I have a method that runs in a background thread using a copy of NSManagedObjectContext which is specially generated when the background thread starts as per Apples recommendations.
In this method it makes a call to shared instance of a class, this shared instance is used for managing property values.
The shared instance that managed properties uses a NSManagedObjectContext on the main thread, now even though the background thread method should not use the NSManagedObjectContext on the main thread, it shouldn't really matter if the shared property manager class does or does not use the such a context as it only returns scalar values back to the background thread (at least that's my understanding).
So, why does the shared property class hang when retrieving values via the main threads context when called from the background thread? It doesn't need to pass an NSManagedObject or even update one so I cannot see what difference it would make.
I can appreciate that my approach is probably wrong but I want to understand at a base level why this is. At the moment I cannot understand this whole system enough to be able to think beyond Apples recommended methods of implementation and that's just a black magic approach which I don't like.
Any help is greatly appreciated.
Does using:
[theContext performBlock:^{
// do stuff on the context's queue, launch asynchronously
}];
-- or --
[theContext performBlockAndWait:^{
// do stuff on the context's queue, run synchronously
}];
-- just work for you? If so, you're done.
If not, take a long, hard look at how your contexts are setup, being passed around, and used. If they all share a root context, you should be able to "move" data between them easily, so long as you lookup any objectIDs always on your current context.
Contexts are bound to threads/queues, basically, so always use a given context as a a reference for where to do work. performBlock: is one way to do this.
Related
Problem Statement
I am having trouble saving server side data asynchronously.
Structure
I am using the following structure of NSManagedObjectContext in order of parent to child:
writerManagedObjectContext (NSPrivateQueueConcurrencyType)
masterManagedObjectContext (NSMainQueueConcurrencyType)
backgroundManagedObjectContext (NSPrivateQueueConcurrencyType)
Code
I am using the following code to save data
[backgroundManagedObjectContext performBlock:^{
[backgroundManagedObjectContext save:nil];
[masterManagedObjectContext performBlock:^{ // Starts blocking UI from here
[masterManagedObjectContext save:nil];
[writerManagedObjectContext performBlock:^{
[writerManagedObjectContext save:nil];
}]
}]
}]
Problem
The code saves fine. The backgroundManagedObjectContext also saves asynchronously. However, both masterManagedObjectContext and writerManagedObjectContext refuse to be saved asynchronously and blocks the UI thread. (I know it blocks the UI thread because I tried to perform actions which have nothing to do with Core Data and they were also blocked. This is not a case of the persistent coordinator not being accessible)
Questions
What would be the reason the above code blocks the main thread?
Am I correct in assuming I can call the above code from anywhere since the save will be called in each respective thread/context?
Any help would be much appreciated.
Edit
http://floriankugler.com/2013/04/29/concurrent-core-data-stack-performance-shootout/
Apparently the freeze comes from the attempt to propagate to the parent NSManagedObjectContext. The article seems to elude to the fact that it is impossible to have a truly asynchronous save to the main context.
The data was heavily linked, 5MB, and took approximately 40s to save to psc. I don't think I'll use a parallel structure as described in the post since the code base is already large as it is. I would appreciate any strategies I can utilize to decrease this freeze.
Even though backgroundManagedObjectContext is a private queue context it still propagates its changes up to masterManagedObjectContext, being its parent. This could be where it's choking up. Merging changes from child contexts still takes up CPU time, the effects of which become more pronounced in a busy queue like the UI's.
You can always use Instruments to analyze what's going on.
If your particular use case permits, try setting backgroundManagedObjectContext.persistentStoreCoordinator to the same psc as writerManagedObjectContext and not make it a child of masterManagedObjectContext.
Better yet, use an awesome framework like MagicalRecord. Doesn't shield you from problems like this, but less code makes things (arguably) a bit easier to debug.
What I'm trying to do:
perform background sync with a web API without freezing the UI. I'm using MagicalRecord but it's not really specific to it.
make sure I'm using contexts & such correctly
What my question really is: is my understanding correct? Plus a couple questions at the end.
So, the contexts made available by MagicalRecord are:
MR_rootSavingContext of PrivateQueueConcurrencyType which is used to persist data to the store, which is a slow process
MR_defaultContext of MainQueueConcurrencyType
and for background you would want to work with a context generated by MR_context(), which is a child of MR_defaultContext and is of PrivateQueueConcurrencyType
Now, for saving in an asynchronous way, we have two options:
MR_saveToPersistentStoreWithCompletion() which will save all the way up to MR_rootSavingContext and write to disk
MR_saveOnlySelfWithCompletion() which will save only up to the parent context (i?e. MR_defaultContext for a context created with MR_context)
From there, I thought that I could do the following (let's call it Attempt#1) without freezing the UI:
let context = NSManagedObjectContext.MR_context()
for i in 1...1_000 {
let user = User.MR_createInContext(context) as User
context.MR_saveOnlySelfWithCompletion(nil)
}
// I would normally call MR_saveOnlySelfWithCompletion here, but calling it inside the loop makes any UI block easier to spot
But, my assumption was wrong. I looked into MR_saveOnlySelfWithCompletion and saw that it relies on
[self performBlock:saveBlock];
which according to Apple Docs
Asynchronously performs a given block on the receiver’s queue.
So I was a bit puzzled, since I would expect it not to block the UI because of that.
Then I tried (let's call it Attempt#2)
let context = NSManagedObjectContext.MR_context()
for i in 1...1_000 {
let user = User.MR_createInContext(context) as User
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) { () -> Void in
context.MR_saveOnlySelfWithCompletion(nil)
}
}
And this does the job, but it doesn't feel right.
Then I found something in the release notes of iOS 5.0
When sending messages to a context created with a queue
association, you must use the performBlock: or performBlockAndWait:
method if your code is not already executing on that queue (for the
main queue type) or within the scope of a performBlock... invocation
(for the private queue type). Within the blocks passed to those
methods, you can use the methods of NSManagedObjectContext freely.
So, I'm assuming that:
Attempt#1 freezes the UI because I'm actually calling it from the main queue and not within the scope of a performBlock
Attempt#2 works, but I'm creating yet another thread while the context already has its own background thread
So of course what I should do is use saveWithBlock:
MagicalRecord.saveWithBlock { (localContext) -> Void in
for i in 1...1_000 {
User.MR_createInContext(context)
}
}
This performs the operation on a direct child of MR_rootSavingContext which is of PrivateQueueConcurrencyType.
Thanks to rootContextChanged, any change that goes up to MR_rootSavingContext will be available to MR_defaultContext.
So it seems that:
MR_defaultContext is the perfect context when it comes to displaying data
edits are preferably done in an MR_context (child of MR_defaultContext)
long running tasks such as a server sync are preferably done using saveWithBlock
What it still don't get is how to work with MR_save[…]WithCompletion(). I would use it on MR_context but since it blocked the main thread in my test cases I don't see when it becomes relevant (or what I missed…).
Thanks for your time :)
Ok, I am rarely using magical records but since you said you question is more general I will attempt an answer.
Some theory: When creating a context you pass an indicator as to whether you want it to be bound on the main or a background thread
let context = NSManagedObjectContext(concurrencyType: NSManagedObjectContextConcurrencyType.PrivateQueueConcurrencyType)
By "bound" we mean that a thread is referenced by the context internally. In the example above a new thread is created and owned by the context. This thread is not used automatically but must be called explicitly as:
context.performBlock({ () -> Void in
context.save(nil)
return
});
So your code with 'dispatch_async' is wrong because the thread the context is bound to can only be referenced from the context itself (it is a private thread).
What you have to infer from the above is that if the context is bound to the main thread, calling performBlock from the main thread will not do anything different that calling context methods straight.
To comment on your bullet points at the end:
MR_defaultContext is the perfect context when it comes to displaying data: An NSManagedObject must be accessed from the context it is
created so it is actually the only context that you can feed the
UI from.
edits are preferably done in an MR_context (child of MR_defaultContext): Edits are not expensive and you should follow
the rule above. If you are calling a function that edits an NSManagedObject's properties from the main thread (like at the tap of a button)
you should update the main context. Saves on the other hand are
expensive and this is why your main context should not be linked to a
persistent store directly but just push its edits down to a root
context with background concurrency owning a persistent store.
long running tasks such as a server sync are preferably done using saveWithBlock Yes.
Now, In attempt 1
for i in 1...1_000 {
let user = User.MR_createInContext(context) as User
}
context.MR_saveOnlySelfWithCompletion(nil)
There is no need to save for every object creation. Even if the UI was not blocked it is wasteful.
About MR_context. In the documentation for magical records I cannot see a 'MR_context' so I am wondering if it is a quick method to access the main context. If it is so, it will block.
NSManagedObjectContext's have had the performBlock: and performBlockAndWait: methods added to help make concurrency easier. I've been using them -- potentially fairly naively -- and I just realized that there's a question I've never really asked.
If I create an NSManagedObject subclass inside one of the performBlock methods, it's 'home' thread is the thread of it's parent context -- which in the case of the NSPrivateQueueConcurrencyType is probably an independent thread I have no other access to.
So do I need to do a performBlock call just to access the data contained inside my managed objects? Or is there a background magic going on to help protect me in the case of using getters? (Or setters, though that seems like a bad idea...)
NSManagedObject is not supposed to be used outside of its managedObjectContexts thread/queue (sometime it works and some times you crash ==> don't do it).
CoreData does not guarantee safe read access to the object.
To access an object owned by a "private queue" context, always use either [context performBlock:...] or [context performBlockAndWait:...], unless you access its objectID or managedObjectContext properties.
You do need to use performBlock: or performBlockAndWait:, but there's one exception. If you're using NSMainQueueConcurrencyType and you are using the managed object on the main queue, you can access it directly, with no block. This can be a great convenience when you need to update your UI from a managed object, or vice versa.
I am creating a NSManagedObject (subclass) with certain attributes. At the same time, I am executing some code/a block that does some network operation given the attributes of my NSManagedObject. Now, some times that network operation might fail or take too long, so I want to add the ability to cancel the execution of that code/block.
I was thinking of making the code/block an NSThread, and then I have the ability to call [theThread cancel]. However, how do I associate the NSThread with my NSManagedObject, given that I cannot add properties to NSManagedObject Categories? Is it OK to just add the property to the definition of the NSManagedObject itself? Seems legal, but subsequent changes to the Core Data model would overwrite my code, I guess.
But maybe there is an entirely different and better way to accomplish what I am trying to do? Any ideas?
First, new code really should prefer GCD or NSOperationQueue over NSThread. If you find yourself using NSThread it's time to slow down and revisit your design and implementation requirements.
Second, using NSManagedObject across threads is really, really bad. If you do anything but exceedingly trivial things, it can get very difficult to do right as well.
Finally, no matter how you do your threaded network access, you should prefer to grab the data from the managed object, and pass that instead of the managed object itself
If you must access the managed object, make sure your managed object context is of either NSMainQueueConcurrencyType or NSPrivateQueueConcurrencyType and access the managed object like by invoking performBlock or performBlockAndWait using the managedObjectContext property of the managed object.
EDIT
Ok, let me check this with you. What I a currently doing is spawning a
backgroundContext, create a new NSManagedObject using performBlock,
then save that background Context, switch to the parent context (using
performblock), obtain the newly created object in that context using
existingObjectWithId:. Then, I create a NSOperation subclass, tie the
NSManagedObject (from the parent context) to that NSOperation subclass
(it's a property on the subclass) and put that operation in a
NSOperationQueue. Within that NSOperation, the NSManagedObject gets
changed. It seems to work fine, does that look ok? – user1013725
Um... maybe??? I didn't follow that. Could you please post the code? That would be much more precise and more easy to understand.
#JodyHagins So I am not using performBlock, but maybe that's ok
because the managedObjectContext is the main context? – user1013725
No.
If the main context is created with either alloc] init] or alloc] initWithConcurrencyType:NSConfinementConcurrencyType then you must use it only when you know you are running on the main thread.
If it is created with alloc] initWithConcurrencyType:NSMainThreadConcurrencyType then you must use it only when you know you are running on the main thread or within one of the performBlock methods.
In iOS 5, NSManagedObjectContext has a couple of new methods, performBlock: and performBlockAndWait:. What are these methods actually used for? What do they replace in older versions? What kind of blocks are supposed to be passed to them? How do I decide which to use? If anyone has some examples of their use it would be great.
The methods performBlock: and performBlockAndWait: are used to send messages to your NSManagedObjectContext instance if the MOC was initialized using NSPrivateQueueConcurrencyType or NSMainQueueConcurrencyType. If you do anything with one of these context types, such as setting the persistent store or saving changes, you do it in a block.
performBlock: will add the block to the backing queue and schedule it to run on its own thread. The block will return immediately. You might use this for long persist operations to the backing store.
performBlockAndWait: will also add the block to the backing queue and schedule it to run on its own thread. However, the block will not return until the block is finished executing. If you can't move on until you know whether the operation was successful, then this is your choice.
For example:
__block NSError *error = nil;
[context performBlockAndWait:^{
myManagedData.field = #"Hello";
[context save:&error];
}];
if (error) {
// handle the error.
}
Note that because I did a performBlockAndWait:, I can access the error outside the block. performBlock: would require a different approach.
From the iOS 5 core data release notes:
NSManagedObjectContext now provides structured support for concurrent operations. When you create a managed object context using initWithConcurrencyType:, you have three options for its thread (queue) association
Confinement (NSConfinementConcurrencyType).
This is the default. You promise that context will not be used by any thread other than the one on which you created it. (This is exactly the same threading requirement that you've used in previous releases.)
Private queue (NSPrivateQueueConcurrencyType).
The context creates and manages a private queue. Instead of you creating and managing a thread or queue with which a context is associated, here the context owns the queue and manages all the details for you (provided that you use the block-based methods as described below).
Main queue (NSMainQueueConcurrencyType).
The context is associated with the main queue, and as such is tied into the application’s event loop, but it is otherwise similar to a private queue-based context. You use this queue type for contexts linked to controllers and UI objects that are required to be used only on the main thread.
They allow you to access the same managedObjectContext accross threads.
I am not really sure I am correct, but this is how I use it.
You use performBlockAndWait is like "usual". You do not need it if you execute the managedObjectContext only on one thread. If you execute it on many threads then yes you will need performBlock.
So, if you're on main thread, you do not need to do performBlockAndWait for the main managedObjectContext. At least I don't and is doing fine.
However if you access that managedObjectContext on other threads then yes you will need to do performBlockAndWait.
So that's the purpose of performBlock and performBlockAndWait.
Would someone please correct me if I am wrong here. Of course if you access the context only on one thread then you can simply use the default.