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.
Related
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.
I'm sorry the title may mislead you, since I'm not so good at English. Let me describe my problem as below (You may skip to the TL;DR version at the bottom of this question).
In Coredata, I design a Product entity. In app, I download products from a server. It return JSON string, I defragment it then save to CoreData.
After sometimes has passed, I search a product from that server again, having some interaction with server. Now, I call the online product XProduct. This product may not exist in CoreData, and I also don't want to save it to CoreData since it may not belong to this system (it come from other warehouse, not my current warehouse).
Assume this XProduct has the same properties as Product, but not belong to CoreData, the developer from before has designed another Object, the XProduct, and copy everything (the code) from Product. Wow. The another difference between these two is, XProduct has some method to interact with server, like: - (void)updateStock:(NSInteger)qty;
Now, I want to upgrade the Product properties, I'll have to update the XProduct also. And I have to use these two separately, like:
id product = anArrayContainsProducts[indexPath.row];
if ([product isKindOfClass:[XProduct class]] {
// Some stuff with the xproduct
}
else {
// Probably the same display to the cell.
}
TL;DR
Basically, I want to create a scenario like this:
Get data from server.
Check existed in CoreData.
2 == true => add to array (also may update some data from server).
2 == false => create object (contains same structure as NSManagedObject from JSON dictionary => add to array.
The object created in step 4 will never exist in CoreData.
Questions
How can I create an NSManagedObject without having it add to NSMangedObjectContext and make sure the app would run fine?
If 1 is not encouragement, please suggest me a better approach to this. I really don't like to duplicate so many codes like that.
Update
I was thinking about inheritance (XProduct : Product) but it still make XProduct the subclass of NSManagedObject, so I don't think that is a good approach.
There are a couple of possibilities that might work.
One is just to create the managed objects but not insert them into a context. When you create a managed object, the context argument is allowed to be nil. For example, calling insertNewObjectForEntityForName(_:inManagedObjectContext:) with no context. That gives you an instance of the managed object that's not going to be saved. They have the same lifetime as any other object.
Another is to use a second Core Data stack for these objects, with an in-memory persistent store. If you use NSInMemoryStoreType when adding the persistent store (instead of NSSQLiteStoreType), you get a complete, working Core Data stack. Except that when you save changes, they only get saved in memory. It's not really persistent, since it disappears when the app exits, but aside from that it's exactly the same as any other Core Data stack.
I'd probably use the second approach, especially if these objects have any relationships, but either should work.
I am using Realm to handle persisting data to disk in my app. I am storing an ArticleCollection object to Realm (this just contains an array of NewsArticle objects).
I am using createOrUpdateInDefaultRealmWithValue to either create a new ArticleCollection or update the existing one (achieved using a primary key on ArticleCollection).
This currently works well when the app fetches new articles it will overwrite the current collection of articles and replace with a new instance of ArticleCollection.
The issue is that whilst ArticleCollection is replaced (in Realm brwoser I can see the count of ArticleCollection is always 1 as expected), the number of NewsArticles always increases. It seems that sub objects of ArticleCollection (NewsArticle) never get replaced/deleted, only appended to.
How can I ensure that when I use createOrUpdateInRealm, it will delete all sub objects?
Make sure that are also using a primary key on your NewsArticle object. Otherwise those news articles won't be updated. That's why you are seeing an increasing number of news articles.
I am in a situation where I allow the user to download a PFObject and modify it locally, and they can then either cancel the changes or hit Done, which will dismiss the editing interface but NOT upload the changes to Parse yet. They need to hit Save on the previous screen to write all changes to the database at once.
The problem is once the PFObject is modified, you cannot revert it to its prior state without refetching from the database. But I cannot always refetch the data from the database every time they hit Cancel because the prior state may not be uploaded to Parse yet (and that's a bad UX making them wait to discard changes that are only stored locally).
For example, imagine the user taps to edit the PFObject, they make changes then hit Done, then tap on it again and further edit the object, then hit Cancel. In this case, the object needs to be reverted to its prior state, but that state has not been uploaded to Parse yet. So I cannot refetch the data from the database to revert changes otherwise it would overwrite the changes they made the first time.
To solve this problem, I would simply fetch the PFObject and store a copy of it. I'd call that the transient object. I would have another property that stores the real object. The user would modify the transient object, and when they hit Cancel I would simply set that to nil, if they instead hit Done I would set the real object equal to the transient object, and once they finally hit Save I would save the real object to the database. That way I can be sure changes aren't being made to the real object until the user commits the changes. The problem is, PFObject does not adopt the NSCopying protocol (not sure why), therefore I cannot create a copy of the PFObject. Any change I make to it affects the real object.
How can this be resolved, without modifying the app's design that allows control over when the data is committed and later saved? Is there a way to extend PFObject and adopt NSCopying, has it been done before?
I did consider storing the attributes of the object in a dictionary and allow the user to edit that instead, then upon commit set each of those attributes on the PFObject. The problem with this solution arises with complex structures. In this app, I allow the user to modify multiple arrays that contain multiple PFObjects. It's just infeasible to try to recreate and later merge changes with complex structures like this beyond a single simple PFObject.
I ran into this same problem. I did not make any changes directly to the PFObject, but rather, saved the updates in an NSDictionary. When the user clicks the done button, I then update the PFObject and saveInBackground. I don't think there is a "discard local changes" option for PFObject. If you don't do this, the only option is to throw out the existing PFObject and fetch again.
Regarding the NSDictionary comment, perhaps NSArray would be better. The implementation really depends on your specific program, but I'll give a quick example. The NSArray we'll call instructionArray. Imagine there are 3 sections in a tableView. Also assume that the data source for each section is an NSArray of PFObjects. Now say you want to set the age property of each PFObject in Section 2 to 35.
Add an NSArray object (corresponding to an instruction to carry out) to the instructionArray. This instruction to carry out could have the form
Section to update
Property to update
Value to update to
So the object you'll add is #[#(2),#"age",#(35)];
Given that the user is probably carrying out a finite amount of instructions, it might not be that performance heavy to loop through the instructionArray in cellForRowAtIndexPath so when a cell uses its corresponding PFObject to figure out what to display, it can loop through the instructions after and change what is displayed as if the PFObject was updated.
When the save button is touched, loop through the instructions and actually edit the PFObjects themselves.
If you need the instructions to handle specific objects rather than sections, then you just have to update the structure of the instructionArray. Maybe you could include an identifier to indicate what type of instruction it is.
I'm currently using iCloud and CoreData to sync data across my app, so every time notification fires, I update my local array of data. The problem I am running into is that my data set is getting large and I don't want to update the entire set of data every time there is a new notification.
Basically, I have an Entity called Photo, and every time the user makes an update to one Photo object on device A, it gets synced with iCloud, which then gets pushed to device B. The device receives the notification through:
persistentStoreDidImportUbiquitousContentChanges:
which looks like this:
notification.userInfo.description:
{
deleted = "{(\n)}";
inserted = "{(\n 0x17045c80 56E70CB19352/Photo/p8431>\n)}";
updated = "{(\n)}";
}
I'd like to grab that specific insertion, update, or deletion and apply it to my local array instead of iterating through the entire set of fetchedObjects.
I tried casting the insertion object to a Photo object, but that didn't work. Any thoughts on how to extract that info?
Thanks!
The objects in that notification are instances of NSManagedObjectID. You can use those with your NSManagedObjectContext to retrieve the managed objects. Use existingObjectWithID:error: (safe, potentially slow) or objectWithID: (fast, potentially less safe).
You probably don't need to do that, though. You can take that notification and pass it to mergeChangesFromContextDidSaveNotification: to merge any changes it contains. If you need to do manual merging, you can, but it's usually not needed.