Overview:
Stanford iOS tutorials contains an implementation (header + implementation file) to help use the table view while using core data.
Link - http://www.stanford.edu/class/cs193p/cgi-bin/drupal/downloads-2011-fall
File Name - CoreDataTableViewController.zip
CoreDataTableViewController.h contains the following text:
// Remember that once you create an NSFetchedResultsController, you CANNOT modify its #propertys. If you want new fetch parameters (predicate, sorting, etc.), create a NEW NSFetchedResultsController and set this class's fetchedResultsController #property again.
Question
It sounds a bit strange, because in docs they mention situation when you change properties, and recommend some thing in this case... Look like some mistake, or I miss something. If i just change predicate and fetch again, everything works...
It depends if you are using a cache or not. As it says in the documentation you link to:
If you are using a cache, you must call deleteCacheWithName: before changing any of the fetch request, its predicate, or its sort descriptors. You must not reuse the same fetched results controller for multiple queries unless you set the cacheName to nil.
If you are changing these properties, then it is probably simpler (this is a beginner's course, and the creation of the fetched results controller is done outside of this sample code) to just tell you to create a new fetched results controller than to go into an explanation of caching, and clearing a cache, and so on.
Related
This question is poorly phased but this can be better explained in code.
We have a Core Data Stack with private and main contexts as defined by Marcus Zarra here: http://martiancraft.com/blog/2015/03/core-data-stack/
We call a separate class to do a fetch request (main context) and return an array of NSManagedObjects:
NSArray *ourManagedObjects = [[Client sharedClient].coreDataManager fetchArrayForClass:[OurObject class] sortKey:#"name" ascending:YES];
We then do some processing and store a reference:
self.ourObjects = processedManagedObjects
Our view contains a UITableView and this data is used to populate it and that works just fine.
We change the data on our CMS, pull to refresh on the UITableView to trigger a sync (private context) and then call this same function to retrieve the updated data. However, the fetch request returns the exact same data as before even though when I check the sqlite db directly it contains the new data. To get the new values to display I have to reload the app.
I have discovered that if I don't assign the processedManagedObjects to self, the fetch request does indeed return the correct data, so it looks like holding a reference to the NSManagedObject stops it from getting new data from the main context. However I have no idea why that would be.
To clarify, we're pretty sure there's nothing wrong with our Core Data Stack, even when these managed objects are not being updated, other are being updated just fine, it's only this one where we store a local reference.
It sounds like what's going on is:
Managed objects don't automatically update themselves to reflect the latest data in the persistent store when changes are made via a different managed object context.
As a result, if you keep a reference to the objects, they keep whatever data they already had.
On the other hand if you don't keep a reference but instead re-fetch them, you get the new data because there was no managed object hanging around with its old data.
You have a few options:
You could keep the reference and have your context refresh the managed objects, using either the refresh(_, mergeChanges:) method or refreshAllObjects().
If it makes sense for your app, use an NSFetchedResultsController and use its delegate methods to be notified of changes.
Don't keep the reference.
The first is probably best-- refreshAllObjects() is probably what you want. Other options might be better based on other details of your app.
Try setting the shouldRefreshRefetchedObjects property of the fetch request to true. According to the documentation:
By default when you fetch objects, they maintain their current property values, even if the values in the persistent store have changed. Invoking this method with the parameter true means that when the fetch is executed, the property values of fetched objects are updated with the current values in the persistent store.
I have an iOS app that I am currently building. I'm using CoreData for the first time and have a question in regards to passing data from one ViewController to the next. Right now I have a tableview set up in VCa, and when I tap a cell I want to get the details of that object (Person) in VCb. I have been thinking of doing this in 2 different ways.
Pass a reference of the Persons name in the prepage function. And over in VCb use a predicate to fetch that person back out of CoreData. Is using another fetch request the correct solution here. There will not be a ton of data in this app, so I don't think it would be very taxing on the system to do another request, but I'm new to CoreData.
Capture the data object in VCa and pass that object over to VCb via the prepare function. This would eliminate me having to do another fetch request in VCb but seems like I could be limiting myself for future expansions.
Again, I'm new to CoreData and just wanted some thoughts on which method of thinking is better and why.
Don't ever fetch an object that you have already fetched. In your case #1 you already have the object that you need, but you'd go ahead and fetch it again for no good reason. It might not be "very taxing" in your case but that just means you're probably not duplicating a lot of work for no reason yet, but it's still a bad design. And anyway-- this is an entity representing a person. Are you certain that there would never, ever be more than one person with the same name?
Your second idea is the better one here.
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 hope this question isn't too general/ambiguous...
I'm writing an iphone quiz game app and am having trouble figuring out the best way to handle data. Currently I am thinking of having a single Model class that holds an array of "User" classes which each have an array of user-specific "Question" classes. I'd like to be able to access the overarching Model from any of my view controllers, but that means I'll probably have to pass the model object to any new view controller, use a singleton, or do something else. What is the best way to access my Model object from other classes? Another factor I'm not sure about is being able to save the data - would I have to use Core Data/SQLite to save my single Model object, or is there a simpler way?
I'd start by designing a schema using CoreData. IMO, its best to start out using CoreData because then you'll never have to convert your data layer to CoreData, in the event that your app scales beyond a simple object or two.
The other route would be to create a web service that returns your data... so you just call the service and it returns a collection of user objects. You can either send down the entire object graph with the questions, or create another service to return a collection of questions for a specific user. If you have a web server handy, this method scales the best because you don't have to rely on app updates to get new questions into your system. I would still use CoreData to cache the results... so that way you're not downloading the same information all the time.
So when it comes to accessing CoreData objects, I use a repository class that's a singleton. This makes it easy for any view controller to grab an instance of the repository and get some data. Here's what something like that might look like;
[[Repository defaultRepository] findFirst:[User class]
where:#"name == 'John'"]
There's a lot of redundant code to fetch data so wrapping that up in an object will help get all that nasty code, like predicates and sorting, out of your view controllers. You can see where I leverage a va_list in the where clause so I can inject that string right into my predicate. Here are some other methods you could implement:
- (NSArray *) findAll:(Class)entity
sortByKey:(NSString *)key
ascending:(BOOL)ascending;
- (NSArray *) findAll:(Class)entity
sortByKey:(NSString *)key
ascending:(BOOL)ascending
where:(NSString *)format, ...;
- (id) findFirst:(Class)entity
where:(NSString *)format, ...;
I'm not sure if this is the preferred way, but I've had a lot of success with this method. Hope this helps!
Check this link, this will help you a lot
Link: http://mobile.tutsplus.com/tutorials/iphone/iphone-sdk_store-data/
This cover 4 major ways to store data in iPhone with sample code.
1) NSUserDeafult
2) Property Lists
3) SQLLite
4) Core Data
My quest to collect the scattered, magical pieces of code needed to create an animated and dynamically updating UITableView - artifacts hidden in the dark and ghastly depths of Apple Inc's feared dungeon, The Documentation - has with the help of the ever-friendly townsfolk and everyday heroes of Stack Overflow finally been completed.
But worry not, the end of this quest is but the beginning of a new one.
I have one UITableView. That tableView is hooked up to a NSFetchedResultsController. The FRC delegate methods are all up and running as pr. Apple's example code.
I have two NSManagedObjectContexts:
The Truth. This MOC is only inserted to / deleted from when the user adds or deletes an object.
The scratch pad. This is the MOC that the tableView's FRC is hooked up to. Any change here is reflected in the tableView with nice animations.
The scratch pad is seeded with objects from The Truth, but it is never saved. This means that I can insert and delete (show and hide) objects here to my heart's content, all while the tableView politely updates.
(To anyone reading this in an attempt to implement something like this I would say: get acquainted with [managedObjectContext objectWithID:id])
My question comes from the need to sort the tableView. As made clear to me by reading about NSSortDescriptor in conjunction with NSFetchRequest, using a sortDescriptor simply won't fly when one's using a SQLite store. The Docs say, "instead you should sort the returned array in memory". All-right then! But how do I go about doing that?
Where, in my logic as described above, do I inject this sorted array? Wherever I turn, there seems to be problems.