Working on a new project, our team decided to use UICollectionViewDiffableDataSource to handle our collection views. It works fine, but we're getting (infrequent) crashes logged via an online tool with the message Invalid parameter not satisfying: initialSnapshot.numberOfSections == initialSections.count. We can't seem to reproduce this locally.
The crash occurs at the point where we update the data source with new data, specifically at dataSource.apply(snapshot). We're unsure as to how this could happen, as the data is always created the same.
Specifically, the unit working on this one view decided to forgo the creation of a section model and instead decided to use an Int as section identifiers, because they didn't want to use sections, just display items. That's one thing I hadn't seen before, but an Int satisfies the requirements for identifiers, so the code does compile correctly.
Here is the code:
Collection View and Data Source creation
These variables are inside the class of a UIView which is programmatically created.
var collectionView = UICollectionView(frame: .zero, collectionViewLayout: createCollectionViewLayout())
lazy var dataSource = UICollectionViewDiffableDataSource<Int, URL>(collectionView: collectionView, cellProvider: provideCell(_:indexPath:item:))
Updating the Datasource
func updateUI() {
var snapshot = dataSource.snapshot()
snapshot.deleteAllItems()
snapshot.appendSections([0])
if let urls = viewModel?.imageUrls {
snapshot.appendItems(urls)
}
dataSource.apply(snapshot)
}
In all (production) cases I have tested, viewModel?.imageUrls is empty on the first call, then contains items on the second call and all calls after that. The number of items usually doesn't change.
I have thought about not using dataSource.snapshot() and instead creating a new one, then I also wouldn't have to call deleteAllItems() every time. But I don't want to just push this as the solution when I can't be sure whether this really fixes the issue.
Has anyone encountered an issue like this before? Is it correct to use an Int as the section identifier? What could be other causes of the crash?
My guess is that you delete only items and each time you add an additional section to your snapshot, which causes an error.
So if your Snapshot already has a "0" section, you do not need to add a new one each time.
In my projects I create a new snapshot every time:
var snapshot = NSDiffableDataSourceSnapshot<Int, URL>()
snapshot.appendSections([0])
if let urls = viewModel?.imageUrls {
snapshot.appendItems(urls, toSection: 0)
}
customDataSource.apply(snapshot, animatingDifferences: animate)
The same approach is used in WWDC videos.
Related
I have a dashboard view that I'd like to display a few variables from my last entered CoreData entry. However, I can't figure out how to fetch only the last data entered into a variable so I can display it. Any ideas?
EDIT: I'm trying to setup a NSFetchRequest inside of a called function that is called only onappear. However, I'm getting errors and am lost.
func singleEntryPull() -> [Item] {
let request: NSFetchRequest<Item> = Item.fetchRequest()
request.sortDescriptors = [NSSortDescriptor(key: "todaysDate", ascending: false)]
request.fetchLimit = 1
let singleEntry = FetchRequest(fetchRequest: request)
return singleEntry
}
And then the return from the function should only show 1 result and I can then use the returned value to display the variables I need?
Well, not sure if this is the cleanest or best way to do this but I got it working like I want for now until a better solution comes up. I'm still using #FetchRequest which im now aware is pulling data live and updating it, but that might work as if someone keeps the app open overnight and updates it in the morning, I'd want it to display that latest entry. I used this:
ForEach(singleEntry.prefix(1)) { item in
A fetch limit of 1 on a fetch request will return you a single value. However, when you're setting up a #FetchRequest, you're doing more than this - you're making the initial fetch and then continuing to monitor the context for changes, so it live updates. This monitoring only uses the predicate of your fetch request.
Depending on your order of operations, you could be seeing the latest data, and then any new data inserted since you started that view. My experiments with the SwiftUI core data template project prove this out - on initial run you get the a single latest entry, but as you add newer ones, the fetch-limited screen picks up the new entries.
Depending on how this view is actually used, you have two choices - you can do an actual fetch request on appear of the view and store the result as an observable object, or you can make sure you only ever use the first record from the fetch request's results array, which will always be the latest record because of your sort ordering:
var body: some View {
if let latest = singleEntry.first {
// Some view describing the latest entry
} else {
Text("No record")
}
}
So my goal is to get rid of these bugs completely. I am in a dilemma where each decision leads to a bug.
The first thing I can do that eventually becomes an issue is use a String-interpolated collection path in all my query functions like so:
func getEventName() {
listener = db.collection("school_users/\(user?.uid)/events").order(by: "time_created", descending: true).addSnapshotListener(includeMetadataChanges: true) { (querySnapshot, error) in
if let error = error {
print("There was an error fetching the data: \(error)")
} else {
self.events = querySnapshot!.documents.map { document in
return EventName(eventName: (document.get("event_name") as! String))
}
self.tableView.reloadData()
}
}
}
The thing with this is, when I run the app on the simulator, I am restricted from pressing buttons and then sometimes I can press them and then sometimes they get restricted again. This bug is so confusing because it makes no sense where it springs from.
The other issue is I can use a Constants value in all the query functions in my collections path.
static let schoolCollectionName = "school_users/\(user?.uid)/events"
This is nested in a Firebase struct within the Constants struct. In order to keep Xcode from giving errors I create a let users = Auth.auth().currentUser variable outside the Constants struct. The issue with this value is that when I put that in all of my query functions collection paths, all the buttons are accessible and selectable all the time, but when a user logs out and I log in as a new user, the previous user's data shows up in the new user's tableview.
It would obviously make more sense to use the Constants value because you prevent typos in the future, but I can't figure out how to get rid of the bug where the old user's data shows up in the new user's tableview. Thanks in advance.
The user id should definitely not be a constant. What it sounds like is that right now, you have no reliable way to change users -- your setup probably depends on which user is logged in at app startup, since that's where your variable gets set.
I would do something more like this:
func getEventName() {
guard let user = Auth.auth().currentUser else {
//handle the fact that you don't have a user here -- don't go on to the next query
return
}
listener = db.collection("school_users/\(user.uid)/events").order(by: "time_created", descending: true).addSnapshotListener(includeMetadataChanges: true) { (querySnapshot, error) in
Note that now, user.uid in the interpolated path doesn't have the ? for optionally unwrapping it (which Xcode is giving you a warning for right now). It will also guarantee that the correct query is always made with the currently-logged-in user.
Regarding being able to press the buttons, that sounds like an unrelated issue. You could run your app in Instruments and check the Time Profiler to see if you have long-running tasks that are gumming up the main/UI thread.
I need to observe changes of an Entity after import occurred.
Currently I have next logic:
Save Entity with temp identifier (NSManagedObject.objectId) to local core data storage.
Send Entity to the server via Alamofire POST request.
Server generates JSON and reply with the almost the same Entity details but with modified identifier which was NSManagedObject.objectId previously. So the local one Entity id will be updated with server id.
Now when I received new JSON I do transaction.importUniqueObjects.
At this step I want to inform my datasource about changes. And refetch data with updated identifiers.
So my DataSource has some Entities in an array, and while I use this datasource to show data it's still static information in that array which I fetched before, but as you see on the step number 4 I already updated core data storage via CoreStore import and want DataSource's array to be updated too.
I found some information regarding ListMonitor in CoreStore and tried to use it. As I can see this method works when update comes
func listMonitorDidChange(_ monitor: ListMonitor)
but I try to refetch data somehow. Looks like monitor already contains some most up to date info.
but when I do this:
func listMonitorDidChange(_ monitor: ListMonitor<MyEntity>) {
let entities = try? CoreStore.fetchAll(
From<MyEntity>()
.orderBy(.ascending(\.name))
) // THERE IS STILL old information in database, but monitor instance shows new info.
}
And then code became like this:
func listMonitorDidChange(_ monitor: ListMonitor<MyEntity>) {
var myEntitiesFromMonitor = [MyEntity]()
for index in 0...monitor.numberOfObjects() {
myEntitiesFromMonitor.append(monitor[index])
}
if myEntitiesFromMonitor.count > 0 {
// HERE we update DataSource
updateData(with: myEntitiesFromMonitor)
}
}
not sure if I am on the right way.
Please correct me if I am wrong:
As I understood each time core data gets updated with new changes, monitor gets updated as well. I have not dive deep into it how this was made, via some CoreData context notification or whatever but after you do something via CoreStore transaction, such as create or update or delete object or whatever you want, monitor gets update. Also it has callback functions that you need to implement in your class where you want to observe any changes with data model:
Your classes such as datasource or some service or even some view controller (if you don't use any MVVP or VIPER or other design patterns) need to conform to ListObserver protocol in case you want to listen not to just one object.
here are that functions:
func listMonitorDidChange(monitor: ListMonitor<MyPersonEntity>) {
// Here I reload my tableview and this monitor already has all needed info about sections and rows depend how you setup monitor.
// So you classVariableMonitor which I provide below already has up to date state after any changes with data.
}
func listMonitorDidRefetch(monitor: ListMonitor<MyPersonEntity>) {
// Not sure for which purposes it. I have not received this call yet
}
typealias ListEntityType = ExerciseEntity
let classVariableMonitor = CoreStore.monitorSectionedList(
From<ListEntityType>()
.sectionBy(#keyPath(ListEntityType.muscle.name)) { (sectionName) -> String? in
"\(String(describing: sectionName)) years old"
}
.orderBy(.ascending(\.name))
.where(
format: "%K == %#",
#keyPath(ListEntityType.name),
"Search string")
)
All other thing documented here so you can find info how to extract info from monitor in your tableview datasource function.
Thanks #MartinM for suggestion!
I currently have a collectionView: UICollectionView that displays contents based on an array of objects called projects. At times I will receive an updated array called newProjects from a server. newProjects should replace projects as the data source for the collectionView.
I have figured out a way to update the collectionView when objects have been deleted from the array with the following code:
var indexPaths = [IndexPath]()
for project in projects {
if(!newProjects.contains(project)) {
let index = projects.firstIndex(of: project)
indexPaths.append(IndexPath(item: index!, section: 0))
}
}
collectionView.deleteItems(at: indexPaths)
projects = newProjects
collectionView.reloadData()
This works for deleted projects. However, I am also trying to react to added projects. My code however seems to fail, since I keep getting the exception:
Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (65) must be equal to the number of items contained in that section before the update (64), plus or minus the number of items inserted or deleted from that section (0 inserted, 0 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).
My code for added project comparison is:
// First, the deletion from the code above is called.
// project = newProjects and data reload doesn't happen after deletion.
var addIndexPaths = [IndexPath]()
for project in newProjects {
if(!projects.contains(project)) {
projects.append(content)
addIndexPaths.append(IndexPath(item: projects.count - 1, section: 0))
}
}
collectionView.insertItems(at: addIndexPaths)
projects = newProject
collectionView.reloadData()
I'm sure this code is somewhat of a hack job (since it doesn't function), but I honestly can't figure out how else to reliably update a UICollectionView. Does anyone have suggestions or helpful links? Cheers!
As Wez pointed out, just taking out all the insertion and deletion of data did end up working. Now the entirety of the call is just reduced to:
projects = newProjects
collectionView.reloadData()
I initially thought that didn't work, but that was due to a problem with the server. I hadn't tried it yet, thinking I needed to manually insert all changed projects.
As rmaddy pointed out, taking reloadData() out of the original code also worked! My initial thought for some reason was that reloadData() would be necessary after adding and deleting items, probably because I also thought UITableViews required you to call endUpdates()... I overcomplicated some essential concepts.
Thanks everyone!
this is my first time working with Core Data in swift. I'm really enjoying it but it's also a challenge making sure my Appdelegate saves etc.
The Problem
Basically I am creating an budgeting app. Once a budget ends I need to take the current budget and store it away into a history entity. Now I have 2 different entities that work here:
NewBudgetCreateMO and HistoryBudgetHolderMO. What should happen is that the HistoryBudgetHolder should add a budget (newBudgetCreateMO) into it's One-To-Many relationship. Here is an image of my graph and their relationship.
Now if I've set this up right I should be allow to have as many NewBudgetCreateMOs in my History as I like by adding them? The code below is the generated code for my History entity which shows that it contains an NSSet
extension HistoryBudgetHolderMO {
#nonobjc public class func fetchRequest() -> NSFetchRequest<HistoryBudgetHolderMO> {
return NSFetchRequest<HistoryBudgetHolderMO>(entityName: "HistoryBudgetHolder");
}
#NSManaged public var budgets: NSSet?
}
extension HistoryBudgetHolderMO {
#objc(addBudgetsObject:)
#NSManaged public func addToBudgets(_ value: NewBudgetCreateMO)
#objc(removeBudgetsObject:)
#NSManaged public func removeFromBudgets(_ value: NewBudgetCreateMO)
#objc(addBudgets:)
#NSManaged public func addToBudgets(_ values: NSSet)
#objc(removeBudgets:)
#NSManaged public func removeFromBudgets(_ values: NSSet)
}
So I assumed that I could just use "addToBudgets" to add a set piece of data and it does seem to work but for only one instance.
Where I'm doing the adding
So I do a fetch request on the HistoryBudgetHolderMO to see if I have any in the data base. If not then I create a new one from my App Delegate (Please NOTE: I have done the app delegate casting etc in a method above and then have passed the App Delegate and Context to this method)
private func SaveAndDeleteCurrentBudget(context : NSManagedObjectContext, appDele : AppDelegate){
let fetchHistory : NSFetchRequest<HistoryBudgetHolderMO> = HistoryBudgetHolderMO.fetchRequest()
//Saves the budget to the history budget. If we don't have oen we created one and add it to that
do{
let historyBudgets : [HistoryBudgetHolderMO] = try context.fetch(fetchHistory)
if historyBudgets.count <= 0{
let newHistoryBudget : HistoryBudgetHolderMO = HistoryBudgetHolderMO(context: context)
newHistoryBudget.addToBudgets(budgetData.first!)
print("entered new historyBudget")
}else{
historyBudgets.first!.addToBudgets(budgetData.first!)
}
appDele.saveContext()
}catch{
print("Error when looking for history fetch result")
}
//Deletes all budget data and budget entries that are currently used
for object in budgetData{
context.delete(object)
}
let fetchAllDataEntries = NSFetchRequest<NSFetchRequestResult>(entityName: "BudgetEntry")
let deleteReq = NSBatchDeleteRequest(fetchRequest: fetchAllDataEntries)
do{
try context.execute(deleteReq)
}catch{
print("Error when deleting budget entries")
}
appDele.saveContext()
}
I do the fetch request and check if a history entity is there. If not then I create a new one, add the budget entry and then save the context.
If not then I grab the first instance of the history holder (as there should only ever be one as it's just a container) and I add the budget entry and then save.
Where it gets bad
So the first time I do this and it's in state 2 I get a value of Optional(1) which means it has stored one entry of the History. However any more additions after this keep saying it's Optional(1). I've tried looking up countless solutions, tried messing around with the extensions etc. I figured this would be a simple Get/Set operation but It's just not working.
Any help with this would be greatly appreciated.
Thanks for taking the time to read this.
Your solution seems good now. I also would have suggested to get rid of the HistoryBudgetHolderMO class. May I suggest to add another field/property to the NewBudget class: a creationDate (Date type). That way you can always fetch the latest one (e.g. fetch all of them and sort by creationDate). You could als add an active/historic boolean property to mark Budgets as active/inactive. Another suggestion is try to avoid force unwrapping. Instead of writing
budgetData.first!.attributeName
try to work with the 'if let' construct
if let budget = budgetData.first {
budget.attributeName
}
Solution For Anyone Interested
As I mentioned before I'm still learning Core Data and I'm grateful for KaraBenNensi for his comment to get me thinking.
Right so there was no need for a "holder" type object. Instead what I have done is I have used the last index of my budgets. So everytime I create a new budget I simply keep them all in the array. So instead of saying:
budgetData.first!.attributeName
I now use
budgetData.last!.attributeName.
This means that my database will grow but it would have grown with the history holder anyway. Now when I want to display history I just fetch all the results from the budgetData core data model. When I want to display my actual budget I just use .last so I get the most recently created budget.
I hope this helps someone and I'm glad I could figure it out. If anyone needs help in the future just reply to this and I'll try to help (But I'm no expert!)