I have a SwiftUI calendaring app with a UI similar to the built-in Calendar.app. I'm getting crashes whenever I try to delete events. The overall lifecycle of my app is as follows:
Download calendar data from server and populate models ([Events], [Users], [Responses] etc)
Transform the source data into a more structured format (see https://stackoverflow.com/a/58583601/2282313)
Render list view of events, each event linking to a Detail View and an Edit modal (very similar to calendar.app)
When an event is deleted, I tell the server to delete the event (if it's a recurring event, the server will delete multiple events), then refresh my data from the server by re-downloading the data, re-populating the models and re-generating the structured data (which causes the list to refresh).
When I do this, I get crashes coming from my calculated values because event data displayed in the detail view is no longer available. For example, I get the array index of a user's RSVP as follows:
var responseIndex: Int {
userData.responses.firstIndex(where: { $0.user == response.user && $0.occurrence == response.occurrence })!
}
I thought this was because I hadn't dismissed the view displaying the deleted event before updating the data, but even if I delay the data refresh until the view is no longer displayed, I still get the crash (SwiftUI seems to keep these views in memory).
What is the right way to handle data deletion? Do I need to keep deleted events in my UserData EnvironmentObject and just mark them as "deleted/hidden" to avoid this issue, or is there a better way to handle it?
There's quite a bit of code involved in this, so it's tricky to provide a sample I'm happy to add relevant bits if asked.
EDIT: I found this article which clarifies something really well: https://jasonzurita.com/swiftui-if-statement/
SwiftUI is perfectly happy to try and render nil views, it just draws nothing. Counter-intuitively, a good way to avoid crashes and make the compiler happy is to set your code up around this.
Original "answer" follows...
I don't know if this is the "right" way to do this, but I ended up making sure that none of my UserData is ever deleted to avoid the crashes. I added a "deleted" bool to my Occurrence (i.e. Event) object, and when I refresh my structured data, I get the latest data from the server, but check to see if any of the old ones are no longer present. Steps are:
Get latest list of occurrences from server
Create a second init() for my structured data which takes the existing data as an argument
Inside the new init(), flatten the structured data, check for deleted items against the new data, update data which hasn't been removed, cull duplicates, then merge in net new data. Once that's done, I call my original init() with the modified data to create new structured data
Code looks like this:
init(occurrences: [Occurrence], existing: [Day]) {
// Create a mutable copy of occurrences (useful so I can delete duplicates)
var occurrences = occurrences
// Flatten the structured data into a plan array of occurrences again
var existingOccurrences = existing.compactMap({ $0.occurrences }).flatMap { $0 }
// Go through existing occurrences and see if they still exist.
existingOccurrences = existingOccurrences.map {
occurrence -> Occurrence in
let occurrenceIndex: Int? = occurrences.firstIndex(where: { $0.id == occurrence.id })
// If the occurrence no longer exists, mark it as "deleted" in the original data
if occurrenceIndex == nil {
var newOccurrence = occurrence
newOccurrence.deleted = true
return newOccurrence
// If it still exists, replace the existing copy with the new copy
// (in case it has changed since the last pull from the server)
// Remove the event from the "new" data so you don't get duplicates
} else {
let newOccurrence = occurrences[occurrenceIndex!]
occurrences.remove(at: occurrenceIndex!)
return newOccurrence
}
}
// Merge the existing data (with deleted items marked) and the updated data (with deleted items removed)
let finalOccurrences = existingOccurrences + occurrences
// Re-initialize the strutured data with the new array of data
self = EventData(occurrences: finalOccurrences)
}
Once this was done, I had to update my code to make sure I'm always using my structured data as the source of truth (which I wasn't doing before because accessing the "source" flat data was often easier, and I've updated my ForEach in my list view to only render a row if deleted is false.
It works! It's perhaps a sub-optimal way to solve the problem, but no more crashes. Still interested to hear better ways to solve the problem.
Related
I am learning pagination with Firebase. I am using a method in which I store the key of the last added item in the previous page, so the next page can continue from there.
The problem is that when using ref.queryStarting(at value: lastItemKey) to keep retrieving items from the last added key, the last item gets repeated twice (since queryStarting is inclusive).
And so if I limit to 5 the query I would end up with only 4 new items as 1 would be a duplicate.
The only solution I came up is requesting one more item and remove the repeated one, but I wonder if it´s efficient at all doing it this way. (since we are wasting one item in each query)
If it´s any help, my code looks like this:
// rest of the pages
if let lastItemID = lastItemKey {
itemPageRef = self.itemsRef.queryOrderedByKey().queryStarting(atValue: lastItemID)
.queryLimited(toFirst: UInt(amount))
} else {
// First page of data: we retrieve the first (amount) items
print("We are in the first page of DATA")
itemPageRef = self.itemsRef.queryOrderedByKey().queryLimited(toFirst: UInt(amount))
}
itemPageRef.observeSingleEvent(of: .value, with: { [weak self] (snapshot) in
Requesting an overlapping child node between the pages is the only way the Firebase API supports. Since there is no other way to do this, there isn't a more efficient way.
That said, it's typically quite efficient, especially if you use a page size of 25+ child nodes, which is also more reasonable on most use-cases I've seen.
I've got an app that uses a Firebase db containing 100,000 items. My app has to process through each of these items which takes several seconds.
What is happening is that every time the app is launched (from a terminated state) those 100,000 items are being processed each time (even if the contents of the db on the Firebase server have not changed). Obviously, I don't want the app to do this if not necessary. Here's some code:
if dbRef == nil {
FirebaseApp.configure();
Database.database().isPersistenceEnabled = true
...
let dbRef = Database.database().reference(withPath: kFirebaseDBName)
_ = spamRef.observe(DataEventType.value, with: { (theSnapshot) in
if let content = theSnapshot.value as? [String : AnyObject]
{
self.processContent(content: content)
}
Each time the app is started then the content snapshot contains the entire database reference contents.
Is there a way of, for example, getting the last date the database was updated (on the server), or only obtaining the delta of changed items between each app launch - can a query return just changed since last queried for example, or something similar?
I don't know how many items have changed so cannot call something like:
queryLimited(toLast: N))
As I don't know what value N is.
I've tried adding keepSynced as follows in the hope it might change things, but no.
if dbRef == nil {
FirebaseApp.configure();
Database.database().isPersistenceEnabled = true
...
let dbRef = Database.database().reference(withPath: kFirebaseDBName)
dbRef.keepSynced(true)
_ = dbRef.observe(DataEventType.value, with: { (theSnapshot) in
if let content = theSnapshot.value as? [String : AnyObject]
{
self.processContent(content: content)
}
I have no idea how much data might have changed so don't know what value to supply to something like toLast or similar to modify the observation parameters.
The database (which was not created nor updated with new content by me) has 100,000 items in a flat structure (i.e. one parent with 100,000 children) and any number of these children in any order might have been deleted and replaced since last time my app ran, but the total will still be 100,000. None of the children have an explicit timestamp or anything like that.
I was under the impression if Firebase kept a local cache of the data (due to isPersistenceEnabled) then next time it connects with the server it would only sync what had changed on the server. Therefore in order to do this Firebase itself must internally have some delta information somewhere, so I was hoping that delta information may available in some form to my app.
Note: My app does not need persistence to be enabled, the above code is doing so just as variations to see if anything will result in the behavior I desire with the observer.
UPDATE
So looking at the documentation more you can set a timestamp for the last time a user was connected to the server using:
lastOnlineRef.onDisconnectSetValue(ServerValue.timestamp())
Take a look at this question Frank explains some issues with persistence and listeners. The question is for Android but the principles are the same.
I still think the problem is your query. Since you already have the data persisted .value is not what you want since this returns all of the data.
I think you want to attach a .childChanged listener to your query. In this case the query will only return the data that has been changed. If you haven't heard of .childChanged before you can read about it here.
I didn't realize this problem is specifically related to persistence. I think you are looking for keepSynced(). Take a look at this.
ORIGINAL ANSWER
The problem is your query. You are asking for all of the data that's why you're getting all of the data. You want to look into limiting your queries using toFirst or toLast. Additionally, I don't think you can query for the last time the database was updated. You could check the last node in your data structure if you have the timestamp saved, but you might as well just get the newest data.
You want something like this:
ref.child("yourChild").queryLimited(toLast: 7).observeSingleEvent(of: .value, with: { snap in
// do something
})
Depending on how you're writing your data you'll want toLast or toFirst. Assuming the newest data is written last toLast is what you want. Also note that the numbers I am limiting to are arbitrary you can use any number that fits your project.
If you already have a key and you want to start querying above that key you can do something like this:
ref.child("YourChild").queryOrderedByKey().queryEnding(atValue: lastVisiblePostKey).queryLimited(toLast: 8).observeSingleEvent(of: .value, with: { snap in
// do something with more posts
})
You may also want to look into this question, this question and pagination.
I'm currently creating an app which stores information in a database which the users device must have in order for certain parts of the app to respond differently.
My question is, when should I be calling for this data from firebase.
e.g.
In one part of my app, the app needs to know if the user is currently "connected" to another user. Currently, it checks this against the database as the user presses on the tab bar icon where this information needs to be known, which takes a couple seconds. (checked in the viewdidload() override func)
Should I be grabbing all data from the database before the first view controller is even displayed?
Is there a way to share this between all the view controllers?
If I could load all data from the database into global variables on the device that all view controllers can see this would seem much easier, however i'm not sure if this is good practice.
What would you recommend?
My database structure:
Basically, right now, when the user opens the app and logs in, I need the 'name' and 'family' of each user to be stored for use across the whole app globally across all classes and view controllers.
In terms of the list. when the user clicks the view controller where the list is, currently i'm just running code like this
self.ref.child("familys").child(email.replacingOccurrences(of: ".", with: "")).child("list").observe(.value, with: { (DataSnapshot) in
if DataSnapshot.hasChildren() == false{
print("No list")
return
}
self.tableList = DataSnapshot.value as! [String]
self.tableView.reloadData()
}) { (Error) in
print(Error)
}
then it goes ahead and updates the list with the array 'tableList'.
This means the first time the user clicks to get to the shopping view, there is some delay before the list populates.
I'm not sure what the standard way is to go about grabbing data like this and when it should be done in a way which minimises data usage and database access frequency.
I think you are on the right track. Although you may think you need all the data stored globally so you can access it from all view controllers, you don't. You can pass the data between viewControllers through segues. If you are making a tab based app, you just pull the data necessary for that ViewController in the viewDidLoad() (just like you are doing).
The name of the game is to structure your Firebase database so that when you pull data, you can pull as little as possible to fill all the fields in the View Controller. Since Firebase uses a JSON structure, there is no shame in saving the same data twice in order to make a search faster.
That being said, I think a currentUser global variable is useful in your case. Assuming you have a current user (one user logged in), I would just create a User class that mimics the Firebase and instantiate one global variable called currentUser. Your currentUser object should contain enough information to go and pull anything you need for filling ViewControllers. For example if your User class has an email attribute, you can do:
self.ref.child("familys").child(currentUser.email.replacingOccurrences(of: ".", with: "")).child("list").observe(.value, with: { (DataSnapshot) in
if DataSnapshot.hasChildren() == false{
print("No list")
return
}
self.tableList = DataSnapshot.value as! [String]
self.tableView.reloadData()
}) { (Error) in
print(Error)
}
Global variables should be avoided when possible, but I think just reducing it to one global variable should be enough to get you going.
As for why you are getting a delay, I'm not sure. When you are pulling that little data, it should be extremely fast. If there really is a noticeable delay when pulling a list of 2 items, the issue might be elsewhere (network, simulator, etc.).
When I call this observe function from in my viewcontroller, the .childadded immediately returns a object that was already stored instead of has just bin added like .childadded would suspect.
func observe(callback: RiderVC){
let ref = DBProvider.Instance.dbRef.child("rideRequests")
ref.observe(DataEventType.childAdded) { (snapshot: DataSnapshot) in
if let data = snapshot.value as? NSDictionary {
let drive = cabRide(ritID: ritID, bestemming: bestemming,
vanafLocatie: vanaf, taxiID: taxiID, status: status)
print(drive)
callback.alertForARide(title: "Wilt u deze rit krijgen?", message: "Van: \(vanaf), Naar: \(bestemming)", ritID: ritID)
}
}
}
When I try this function with .childchanged, I only get a alert when it is changed like it suppose to do, but when doing .chiladded, it just gets all the requests out of the database and those requests were already there.
When I add a new request, it also gives an alert. So it works, but how can I get rid of the not added and already there requests?
Does anybody know this flaw?
This is working exactly as promised. From the documentation:
Retrieve lists of items or listen for additions to a list of items.
This event is triggered once for each existing child and then again
every time a new child is added to the specified path. The listener is
passed a snapshot containing the new child's data.
That might seem weird at first, but this is generally what most developers want, as it's basically a way of asking for all data from a particular branch in the database, even if new items get added to it in the future.
If you want it to work the way you're describing, where you're only getting new items in the database after your app has started up, you'll need to do a little bit of work yourself. First, you'll want to add timestamps to the objects you're adding to the database. Then you'll want to do some kind of call where you're asking to query your database by those timestamps. It'll probably look something like this:
myDatabaseRef.queryOrdered(byChild: "myTimestamp").queryStarting(atValue: <currentTimestamp>)
Good luck!
I have an iOS app that synchronises to Parse.com.
It can find anything that was added to Parse and add it to Core Data using PFQuery. It can also check for any data that has been updated and update accordingly.
However, I'm not sure how to find objects that have been deleted on Parse.com.
Does anyone know of a query that will list the ObjectIDs that have been deleted and the date of their deletion? I can then remove them from the Core Data on the app.
I needed this function, too, but figured that marking rows as deleted will bloat the data and add a condition to every query. So I created a Deletion class. It records only the class name and ID of any deleted row, so it stays pretty small:
function recordDeletion(klass, identifier) {
var Deletion = Parse.Object.extend("Deletion");
var deletion = new Deletion();
deletion.set("klass", klass);
deletion.set("identifier", identifier);
return deletion.save();
}
// for every class that you want deletions recorded, add one of these...
Parse.Cloud.beforeDelete("MyClass", function(request, response) {
recordDeletion("MyClass", request.object.id).then(function() {response.success();});
});
My iOS clients record the date when they last fetched data, then get everything newly created/updated from MyClass (+ others) and Deletion. With that, the can delete the Deletions locally.
Over a longer period, the clients remove all of the locally cached data and get a fresh copy of everything (except Deletions). This allows me to have a scheduled job on the server that will empty the Deletion table (on a cycle that's much longer than the client's cycle).
There is no provided API for this.
As per the comment from #Fogmeister you can tag objects as deleted and update like that. Alternatively you can maintain a specific list of deleted ids (potentially using Parse.Cloud.beforeDelete) and then make a specific request to get only the deletions.
In either case you will need to explicitly manage the scheme you choose and also decide how and when to clean up the deleted objects / deletion records.