I watched a video tutorial on CoreData. I saw a that an option
Uses Lazy fetching
To map an Object Model with Array Controller.
But did not understand what is Lazy fetching?
That means that data would be actually fetched (automatically) not a the time of request, but at the time when controller would ask for data. For example consider large tableview with thousands of lines - they are not fetched all at the same time when request is executed, rather they are fetched by small parts dynamically when users scrolls tableview.
Related
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
Here is my context:
When I launch my app I fetch local data from CoreData and fill a tableview with it. At the same time I send an asynchronous request to my webservice to fetch what will be the new content of my tableview.
As soon as the request sends me a response I delete all the instances of the current NSManagedObjects and create new ones with the new data I got. Then I replace the datasource of my tableview to an array of the new NSManagedObjectContexts instances.
My problem:
I'm getting an error : CoreData could not fulfill a fault for ... if I scroll my tableview when the request finished and is triggering the deletion/creation of my tableview's data source.
I understand that this problem occurs because I'm trying to access an old NSManagedObject instance while doesn't exist anymore as it is explained in the doc : Apple doc. But I have no idea of what are the best practices in my case.
I don't want to block the user until my request finished but I have to prevent any error if he accesses "old" data while the request didn't finish (e.g : what if the user taps on a cell and I pass an instance of an NSManagedObject to another viewcontroller but when the request finishes this object doesn't exist anymore ?)
I would appreciate any help !
I highly recommend you to use NSFetchedResultsController since it's sole purpuse is:
You use a fetched results controller to efficiently manage the results returned from a Core Data fetch request to provide data for a UITableView object.
When using a fetched results controller it is much easier to handle the Core Data events like insert, delete, update.
You say you have three sections in your table view? That's no problem, NSFetchedResultsController can handle all of that.
Take a look at this. Apple provides a very nice set of instructions on how to configure and use NSFetchedResultsController.
What would be the best practise and best for user experience to achieve the following?
1:) Retrieve data from JSON
2:) Store in Core Data
3:) Display in UITableViewController
Do i store the JSON first, then populate the table using the stored data? OR Do i store it in the Core Data (background process) and populate the table using the JSON for the first time?
I want the user to be presented with a UITableview with minimum load time.
Thanks
This is what I would do:
Create your Core Data database and model.
Create a data access layer that will contain the read and write methods for each of your objects
In the read functions you can query the core data, if there is data then return that. Then in the background call the web server and and update your core data with the new JSON.
If there is no data go and request it from the web server, populate your core data tables using the JSON and then return the data from the core data so it is always consistent.
You can also have a last update date on your data so you are only requesting the new data from the web server that isnt already in your local core data DB. This will reduce the amount of data coming down to your ios device.
If you want minimum load time then I'd serve from JSON and that save to CoreData afterwards. That way the user can see content straight away without first having to wait for all the data to be saved (and parsed).
The course of action in this matter heavily depends on:
A. The amount of JSON data you are downloading
B. How effective your backend is at only sending necessary JSON results (rather than sending everything in bulk)
C. How you are attaching Core Data to your UITableViewController.
I recently made a pretty big project that does exactly this, which involved fetching a pretty big chunk of JSON, parsing it, and inserting it into Core Data. The only time there is any delay is during the initial load. This is how I accomplished it:
Download JSON. Cast as [[String: AnyObject]]: if let result = rawJSON as? [[String: AnyObject]] {}
Check to see if IDs for objects already exist in Core Data. If that object already exists, check if it needs update. If it doesn't exist, create it. Also check if IDs have been deleted from the JSON, if so remove them from.
Use NSFetchedResultsController to manage data from Core Data and populate the UITableView. I use NSFetchedResultsController rather than managedObjectContext.executeFetchRequest() because NSFetchedResultsController has delegate methods that are called every time the managedObjectContext is updated.
I'm displaying some images in a collection view, and using SDWebImage to prefetch thumbnails for all of them.
I'm using a batch size of 20 in the NSFetchRequest, but when iterating over every object to get the url of the image I need, the batch size performance gain is wasted. Reloading the data takes 0.3s instead of 0.000295s, which results in a obvious delay in the UI.
I've tried setting [request setPropertiesToFetch:#[#"propertyName"]] but it doesn't seem to make a difference. I guess it's not the method I'm looking for.
Any suggestions ?
Edit:
I am using using an UICollectionView backed by a NSFetchedResultsController (and delegate) data-source. I am making a NSFetchRequest with a batch size of 20 and fetching it via the NS-FRC. I also need to get the list of urls from ALL the fetched objects, ie:
for(NSManagedObjectSubclass *object in frc.fetchedObjects)
// this is causing the slow-down, because it's faulting all the objects
// not what you'd want if you have many objects!
{
[urlList addObject:(object.url)]
}
[SDWebImagePrefetcher prefetchUrls:urlList];
// this runs in the background and downloads/gets from cache a list of images
See NSFetchRequest's class reference, -setPropertiesToFetch:. "This value is only used if resultType is set to NSDictionaryResultType."
You're correct that iterating over all of the fetched objects kills the benefit of the small batch size. I'm curious, though, why you need to prefetch all of the image URLs at once. If you're doing this to trigger a download, look into doing that in -awakeFromFetch: pass the object.url off to your SDWebImagePrefetcher, which will enqueue the request and start or continue processing. You'll have to subclass NSManagedObject to do this.
I am trying to fetch 10000records from CoreData in a UITableView using NSFetchedResultsController and trying to make it as fast as possible (since the request has a sort descriptor it takes longer to fetch this amount of data).
I am trying to fetch 100records from CoreData with the first and main NSFetchedResultsController(used in the delegate methods of the table), that is displayed on the table while in another queue I started in viewDidAppear another fetch for all 10000records on an auxiliarFRC. After the fetch in the AuxFRC ends, I assign to the main FRC the AuxFRC so all the records get transfered and I reload the table.
My problem is that the UITableView gets stuck at the first loaded rows till the AuxFRC ends the fetch even if I dispatch the performFetch, and I can not understand why this happens, or if this way is wrong what other way can be used to fetch 10000records and stay up to date if the data changes?
I guess the problem was the NSManagedObjectContext - that is not thread safe and I used the same one for both the fetches. I created a copy of the original one and changed the AuxFRC on the second context. This solved everything.
The trick is to use the cache mechanism of FRC and coordinate the asynchronous fetch with the display thread.
All is summarised in this answer.