When using NSPrivateQueueConcurrencyType and NSMainQueueConcurrencyType types for NSManagedObjectContext,
is it safe to make nested performBlock calls on the same context ?
[backgroundContext performBlock:^{
NSFetchRequest *myRequest = ...;
__block NSArray *result= nil;
[backgroundContext performBlockAndWait:^{
results = [backgroundContext executeFetchRequest:myRequest error:NULL];
}];
}];
It may seem stupid but I have an existing codebase with a lot of helpers methods which encapsulate the executeFetchRequest calls. I don't want to make assumptions about whether the caller has already used performBlock or not.
For example:
-(void)updateObjects:(BOOL)synchronous
{
if (YES == synchronous)
[self fetchHelper];
else
{
[backgroundContext performBlock:^{
[self fetchHelper];
}];
}
}
-(NSArray*)fetchHelper
{
[self.backgroundContext performBlockAndWait:^{
//Fetch the objects...
[self.backgroundContext executeFetchRequest: (...)];
}];
}
I have tried it and it works. But I have learned (the hard way) to be very careful with Core Data and multi-threading.
Yes, performBlockAndWait is reentrant. Directly from Apple's release notes...
Core Data formalizes the concurrency model for the
NSManagedObjectContext class with new options. When you create a
context, you can specify the concurrency pattern to use with it:
thread confinement, a private dispatch queue, or the main dispatch
queue. The NSConfinementConcurrencyType option provides the same
behavior that was present on versions of iOS prior to 5.0 and is the
default. 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. The
performBlockAndWait: method supports API reentrancy. The performBlock:
method includes an autorelease pool and calls the
processPendingChanges method upon completion.
Related
I'm trying to separate my application work when there is a bigger work to do to optimize performance. My problem is about a NSManagedObjectContext used in another thread than the main one.
I'm calling:
[NSThread detachNewThreadSelector:#selector(test:) toTarget:self withObject:myObject];
On the test method there are some stuff to do and I have a problem here:
NSArray *fetchResults = [moc
executeFetchRequest:request
error:&error];
Here is my test method:
-(void) test:(MyObject *)myObject{
#autoreleasepool {
//Mycode
}
}
The second time I call the test method, my new thread is blocked when the executeFetchRequest is called.
This problem arrived when my test method is called more than one time in succession. I think the problem comes from the moc but I can't really understand why.
Edit:
With #Charlie's method it's almost working. Here is my code to save my NSManagedObjectContext (object created on my new thread).
- (void) saveContext:(NSManagedObjectContext *) moc{
NSError *error = nil;
if ([moc hasChanges] && ![moc save:&error]) {
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
}
}
This method is called on the new thread. My problem now is that with this save, I have a deadlock and I don't really understand why. Without it's perfectly working.
Edit2
I'm working on this issue but I still can't fix it. I changed my code about the detachNewThreadSelector. Here is my new code:
NSManagedObjectContext* context = [[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];
context.persistentStoreCoordinator = self.persistentStoreCoordinator;
context.undoManager = nil;
[context performBlock:^
{
CCImages* cachedImage;
NSManagedObjectContext *childContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
childContext.parentContext = context;
cachedImage=[CCImages getCCImageForKey:path inManagedObjectContext:childContext];
UIImage *image = [self getImageFromCacheWithPath:path andCachedImage:cachedImage atDate:now];
if (image != nil){
if(![weakSelf.delegate respondsToSelector:#selector(CacheCacheDidLoadImageFromCache:)])
[weakSelf setDelegate:appDelegate.callbacksCollector];
//[weakSelf useCallbackCollectorForDelegate:weakSelf inMethod:#"initPaginatorForListMoments"];
[weakSelf.delegate CacheCacheDidLoadImageFromCache:image];
}
}
- (UIImage*) getImageFromCacheWithPath:(NSString*) path andCachedImage:(CCImages *) cachedImage atDate: (NSDate *) now{
NSURL* localURL=[NSURL URLWithString:cachedImage.path relativeToURL:[self imageCacheDirectory]];
UIImage * image;
//restore uiimage from local file system
if (localURL) {
image=[UIImage imageWithContentsOfFile:[localURL path]];
//update cache
[cachedImage setLastAccessedAt:now];
[self saveContext];
if(image)
return image;
}
return nil;
}
Just after that, I'm saving my contexts (manually for now)
[childContext performBlock:^{
NSError *error = nil;
if (![childContext save:&error]) {
DDLogError(#"Error during context saving when getting image from cache : %#",[error description]);
}
else{
[context performBlock:^{
NSError *error = nil;
if (![context save:&error]) {
DDLogError(#"Error during context saving when getting image from cache : %#",[error description]);
}
}];
}
}];
There is a strange problem. My call back method is called without any problem on my controller (which implements the CacheCacheDidLoadImageFromCache: method). On this method I attest the reception of the image (DDLogInfo) and say that I want my spinner to stop. It does not directly but only 15secondes after the callback method was called.
My main problem is that my context (I guess) is still loading my image from the cache while it was already found. I said 'already' because the callback method has been called and the image was present. There is no suspicious activity of the CPU or of the memory. Instruments didn't find any leak.
I'm pretty sure that I'm using wrongly the NSManagedObjectContext but I can't find where.
You are using the old concurrency model of thread confinement, and violating it's rules (as described in the Core Data Concurrency Guide, which has not been updated yet for queue confinement). Specifically, you are trying to use an NSManagedObjectContext or NSManagedObject between multiple threads.
This is bad.
Thread confinement should not be used for new code, only to maintain the compatibility of old code while it's being migrated to queue confinement. This does not seem to apply to you.
To use queue confinement to solve your problem, first you should create a context attached to your persistent store coordinator. This will serve as the parent for all other contexts:
+ (NSManagedObjectContent *) parentContextWithPersistentStoreCoordinator:(NSPersistentStoreCoordinator *)coordinator {
NSManagedObjectContext *result = nil;
result = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[result setPersistentStoreCoordinator:coordinator];
return result;
}
Next, you want the ability to create child managed object contexts. You will use these to perform work on the data, wether reading or writing. An NSManagedObjectContext is a scratchpad of the work you are doing. You can think of it as a transaction. For example, if you're updating the store from a detail view controller you would create a new child context. Or if you were performing a multi-step import of a large data set, you would create a child for each step.
This will create a new child context from a parent:
+ (NSManagedObjectContext *) childContextWithParent:(NSManagedObjectContext *)parent {
NSManagedObjectContext *result = nil;
result = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[result setParent:parent];
return result;
}
Now you have a parent context, and you can create child contexts to perform work. To perform work on a context, you must wrap that work in performBlock: to execute it on the context's queue. I do not recommend using performBlockAndWait:. That is intended only for re-rentrant methods, and does not provide an autorelease pool or processing of user events (user events are what drives nearly all of Core Data, so they're important. performBlockAndWait: is an easy way to introduce bugs).
Instead of performBlockAndWait: for your example above, create a method that takes a block to process the results of your fetch. The fetch, and the block, will run from the context's queue - the threading is done for you by Core Data:
- (void) doThingWithFetchResults:(void (^)(NSArray *results, NSError *error))resultsHandler{
if (resultsHandler != nil){
[[self context] performBlock:^{
NSArray *fetchResults = [[self context] executeFetchRequest:request error:&error];
resultsHandler(fetchResults, error);
}];
}
}
Which you would call like this:
[self doThingsWithFetchResults:^(NSArray *something, NSError *error){
if ([something count] > 0){
// Do stuff with your array of managed objects
} else {
// Handle the error
}
}];
That said, always prefer using an NSFetchedResultsController over using executeFetch:. There seems to be a belief that NSFetchedResultsController is for powering table views or that it can only be used from the main thread or queue. This is not true. A fetched results controller can be used with a private queue context as shown above, it does not require a main queue context. The delegate callbacks the fetched results controller emits will come from whatever queue it's context is using, so UIKit calls need to be made on the main queue inside your delegate method implementations. The one issue with using a fetched results controller this way is that caching does not work due to a bug.
Again, always prefer the higher level NSFetchedResultsController to executeFetch:.
When you save a context using queue confinement you are only saving that context, and the save will push the changes in that context to it's parent. To save to the store you must recursively save all the way. This is easy to do. Save the current context, then call save on the parent as well. Doing this recursively will save all the way to the store - the context that has no parent context.
Example:
- (void) saveContextAllTheWayBaby:(NSManagedObjectContext *)context {
[context performBlock:^{
NSError *error = nil;
if (![context save:&error]){
// Handle the error appropriately.
} else {
[self saveContextAllTheWayBaby:[context parentContext]];
}
}];
}
You do not, and should not, use merge notifications and mergeChangesFromContextDidSaveNotification: with queue confinement. mergeChangesFromContextDidSaveNotification: is a mechanism for the thread confinement model that is replaced by the parent-child context model. Using it can cause a whole slew of problems.
Following the examples above you should be able to abandon thread confinement and all of the issues that come with it. The problems you are seeing with your current implementation are only the tip of the iceberg.
There are a number of Core Data sessions from the past several years of WWDC that may also be of help. The 2012 WWDC Session "Core Data Best Practices" should be of particular interest.
if you want to use managed object context in background thread, there are two approaches,
1 Create a new context set concurrency type to NSPrivateQueueConcurrencyType and set the parentContext to main thread context
2 Create a new context set concurrency type to NSPrivateQueueConcurrencyType and set persistentStoreCoordinator to main thread persistentStoreCoordinator
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
NSManagedObjectContext *privateContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
privateContext.persistentStoreCoordinator = mainManagedObjectContext.persistentStoreCoordinator;
[[NSNotificationCenter defaultCenter] addObserverForName:NSManagedObjectContextDidSaveNotification object:nil queue:nil usingBlock:^(NSNotification* note) {
NSManagedObjectContext *moc = mainManagedObjectContext;
if (note.object != moc) {
[moc mergeChangesFromContextDidSaveNotification:note];
}
}];
// do work here
// remember managed object is not thread save, so you need to reload the object in private context
});
before exist the thread, make sure remove the observer, bad thing can happen if you don't
for more details read http://www.objc.io/issue-2/common-background-practices.html
I'm having problems with Core Data concurrency on my iOS app.
On my executeFetchRequest I tried to synchronize the managedObjectContext request, but some times this method makes my app freeze.
- (NSArray *)synchronizedWithFetchRequest:(NSFetchRequest *)request andError:(NSError **)error
{
#synchronized(self.managedObjectContext)
{
return [self.managedObjectContext executeFetchRequest:request error:error];
}
}
I've already tried many things like lock/unlock, performBlock/performBlockAndWait, dispatch_sync/dispatch_async and nothing seems to work.
Managed Object Context creation:
...
_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_managedObjectContext setPersistentStoreCoordinator:self.persistentStoreCoordinator];
Is there some way around this? and keep my request returning the results objects on this method?
Thanks!
Synchronising on the MOC suggests that there is more than one thread accessing the same MOC.
That in itself is a violation of CoreData concurrency protocols.
This access is prohibited unless it is wrapped in the context performBlock: method (or its "wait" counterpart). this will negate the need for the #synchronized block altogether.
This thread/queue "boundness" extends to the contexts fetched/registered managed objects, and so, you will not be able to access them as the return values of your method.
I have been using Core Data with a single NSManagedObjectContext for a long time, all fetching, saving, background update operations will be done on single context through helper classes, I was planning to implement a multiple NSManagedObjectContext approach (which is the recommended solution in most of my searching).
My question is: is performBlock the only was to execute code for that context? Can't we do something like below:
- (void) checkSyncServer {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
//do something here, check and fetch data
// create NSManagedObject's
[_tempContext save:&error];
//masterContext will merge changes through notification observers
});
}
(i.e) execute code apart from -performBlock method. How can I execute multiple asynchronous methods and perform a save?
However, I find a single context (which is managed by one singleton NSObject class) simpler to use.
This multiple context with ContextConcurrencyType looks more complicated (in terms of execution flow). Is there a better solution?
You can access contexts in one of two ways:
On its Thread/Queue. This applies to confined contexts and main queue contexts. You can access them freely from their own thread.
With -performBlock: if it is a private queue context or if you are touching the context from a thread other than the one it belongs on.
You cannot use dispatch_async to access a context. If you want the action to be asynchronous then you need to use -performBlock:.
If you were using a single context before and you were touching it with a dispatch_async you were violating the thread confinement rule.
Update
When you call [[NSManagedObjectContext alloc] init] that is functionally equivalent to [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType].
The NSManagedObjectContext has always been thread confined.
As for executing multiple methods you can just call them all in the same block:
NSManagedObjectContext *moc = ...;
[moc performBlock:^{
//Fetch something
//Process data
//Save
}];
Or you could nest them if you wanted them to be all async of each other:
NSManagedObjectContext *moc = ...;
[moc performBlock:^{
//Fetch Something
[moc performBlock:^{
//Process Data
}];
[moc performBlock:^{
//Save
}];
}];
Since -performBlock: is re-entrant safe you can nest them all you want.
Update Async save
To do an async save you should have two contexts (or more):
Main Queue context that the UI talks to
Private Queue context that saves
Private context has a NSPersistentStoreCoordinator and the main queue context has the private as its parent.
All work is done in the main queue context and you can save it safely, normally on the main thread. That save will be instantaneous. Afterwards, you do an async save:
NSManagedObjectContext *privateMOC = ...;
NSManagedObjectContext *mainMOC = ...;
//Do something on the mainMOC
NSError *error = nil;
if (![mainMOC save:&error]) {
NSLog(#"Main MOC save failed: %#\n%#", [error localizedDescription], [error userInfo]);
abort();
}
[privateMOC performBlock:^{
NSError *error = nil;
if (![privateMOC save:&error]) {
NSLog(#"Private moc failed to write to disk: %#\n%#", [error localizedDescription], [error userInfo]);
abort();
}
}];
If you already have an app, all you need to do is:
Create your private moc
Set it as the parent of your main
Change your main's init
Add the private block save method whenever you call save on your main
You can refactor from there but that is all you really need to change.
I get runtime errors which seem to result from my incorrect implementation of GCD in combination with my custom NSManagedObjects.
Nested in a GCD call, I am using custom NSManagedObjects which (seem to) have their own managed object contexts (= self.managedObjectContext).
I am creating the managed object context in the app delegate by using the managed object context provided by UIManagedDocument: self.managedDocument.managedObjectContext.
I don't understand how to pass the correct managed object context down to my custom NSManagedObjects. How would I need to change my code to use the correct managed object context?
This is my main method (inside a view controller):
dispatch_queue_t queue;
queue = dispatch_queue_create("queue", NULL);
dispatch_async(queue, ^{
// ...
NSDecimalNumber *value = [reportedPeriod
valueForCoa:figure.code
convertedTo:self.currencySymbol];
// ...});
}
In this main method I do not have any reference to a managed object context, I do just call valueForCoa:convertedTo: (which is coded as follows):
- (NSDecimalNumber*)valueForCoa:(NSString*)coaStr
convertedTo:(NSString*)targetCurrencyStr {
// ...
CoaMap *coa = [CoaMap coaItemForString:coaStr
inManagedObjectContext:self.managedObjectContext];
// ...
}
valueForCoa is a method in my custom subclassed NSManagedObject ReportedPeriod and uses its (default) managed object context self.managedObjectContext.
The app then usually crashes in the custom subclassed NSManagedObject CoaMap in the following method when it executes the fetch request:
+ (CoaMap*)coaItemForString:(NSString*)coaStr
inManagedObjectContext:(NSManagedObjectContext*)context {
NSFetchRequest *request = [NSFetchRequest
fetchRequestWithEntityName:NSStringFromClass([self class])];
NSPredicate *predicate =
[NSPredicate predicateWithFormat:#"coa == %#",coaStr];
request.predicate = predicate;
// ** The runtime error occurs in the following line **
NSArray *results = [context executeFetchRequest:request error:nil];
// ...
}
The error message is: Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSCFSet: 0x9a8a4a0> was mutated while being enumerated.
Could you please help me with this issue and give me some suggestions on how to improve my code to pass the correct managed object contexts (or on how to make sure that the correct context is used in all methods)?
Thank you very much!
That error generally relates to using a managed object incorrectly context across different threads or queues. You created the MOC on the main queue, but you're using it on a background queue without considering that fact. It's not wrong to use the MOC on a background queue, but you need to be aware of that and take preparations.
You didn't say how you're creating the MOC. I suggest that you should be doing this:
NSManagedObjectContext *context = [[NSManagedObjectContext alloc]
initWithConcurrencyType: NSMainQueueConcurrencyType];
With main queue concurrency you can just use it normally on the main thread. When you're in your dispatch queue though, do this:
[context performBlockAndWait:^{
NSFetchRequest *request = [NSFetchRequest
fetchRequestWithEntityName:NSStringFromClass([self class])];
NSPredicate *predicate =
[NSPredicate predicateWithFormat:#"coa == %#",coaStr];
request.predicate = predicate;
NSArray *results = [context executeFetchRequest:request error:nil];
// ...
}];
This will ensure that the MOC's work occurs on the main thread even though you're on a background queue. (Technically what it actually means is that the MOC's work in the background will be correctly synchronized with work it does on the main queue, but the result is the same: this is the safe way to do this).
A similar approach would be to use NSPrivateQueueConcurrencyType instead. If you do that, you'd use performBlock or performBlockAndWait everywhere for the MOC, not just on background threads.
First,
"how to pass the correct managed object context down to my custom NSManagedObjects."
We create NSManagedObject with NSManagedObjectContext. not the other way around. So, when you have a NSManagedObject, you can access the NSManagedObjectContext by asking its property: – managedObjectContext as listed in Apple Document
Second,
When working with CoreData, multi-threading could be a little tricky. especially for the beginner. The are all kind of details that you need to take care of.
I highly recommend you checkout the Parent-Child NSManagedContext. then, using MagicRecord.
By using MagicRecord, you can simply Grand Central Dispatch with a Block like this:
[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext){
// here use the `localContext` as your NSManagedContext and do things in the background.
// it will take care of all the rest.
}];
If you need to pass NSManagedObject into this block, remember to only pass the NSManagedObjectID instead of the proprety.
this is a example.
NSManagedObjectID *coaMapID = CoaMap.objectID;
[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext){
coaMap *aCoaMap = (coaMap *)[localContext existingObjectWithID:coaMapID error:&error];
// then use aCoaMap normally.
}];
Hope this helped.
Since two days I'm trying to get Core Data to work with multiple threads. I tried standard thread confinement method with NSOperations, merging notifications, using objectWithId, dictionaries of contexts per thread and still I get strange deadlocks, inconsistency exceptions and a bunch of other nasty stuff. It's driving me crazy... moreover I can't find a single example or explanation on how to manage context in two threads when both threads may make changes to the shared persistent store...
I tried to use new iOS 5 method, that supposed to be easier, but still I get errors. The first problem is the deadlock when saving context. I removed all the unnecessary code and stil get deadlocks when executing this code fast enough (by quickly tapping a button):
NSManagedObjectContext *context = [StoreDataRetriever sharedRetriever].managedObjectContext;
for (int i = 0; i < 5; i++) {
NSError *error = nil;
NSLog(#"Main thread: %#, is main? %d", [NSThread currentThread], [NSThread isMainThread]);
BOOL saveOK = [context save:&error];
if (!saveOK) {
NSLog(#"ERROR!!! SAVING CONTEXT IN MAIN");
}
[context performBlock:^{
NSLog(#"Block thread: %#", [NSThread currentThread]);
NSError *error = nil;
BOOL savedOK = NO;
savedOK = [context save:&error];
if (!savedOK) {
NSLog(#"ERROR!!! SAVING CONTEXT IN BLOCK");
}
}];
}
There are no other changes to the database, nothing, only saving context. What is wrong with this code? How should it look like?
Note: [StoreDataRetriever sharedRetriever].managedObjectContext is created in appDelegate using initWithConcurrencyType:NSPrivateQueueConcurrencyType.
What's going on with that code? You are saving the context on a thread synchronously, then you schedule a save on the context private queue. 5 times. So basically, you may well have two save operations, one synchronous and one asynchronous, colliding with each other.
This is clearly an issue. You aren't supposed to save a context with a private queue outside of that queue. It will work with the current context implementation provided there is no scheduled block on the context queue. But this is wrong nevertheless.
…
for (int i = 0; i < 5; i++) {
NSLog(#"Main thread: %#, is main? %d", [NSThread currentThread], [NSThread isMainThread]);
__block NSError *error = nil;
__block BOOL saveOK = YES;
[context performBlockAndWait: ^{
saveOK = [context save: &error];
}];
if (!saveOK) {
NSLog(#"ERROR!!!");
}
…
With that code, you execute the save operation synchronously and most certainly on the same thread - thanks GCD - sparing context switches and synchronization stuff, and without any risk of having two operations running on that context at the same time.
The same rule applies when using NSMainQueueConcurrencyType, with an exception. That queue is bound to the main thread and the main thread only. You can schedule blocks on a context using the main queue from any thread with performBlock and performBlockAndWait like NSPrivateQueueConcurrencyType, and (the exception:) you can use the context directly on the main thread.
NSConfinementConcurrencyType binds the context to a specific thread and you cannot use GCD or blocks to deal with such a context, only the bound thread. There is very little reasons to use that concurrency model as of today. If you have to, use it, but if you do not absolutely have to, don't.
edit
Here is a very nice article about multi-contextes setups: http://www.cocoanetics.com/2012/07/multi-context-coredata/