
}
return TransactionResult.success(withValue: currentData)
}) { (error, committed, snapshot) in
if let error = error {
print(error.localizedDescription)
}
}
Note: I used "latest" in quotes because the first time a transaction is tried, the locally cached data is used to process the transaction. If the server's data is different from what's cached locally, the transaction will be processed a second time.
Edit 1: Added #Jay's contributions and removed brief references to the log messages being abnormal, as if called in a loop, multiple logs are to be expected.
I'm trying to get my firebase database data into a variable to use it in my project
var someArray = [Array]()
let dbRef = Database.database().reference().child("SomeDatabase")
func loadSomeDatabaseData {
dbRef.observeSingleEvent(of: .value) { (snapshot) in
let someDict = snapshot.value as! [String:Any]
let keysOfSomeDict = Array(someDict.keys)
self.someArray.append(contentsOf: keysOfSomeDict)
self.collectionView?.reloadData()
}
}
I've tried calling loadSomeDatabaseData() in my viewDidload, followed by printing someArray, which results in an empty array. I know the keysOfSomeDict array has the correct data that I want, since i tried printing this array directly inside the closure. I would however also like to be able to print and use this data elsewhere in my app.
The Firebase observeSingleEvent method is asynchronous method. it executive in background only because it take time to fetch data from Firebase.
if you print array immediately means you get only empty array.
so print array once you get the data from Firebase. for that you can use escaping closure
Function Declaration:
func loadSomeDatabaseData(resultArray : #escaping([Array])->()) {
dbRef.observeSingleEvent(of: .value) { (snapshot) in
let someDict = snapshot.value as! [String:Any]
let keysOfSomeDict = Array(someDict.keys)
self.someArray.append(contentsOf: keysOfSomeDict)
resultArray(self.someArray)
}
}
Func Call:
self.loadSomeDatabaseData{(firebaseReposne) in
print("FirebaseData" , firebaseReposne) // Hope here you will get your firebase data.
self.collectionView?.reloadData()
}
The observeSingleEvent is asynchronous. So immediately printing someArray after calling loadSomeDatabaseData will result in an empty array. It takes sometime to retrieve the data from Firebase api.
To use this data elsewhere in the app, you could set a flag indicating the data is loaded or send a notification to inform the data is available.
Okay I am reading from a database and when I print the individual variables they print out correctly. However it seems like the data refuses to append to the array. Anyone know why? I can't figure it out at all.
let commuteBuilder = Commutes()
Database.database().reference().child("Users").child(user).child("Trips").observe(DataEventType.childAdded, with: { (snapshot) in
//print(snapshot)
if let dict = snapshot.value as? NSDictionary {
commuteBuilder.distance = dict["Distance"] as! Double
commuteBuilder.title = dict["TripName"] as! String
commuteBuilder.transportType = (dict["Transport"] as? String)!
}
commuteArray.append(commuteBuilder)
})
print("helper")
print(commuteArray.count)
return commuteArray
The data is correctly added to the array, just not at the time that you print the array's contents.
If you change the code like this, you can see this:
let commuteBuilder = Commutes()
Database.database().reference().child("Users").child(user).child("Trips").observe(DataEventType.childAdded, with: { (snapshot) in
if let dict = snapshot.value as? NSDictionary {
commuteBuilder.distance = dict["Distance"] as! Double
commuteBuilder.title = dict["TripName"] as! String
commuteBuilder.transportType = (dict["Transport"] as? String)!
}
commuteArray.append(commuteBuilder)
print("added one, now have \(commuteArray.count)")
})
print("returning \(commuteArray.count)")
return commuteArray
You'll see it print something like this:
returning 0
added one, now have 1
added one, now have 2
etc.
This is likely not the output you expected. But it is working as intended. Firebase loads data from its database asynchronously. Instead of blocking your code, it lets the thread continue (so the user can continue using the app) and instead calls back to the code block you passed to observe when new data is available.
This means that by the time this code returns the array it is still empty, but it later adds items as they come in. This means that you cannot return data from a function in the way you are trying.
I find it easiest to change my way of thinking about code. Instead of "First get the data, then print it", I frame it as "Start getting the data. When data comes back, print it".
In the code above, I did this by moving the code that prints the count into the callback block. Instead of doing this, you can also create your own callback, which is called a completion handler or closure in Swift. You can find examples in this article, this article, this question Callback function syntax in Swift or of course in Apple's documentation.
I just want to ask about firebase retrieve data. How can i handle firebase retrieve data finished? I don't see any completion handler.
I want to call some function after this firebase data retrieve finished. How can i handle???
DataService.ds.POST_REF.queryOrderedByChild("created_at").observeEventType(.ChildAdded, withBlock: { snapshot in
if let postDict = snapshot.value as? Dictionary<String, AnyObject> {
let postKey = snapshot.key
let post = Post(postKey: postKey, dictionary: postDict)
self.posts.append(post)
}
})
In Firebase, there isn't really a concept of 'finished' (when listening to 'child added'). It is just a stream of data (imagine someone adds a new record before the initial data is 'finished'). You can use the 'value' event to get an entire object, but that won't give you new records as they're added like 'child added' does.
If, you really need to use child added and get notified when it's probably finished, you can set a timer. I don't know swift, but here's the logic.
Set up your 'child added' event.
Set a timer to call some finishedLoading() function in 500ms.
Each time the 'child added' event is triggered, destroy the timer set in step two and create another one (that is, extend it another 500ms).
When new data stops coming in, the timer will stop being extended and finsihedLoading() will be called 500ms later.
500ms is just a made up number, use whatever suits.
Do one request for SingleEventOfType(.Value). This will give you all info initially in one shot, allowing you to then do whatever function you want to complete once you have that data.
You can create a separate query for childAdded and then do anything there you want to do when a new post has been added
Write your entire block of code in a function which has a completion handler like so:
func aMethod(completion: (Bool) -> ()){
DataService.ds.POST_REF.queryOrderedByChild("created_at").observeEventType(.ChildAdded, withBlock: { snapshot in
if let postDict = snapshot.value as? Dictionary<String, AnyObject> {
let postKey = snapshot.key
let post = Post(postKey: postKey, dictionary: postDict)
self.posts.append(post)
}
completion(true)
})
}
Then call it somewhere like so:
aMethod { success in
guard success == true else {
//Do something if some error occured while retreiving data from firebase
return
}
//Do something if everything went well.
.
.
.
I've looked over hours of code and notes and I'm struggling to find any documentation that would help me with upvoting and downvoting an object in a swift app with firebase.
I have a gallery of photos and I'm looking to add an instagram style upvote to images. The user has already logged with firebase auth so I have their user ID.
I'm just struggling to figure the method and what rules need to be set in firebase.
Any help would be awesome.
I will describe how I implemented such a feature in social networking app Impether using Swift and Firebase.
Since upvoting and downvoting is analogous, I will describe upvoting only.
The general idea is to store a upvotes counter directly in the node corresponding to an image data the counter is related to and update the counter value using transactional writes in order to avoid inconsistencies in the data.
For example, let's assume that you store a single image data at path /images/$imageId/, where $imageId is an unique id used to identify a particular image - it can be generated for example by a function childByAutoId included in Firebase for iOS. Then an object corresponding to a single photo at that node looks like:
$imageId: {
'url': 'http://static.example.com/images/$imageId.jpg',
'caption': 'Some caption',
'author_username': 'foobarbaz'
}
What we want to do is to add an upvote counter to this node, so it becomes:
$imageId: {
'url': 'http://static.example.com/images/$imageId.jpg',
'caption': 'Some caption',
'author_username': 'foobarbaz',
'upvotes': 12,
}
When you are creating a new image (probably when an user uploads it), then you may want to initialize the upvote counter value with 0 or some other constant depending on what are you want to achieve.
When it comes to updating a particular upvotes counter, you want to use transactions in order to avoid inconsistencies in its value (this can occur when multiple clients want to update a counter at the same time).
Fortunately, handling transactional writes in Firebase and Swift is super easy:
func upvote(imageId: String,
success successBlock: (Int) -> Void,
error errorBlock: () -> Void) {
let ref = Firebase(url: "https://YOUR-FIREBASE-URL.firebaseio.com/images")
.childByAppendingPath(imageId)
.childByAppendingPath("upvotes")
ref.runTransactionBlock({
(currentData: FMutableData!) in
//value of the counter before an update
var value = currentData.value as? Int
//checking for nil data is very important when using
//transactional writes
if value == nil {
value = 0
}
//actual update
currentData.value = value! + 1
return FTransactionResult.successWithValue(currentData)
}, andCompletionBlock: {
error, commited, snap in
//if the transaction was commited, i.e. the data
//under snap variable has the value of the counter after
//updates are done
if commited {
let upvotes = snap.value as! Int
//call success callback function if you want
successBlock(upvotes)
} else {
//call error callback function if you want
errorBlock()
}
})
}
The above snipped is actually almost exactly the code we use in production. I hope it helps you :)
I was very surprised, but this code from original docs works like a charm. There is one disadvantage with it: the json grows pretty big if there are a lot of likes.
FirebaseService.shared.databaseReference
.child("items")
.child(itemID!)
.runTransactionBlock({ (currentData: MutableData) -> TransactionResult in
if var item = currentData.value as? [String : AnyObject] {
let uid = SharedUser.current!.id
var usersLikedIdsArray = item["liked_who"] as? [String : Bool] ?? [:]
var likesCount = item["likes"] as? Int ?? 0
if usersLikedIdsArray[uid] == nil {
likesCount += 1
usersLikedIdsArray[uid] = true
self.setImage(self.activeImage!, for: .normal)
self.updateClosure?(true)
} else {
likesCount -= 1
usersLikedIdsArray.removeValue(forKey: uid)
self.setImage(self.unactiveImage!, for: .normal)
self.updateClosure?(false)
}
item["liked_who"] = usersLikedIdsArray as AnyObject?
item["likes"] = likesCount as AnyObject?
currentData.value = item
return TransactionResult.success(withValue: currentData)
}
return TransactionResult.success(withValue: currentData)
}) { (error, committed, snapshot) in
if let error = error {
self.owner?.show(error: error)
}
}
Not a Swift fella myself (pun!) but I think this stackoverflow question has most of your answers.
Then you would simply use a couple of if statements to return the correct value from the transaction based on whether you want to up vote or down vote.