I'd like to pass to my model the newest fetched data of my core data entities, in order to have them synched.
Is this possible?
The reason is that I have many variables that have to be calculated from the data saved in core data. These values are used in my views, so they should update at the same time.
(Until now I just found a way to pass them around every time with functions, but I find this very chaotic...)
Until now:
func doSomethingWithFetchedData(fetchedData: FetchedResults<Entity>) {
//return what I need
}
Thanks!
NSFetchedResultsController Subscribing to updates for many objects matching a fetch request has been easier than subscribing to updates from a single managed object, thanks to NSFetchedResultsController. It comes with a delegate that informs us about changes to the underlying data in a structured way, because it was designed to integrate with tables and collection views
Here is a good link to start with
I am using Core Data to see whether messages in a table view have been seen before by the user. The way I do this is to save the message Id to Core Data the first time it is seen, and then I run a fetch request when I update the table view to see if there is an entry in the persistent memory with the same Id.
Now what I want to know is how I should most effectively implement my fetch request, based on how time consuming it is. Should I either run a request that returns all saved message Ids as an array when the view is loaded, and then in cellForRowAtIndexPathcheck if that array contains that cell's message Id, or run the fetch request with a predicate in cellForRowAtIndexPath? The latter would be my preferred method, but If i have 100 or so cells I wondered if this would be poor etiquette.
Any help would be very much appreciated.
This is my fetch Request :
func persistQuery(predicateValueString: String!) -> Bool! {
let fetchRequest = NSFetchRequest(entityName: "LogItem")
let predicate = NSPredicate(format: "itemText == %#", predicateValueString)
fetchRequest.predicate = predicate
var didFindResult = true
if let fetchResults = managedObjectContext!.executeFetchRequest(fetchRequest, error: nil) as? [LogItem] {
if fetchResults.count == 0 {
didFindResult=false
}
}
return didFindResult
}
The best way is to use a NSFetchedResultsController. It will optimize the fetching and the memory footprint as well. It is specifically designed for table views.
To get started, take a look at the Xcode template (Master/Detail, check Core Data). It is really quite simple.
Make sure you also implement the delegate methods - they will automatically be called when your managed objects change, so there is only minimal code that is executed to update the UI (an only if the object is actually on screen).
Presumably each of your table view cells represent a LogItem (the NSManagedObject subclass) with a property to indicate the read status. Once you change that, the delegate method will try to update it based on the index path.
That's all there is to it. With the fetched results controller you get a lot of optimization for free, so I would strongly recommend using it whenever you populate a table view with Core Data entities.
I use a collection view to display images stored in CoreData, only a few (around 15) but when I scroll down it lags, It seams to be because I am not fetching my data asyncrhronously, is it possible with only 15 UIImages?
So here is my problem I cannot find a descent tutorial on asynchronous fetching in swift anywhere ,3 days I am looking. Sorry if I didn't search well.
This is what arrived to do with some bits from tutorials
let entity = NSEntityDescription.entityForName("Dreams", inManagedObjectContext: contxt)
let fetchRequest = NSFetchRequest()
fetchRequest.entity = entity
let asyncFetch = NSAsynchronousFetchRequest(fetchRequest: fetchRequest) {
(result:NSAsynchronousFetchResult!) -> Void in
if result.finalResult!.count > 0 {
self.dreams = result.finalResult! as [Dream]
}
NSLog("Done.")
}
Dreams is an array of type [Dream] but when I display the collection view nothing appears
This is how I did it previously
let fetchRequest = NSFetchRequest(entityName: "Dreams")
if let fetchResults = contxt.executeFetchRequest(fetchRequest, error: nil) as? [Dream] {
dreams = fetchResults
}
It worked very well,
Thank you very much for your help
Unfortunately, I don't have time to offer specific code in this instance. I did want to present a basic strategy, however. Fetching on the main thread is exceedingly unlikely to be your problem here, but fetching everything at once definitely could be, particularly with large binary data involved. This should be sped up significantly if:
You start using an NSFetchedResultsController with a good batch size rather than a raw fetch request. NSFetchedResultsController is a little finicky about updates with collection views, in ways which are beyond the specific scope of this question, but they work very well once set up correctly.
You store the images as files instead of in Core Data itself, and store the path to the file instead. Then that image data won't get loaded into memory every time you fault in the object, and you can simply load and display the images as the cells come on screen using that file URL.
People who insist on storing binary data in Core Data usually find that storing the data as the only attribute in a separate "Image" entity with a relationship to the entity you are displaying helps. The image object (along with its binary image data) can then be kept a fault, and thus not in memory, even when the object you fetch for the collection view is in memory. I, however, prefer not to store them in Core Data, and simply handle the resulting atomicity gotchas and the possibility the file Core Data points to got deleted somehow.
This has been asked before, but no solution described that is fast enough for my app needs.
In the communications protocol we have set up, the server sends down a new set of all customers every time a sync is performed. Earlier, we had been storing as a plist. Now want to use Core Data.
There can be thousands of entries. Deleting each one individually takes a long time. Is there a way to delete all rows in a particular table in Core Data?
delete from customer
This call in sqlite happens instantly. Going through each one individually in Core Data can take 30 seconds on an iPad1.
Is it reasonable to shut down Core Data, i.e. drop the persistence store and all managed object contexts, then drop into sqlite and perform the delete command against the table? No other activity is going on during this process so I don't need access to other parts of the database.
Dave DeLong is an expert at, well, just about everything, and so I feel like I'm telling Jesus how to walk on water. Granted, his post is from 2009, which was a LONG time ago.
However, the approach in the link posted by Bot is not necessarily the best way to handle large deletes.
Basically, that post suggests to fetch the object IDs, and then iterate through them, calling delete on each object.
The problem is that when you delete a single object, it has to go handle all the associated relationships as well, which could cause further fetching.
So, if you must do large scale deletes like this, I suggest adjusting your overall database so that you can isolate tables in specific core data stores. That way you can just delete the entire store, and possibly reconstruct the small bits that you want to remain. That will probably be the fastest approach.
However, if you want to delete the objects themselves, you should follow this pattern...
Do your deletes in batches, inside an autorelease pool, and be sure to pre-fetch any cascaded relationships. All these, together, will minimize the number of times you have to actually go to the database, and will, thus, decrease the amount of time it takes to perform your delete.
In the suggested approach, which comes down to...
Fetch ObjectIds of all objects to be deleted
Iterate through the list, and delete each object
If you have cascade relationships, you you will encounter a lot of extra trips to the database, and IO is really slow. You want to minimize the number of times you have to visit the database.
While it may initially sound counterintuitive, you want to fetch more data than you think you want to delete. The reason is that all that data can be fetched from the database in a few IO operations.
So, on your fetch request, you want to set...
[fetchRequest setRelationshipKeyPathsForPrefetching:#[#"relationship1", #"relationship2", .... , #"relationship3"]];
where those relationships represent all the relationships that may have a cascade delete rule.
Now, when your fetch is complete, you have all the objects that are going to be deleted, plus the objects that will be deleted as a result of those objects being deleted.
If you have a complex hierarchy, you want to prefetch as much as possible ahead of time. Otherwise, when you delete an object, Core Data is going to have to go fetch each relationship individually for each object so that it can managed the cascade delete.
This will waste a TON of time, because you will do many more IO operations as a result.
Now, after your fetch has completed, then you loop through the objects, and delete them. For large deletes you can see an order of magnitude speed up.
In addition, if you have a lot of objects, break it up into multiple batches, and do it inside an auto release pool.
Finally, do this in a separate background thread, so your UI does not pend. You can use a separate MOC, connected to a persistent store coordinator, and have the main MOC handle DidSave notifications to remove the objects from its context.
WHile this looks like code, treat it as pseudo-code...
NSManagedObjectContext *deleteContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateConcurrencyType];
// Get a new PSC for the same store
deleteContext.persistentStoreCoordinator = getInstanceOfPersistentStoreCoordinator();
// Each call to performBlock executes in its own autoreleasepool, so we don't
// need to explicitly use one if each chunk is done in a separate performBlock
__block void (^block)(void) = ^{
NSFetchRequest *fetchRequest = //
// Only fetch the number of objects to delete this iteration
fetchRequest.fetchLimit = NUM_ENTITIES_TO_DELETE_AT_ONCE;
// Prefetch all the relationships
fetchRequest.relationshipKeyPathsForPrefetching = prefetchRelationships;
// Don't need all the properties
fetchRequest.includesPropertyValues = NO;
NSArray *results = [deleteContext executeFetchRequest:fetchRequest error:&error];
if (results.count == 0) {
// Didn't get any objects for this fetch
if (nil == results) {
// Handle error
}
return;
}
for (MyEntity *entity in results) {
[deleteContext deleteObject:entity];
}
[deleteContext save:&error];
[deleteContext reset];
// Keep deleting objects until they are all gone
[deleteContext performBlock:block];
};
[deleteContext preformBlock:block];
Of course, you need to do appropriate error handling, but that's the basic idea.
Fetch in batches if you have so much data to delete that it will cripple memory.
Don't fetch all the properties.
Prefetch relationships to minimize IO operations.
Use autoreleasepool to keep memory from growing.
Prune the context.
Perform the task on a background thread.
If you have a really complex graph, make sure you prefetch all the cascaded relationships for all entities in your entire object graph.
Note, your main context will have to handle DidSave notifications to keep its context in step with the deletions.
EDIT
Thanks. Lots of good points. All well explained except, why create the
separate MOC? Any thoughts on not deleting the entire database, but
using sqlite to delete all rows from a particular table? – David
You use a separate MOC so the UI is not blocked while the long delete operation is happening. Note, that when the actual commit to the database happens, only one thread can be accessing the database, so any other access (like fetching) will block behind any updates. This is another reason to break the large delete operation into chunks. Small pieces of work will provide some chance for other MOC(s) to access the store without having to wait for the whole operation to complete.
If this causes problems, you can also implement priority queues (via dispatch_set_target_queue), but that is beyond the scope of this question.
As for using sqlite commands on the Core Data database, Apple has repeatedly said this is a bad idea, and you should not run direct SQL commands on a Core Data database file.
Finally, let me note this. In my experience, I have found that when I have a serious performance problem, it is usually a result of either poor design or improper implementation. Revisit your problem, and see if you can redesign your system somewhat to better accommodate this use case.
If you must send down all the data, perhaps query the database in a background thread and filter the new data so you break your data into three sets: objects that need modification, objects that need deletion, and objects that need to be inserted.
This way, you are only changing the database where it needs to be changed.
If the data is almost brand new every time, consider restructuring your database where these entities have their own database (I assume your database already contains multiple entities). That way you can just delete the file, and start over with a fresh database. That's fast. Now, reinserting several thousand objects is not going to be fast.
You have to manage any relationships manually, across stores. It's not difficult, but it's not automatic like relationships within the same store.
If I did this, I would first create the new database, then tear down the existing one, replace it with the new one, and then delete the old one.
If you are only manipulating your database via this batch mechanism, and you do not need object graph management, then maybe you want to consider using sqlite instead of Core Data.
iOS 9 and later
Use NSBatchDeleteRequest. I tested this in the simulator on a Core Data entity with more than 400,000 instances and the delete was almost instantaneous.
// fetch all items in entity and request to delete them
let fetchRequest = NSFetchRequest(entityName: "MyEntity")
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
// delegate objects
let myManagedObjectContext = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
let myPersistentStoreCoordinator = (UIApplication.sharedApplication().delegate as! AppDelegate).persistentStoreCoordinator
// perform the delete
do {
try myPersistentStoreCoordinator.executeRequest(deleteRequest, withContext: myManagedObjectContext)
} catch let error as NSError {
print(error)
}
Note that the answer that #Bot linked to and that #JodyHagins mentioned has also been updated to this method.
Really your only option is to remove them individually. I do this method with a ton of objects and it is pretty fast. Here is a way someone does it by only loading the managed object ID so it prevents any unnecessary overhead and makes it faster.
Core Data: Quickest way to delete all instances of an entity
Yes, it's reasonable to delete the persistent store and start from scratch. This happen fairly quick. What you can do is remove the persistent store (with the persistent store URL) from the persistent store coordinator, and then use the url of the persistent store to delete the database file from your directory folder. I did it using NSFileManager's removeItemAtURL.
Edit: one thing to consider: Make sure to disable/release the current NSManagedObjectContext instance, and to stop any other thread which might be doing something with a NSManagedObjectContext which is using the same persistent store. Your application will crash if a context tries to access the persistent store.
After reading lots of documentation and examples of Core Data framework, I still don't quite get it. What happens is that I sort of understand parts of a sample code or documentation, but often can't fit it into a larger picture. The second time I read the same code, I still need lots of time to figure it out just like the first time. It's so frustrating.
The NSFetchedResultsControllerDelegate vs NSFetchedResultsController is one of those concepts in Core Data that confuse me.
I think what I need is a simple and conceptual explanation, maybe analogy will be helpful.
All your core data objects have to be accessed via managed object context.
Think of NSFetchRequest as a representation of the database query. When you tell the NSManagedObjectContext (MOC) to fetch data, you give it a fetch request, so it knows what to fetch.
Now, let's say you have a table view. You make a fetch, and have all the data, even for the stuff you don't need to display. Each time the table view changes, and you need to refetch the data. There are a few other issues with just using a fetch request, though it can be done.
To solve some of these problems, enter NSFetchedResultsController (FRC). It manages the database so that you only have the objects in memory that are actually needed at the time. Furthermore, it hooks into the MOC so that when the database changes, it automatically changes its own data.
So, you create a FRC and give it a fetch request. Now, it manages the data so it only has in memory what you want. But, you need to tell it what to grab, and it need to tell you when it has data.
Thus is where the NSFetchedResultsControllerDelegate comes in.
The delegate is the glue between the table view (or some other component) and the FRC. The delegate methods are the communication channel that informs the FRC what data to get, when to get it, then hand it off to the table view.
EDIT
Yes, FRC manages the actual data part. However, when its data changes, it has to have some way to notify the guy who is watching the data. That's what the delegate is for. Let's take a different attack by looking at the delegate methods.
Imagine that the database has been changed in some way. The FRC, through its special magical incantations, has noticed the change, and, like a teenager with a new iPhone, needs to tell someone.
Specifically, again, in your case, it needs to tell the table view that is responsible for displaying the data to the user. Well, how is it going to tell the table view that the data has changed? There are actually several patterns used in iOS, and in this case, we use a delegate.
The guy who is interested in receiving this information from the FRC give him a pointer to an object that implements the delegate methods. When the FRC wants to notify the guy interested, it calls the appropriate methods on the object that is was given as the delegate.
Consider a change has happened. The FRC code would look something like this (ultra-simplified but to give the algorithmic idea).
[delegate controllerWillChangeContent:self];
// Process all the changes...
for (SectionChangeInfo *info in changedSections) {
[delegate controller:self didChangeSection:info.sectionInfo atIndex:info.index forChangeType:info.changeType];
}
for (ObjectChangeInfo *info in changedObjects) {
[delegate controller:self didChangeObject:info.object atIndexPath:info.indexPath forChangeType:info.changeType newIndexPath:index.newIndexPath];
}
[delegate controllerDidChangeContent:self];
Thus, the FRC can tell "somebody" about changes when they occur. In your case, when you give the FRC a delegate, it will call those methods, thus giving you a chance to handle the changes when they happen.
The other delegate method is called to ask the delegate what it wants to use as a section title. So, assume the FRC needs to know what to use, it will call...
NSString *sectionTitle = [[section substringToIndex:1] uppercase];
if ([delegate respondsToSelector:#selector(controller:sectionIndexTitleForSectionName:)]) {
sectionTitle = [delegate controller:self sectionIndexTitleForSectionName:section];
}