I'm trying to dynamically update cells in a UITableView. The events are triggered by data received by background threads, so everything is asynchronous and it's impossible to predict the order of the events.
I'm not sure what the workflow is supposed to be in this case. How can I make sure that the row count and indexpath are correct when calling reloadRowsAtIndexPaths:withRowAnimation? There could still be an animation going on by a previous call to deleteRowsAtIndexPaths:withRowAnimation.
If all the changes happen at the same time, I can use beginUpdates and endUpdates, but what do I do in the above situation?
Hanno, I recommend using a command object model to queue the changes on the main thread.
Here's how:
On the callback in the main thread, append the add or delete as an object to a queue (mutable array).
Walk the queue to add and remove objects on the main thread, with each operation being atomic (i.e gets the table status first, processes it without interruption, removes the head of the queue)
Decide if you want to interrupt the animation if there are more commands on the queue, or wait for the animation to end then process the next queued object.
A mix of a timer and animationDidFinish callbacks should do it.
Since you would have to do all the table updates in the main thread anyway (because UIKit has to be called on the main thread), I don't see a concurrency problem here. Whenever you receive new data, call a method on the main thread that updates the table. This method will block the main thread until it has finished. This should make sure that the table view retains its integrity.
Or are you concerned about overlapping row animations?
Related
I have two actors that are overlapping at begin play. I can get the Overlapping Component(s) easy enough but, is there a way to manually call an OnComponentBeginOverlap event since it doesn't get called at BeginPlay if they are already Overlapping?
No, just create a separate event that follows the same execution as your OnComponentBeginOverlap event.
I'm currently having an issue where creating a new object on a background child thread (whose parent is the main UI thread context) and saving causes my NSFetchedResultsController to show two new objects: one with a temporary objectID, and one with a permanent objectID. This seems to be a bug of some sort, unless I'm missing something.
So I thought I would manually obtain permanent IDs for any new objects I create. This fixes the duplicate row issue, but introduces new random errors (such as "could not fulfill fault for object", refering to the new object I created). If anyone has any ideas as to why any of the previously mentioned is happening, please share.
I'm guessing obtainPermanentIDs is a step in the right direction. But when do I call this method? Before saving to the child context? After saving the child and before the parent? After the parent?
Currently my setup is this:
masterMOC - private queue tied to the persistent store, so physical saves happen here
----mainMOC - main queue tied to the UI, child of masterMOC
-------backgroundMOC - private queue, child of mainMOC
So if I create a new object on backgroundMOC, and I intend to immediatly save to disk (which means I'll have to call save: on all three contexts), where should I be calling obtainPermanentIDs?
(or if anyone has a different solution other than calling obtain permanent ids? What problem was this method introduced to solve anyway? Why would I want to call this method?)
Update:
I think I figured out what's going on (it's only a theory though), though not how to solve it. Core Data apparently generates permanent IDs for objects when they are saved physically to disk. So in my case, this won't happen until I call save on the masterMOC. Currently what I do when creating a new object on the backgroundMOC is:
save on backgroundMOC (so that changes are pushed up one level to the mainMOC and the my table view can insert the new rows)
save on mainMOC (so that I can prepare for saving to disk)
save on masterMOC (which finally saves to disk)
What's happening here is that calling save on the backgroundMOC triggers a UI update, and causes the fetched results controller to insert a new object that still has only a temporary ID. But then calling save on masterMOC causes all objects to get assigned permanent IDs, which causes another UI update, inserting another row for this "new" object! By commenting out the last masterMOC save, I no longer see duplicate entries. Am I doing something wrong here, or is this some kind of bug?
Another update: I think I've pretty much confirmed the bug. I call save on the backgroundMOC and then set up a timer to call save on the mainMOC and masterMOC 5 seconds later. Immediatley upon saving to the backgroundMOC, a new row is inserted into my table. 5 seconds later (upon saving main and master), another new row is inserted. (the row inserted first has a temp id, and the newest insert has permanent id).
I had the exact same issue, of course after a particularly difficult and dispiriting day of debugging everything to find out the issue was temporary IDs. :)
I have the exact same structure as you, and I also have subclasses of NSManagedObjectContext to codify the behavior I expect of saves in the background and main contexts – namely, a save in the background context should save the main context (and the main context should sync any objects that changed with the external service, which is irrelevant but worth mentioning as an explanation for why I have two subclasses), and a save in the main context should save the master context.
In my RFSImportContext subclass (equivalent to your backgroundMOC), I implement - save: to call [super save:], then call [self.parentContext performBlock:] (self.parentContext here is equivalent to your mainM)C, where the block calls obtainPermanentIDsForObjects: with the contents of the main context's - updatedObjects and - insertedObjects arrays, then I save the main context.
I no longer have the leaking of temporary objects into my NSFetchedResultsController as you describe. A way to improve the situation a bit would be to use the RFSMainContext subclass (again, equivalent to your mainMOC) to implement - save: to obtain permanent object IDs, save itself, then save the master context. This codifies the behavior that we always want the main context to have permanent IDs for objects in it when it is saved.
just trying to figure out what would be the best way to design such functionality? Basically i send an asynchronous NSURLConnection which hits a server that sends back a bunch of data. Once i get the data i have to perform some work on it which is pretty expensive and which i would rather do in a bg thread to prevent the UI from locking. Lastly i also need to have a uitable update dynamically as the response from the server is received and processed.
My question is how would i go about doing that work in a background thread as the data arrives so that the table doesnt wait until all the data has loaded before being updated??
This is my pseudo code I have so far. In my ViewController i would have two BOOL flags newDataReceived and dataFinishedDownloading. I would also have two variables, a string that contains the current data and a nsarray that kept the results of processing the data. Then,
in didReceiveResponse: i would spawn a new thread by calling performSelectorInBackground: with the processing method as the selector.
in that method i would have a loop that would first check newDataReceived to see if new data has arrived and if so do some work on it.
once finished processing i would then set the nsarray with the results and then call another method that updates the table datasource and reloads the table using performSelectorOnMainThread:
Lastly i would check the dataFinishedLoading flag to see if there is any more data to process
if there is still data and would start all over again, otherwise cleanup the thread and exit
Also the newDataReceived flag would be set in didReceiveData: as well as the actual data received. Finally in didFinishLoading i would set the dataFinishedLoading flag to signal that all the data has been loaded.
I plan on using NSLock's in #2-4 when checking the status flags as well as getting and setting the received data string and results nsarray.
Im sure there are a number of ways to do the same thing but does this seem like a good way to go about it?
thx
You could take a look at NSOperation and NSOperationQueue. NSOperation is a perfect alternative for doing heavy calculations and operations in the background. If you need continuously updates to the tableview you could implement some protocol in your Operation to handle callbacks to the tableview.
What you are looking to do can be achieved with the performSelectorOn... methods. Have a look at this: http://arstechnica.com/civis/viewtopic.php?f=20&t=49035
Just keep in mind that UI updates should be done in the main thread (so use performSelectorOnMainThread for UI updates).
I am having a few nagging issues with NSFetchedResultsController and CoreData, any of which I would be very grateful to get help on.
Issue 1 - Updates: I update my store on a background thread which results in certain rows being delete, inserted or updated. The changes are merged into the context on the main thread using the "mergeChangesFromContextDidSaveNotification:" method. Inserts and deletes are updated properly, but updates are not (e.g. the cell label is not updated with the change) although I have confirmed the updates to come through the contextDidSaveNotifcation, exactly like the inserts and deleted. My current workaround is to temporarily change the staleness interval of the context to 0, but this does not seem like the ideal solution.
Issue 2 - Deleting objects: My fetch batch size is 20. If an object is deleted by the background thread which is in the first 20 rows, everything works fine. But if the object is after the first 20 rows and the table is scrolled down, a "CoreData could not fulfill a fault" error is raised. I have tried resaving the context and reperforming the frc fetch - all to no avail. Note: In this scenario, the frc delegate method "didChangeObject...." is not called for the delete - I assume this is because the object in question had not been faulted at that time (as it is was outside the initial fetch range). But for some reason, the context still thinks the object is around, although is has been deleted from the store.
Issue 3 - Deleting sections : When the deletion of a row leads to the deletion of a section, I have gotten the "invalid number of rows in section???" error. I have worked around this by removing the "reloadSection" line from the NSFetchedResultsChangeMove: section and replacing it with "[tableView insertRowsAtIndexPaths...." This seems to work, but once again, I am not sure if this is the best solution.
Any help would be greatly appreciated. Thank you!
I think all your problems relate to the fetched results controller's cache.
Issue 1 is caused by the FRC using the cached objects (whose IDs have not changed.) When you add or remove an object that changes the IDs and forces an update of the cache but changing the attributes of an object doesn't do so reliably.
Issue 2 is caused by the FRC checking for the object in cache. Most likely, the object has an unfaulted relationship that persist in the cache. When you delete it in the background the FRC tries to fault in the object at the other end of the relationship and cannot.
Issue 3: Same problem. The cache does not reflect the changes.
You really shouldn't use a FRC's cache when some object other than the FRC is modifying the data model. You have two options:
(Preferred) Don't use the cache. When creating the FRC set the cache property to nil.
Clear the cache anytime the background process alters the data model.
Of course, two defeats the purpose of using the cache in the first place.
The cache is only useful if the data is largely static and/or the FRC manages the changes. In any other circumstance, you shouldn't use it because FRC need to check the actual data model repeatedly to ensure that it has a current understanding of the data. It can't rely on the object copies it squirreled away because another input may have changed the real objects.
My advice:
Detect the changes needed on the background thread
Post the changes to the main thread as a payload
Make the actual changes and save on the main thread (Managed Object Context on the main thread)
DO use the cache for the FRC; you'll get better performance
Quote from "Pro Core Data for iOS" by Michael Privat, Robert Warner:
"Core Data manages its caches intelligently so that if the results are updated by another call, the cache is removed if affected."
Setup:
I have several UITableViewControllers (tvc) that all instantiate a separate NSFetchedResultsController (frc). All these frc-s, though, use the same ManagedObjectContext (moc).
Scenario:
Sometimes, from one of the tvc-s, I launch some background process to update data. The background process updates the data in a separate thread and finally calls [moc mergeChangesFromContextDidSaveNotification:] on the main thread. These changes should then propagate to frc, and [frcDelegate controllerDidChangeContent:] should be called. This works fine... but only for the frcDelegate whose associated tvc initiated the update. (all the tvc-s are acting as frcDelegate for their frc.)
Expected behavior:
since all the frc-s are bound to the same moc, all the frc-s should pick up the changes, and [frcDelegate controllerDidChangeContent:] should be called for all of them.
Actual behavior:
only one [frcDelegate controllerDidChangeContent:] gets called.
Why am I seeing actual behavior instead of expected? How should I debug this? Or is this the correct behavior?
This is the expected behavior.
Each fetched results controller will only observe changes in the entity that is specified in its fetch request. Presumably you are only modifying objects of the entity that corresponds to the controller that initiated the update.