When are NSManagedObjects deallocated? - ios

I have problem with deallocating my controller when I navigate back in hierarchy. I found that the problem would be with objects which are loaded from Core Data. When I comment out line with // * the controller is successfully deallocated but with line in place the NSLog isn't printed.
I extract code from controller:
#implementation ModulesListViewController {
NSArray *_modules;
}
- (void)viewDidLoad {
[super viewDidLoad];
_modules = [[StorageManager manager] getModulesCDByUserEmail:userEmail]; // *
...
}
- (void)dealloc {
NSLog(#"ModulesListViewController dealloc");
}
getModulesCDByUserEmail is method which fetch data from Core Data and returns NSArray. Nothing unusual. I guess the problem is with NSManagedObjectContext.
- (NSManagedObjectContext *)managedObjectContext{
if (_managedObjectContext != nil) {
return _managedObjectContext;
}
_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
_managedObjectContext.parentContext = [self writerManagedObjectContext];
return _managedObjectContext;
}
I have it in singleton and it's globally accessible so it's not deallocated. Is this a problem? Should I create own NSManagedObjectContext for each controller if I want to be properly deallocated? Could these contextes be NSMainQueueConcurrencyType? Or is there a way how can I deallocate objects fetched from context which isn't deallocated?
Thanks for help

Do not create a NSManagedObjectContext for each view controller. There is no value in that.
Instead of a singleton, consider using dependency injection. It is a better design that will save you headaches when you add testing or try and reuse view controllers.
The array is not retaining your view controller so the array is not causing the retain issue. However, with that line of code commented out your _modules variable is nil which means other activity against that variable would have a nil response. I would suggest tracing that variable through your view controller and find out where you are causing a retain loop. It is not in that line of code.
Also, consider using a #property instead of a direct ivar like this. It will allow more optimization from the compiler as well as letting ARC do its job better.
Lastly, if this is a table view that you are working with, consider using a NSFetchedResultsController instead of your own array. You will get far superior performance with less code.

Related

General design - Where do I put centrally accessed objects

I have my main app delegate
I have a few UIViewController derived instances driven by a Storyboard
Say I'd like to provide a centralized persistence layer for my application - perhaps Core Data of SQLite. Where would I put those objects? I'm missing some centrally accessible "Application" class you can access from all the UIViewController instances.
Is there a pattern to follow here?
you should check the singleton pattern:
In software engineering, the singleton pattern is a design pattern
that restricts the instantiation of a class to one object. This is
useful when exactly one object is needed to coordinate actions across
the system. The concept is sometimes generalized to systems that
operate more efficiently when only one object exists, or that restrict
the instantiation to a certain number of objects. The term comes from
the mathematical concept of a singleton.
here is a source for a example implementation: What should my Objective-C singleton look like?
and here is the direct link for the modern solution:
https://stackoverflow.com/a/145395/644629
What you're describing is your model layer. There are two main ways to manage the model:
At application startup, create the main model object and hand it to the first view controller.
Make the main model object a Singleton.
The "main model object" in both cases is generally some kind of object manager. It could be a document, or it could be a PersonManager if you have a bunch of Person objects. This object will vend model objects from your persistence store (generally Core Data).
The advantage of a Singleton here is that it's a little easier to implement and you don't have to pass around the manager. The advantage of a non-Singleton is that it's easier to have more than one (for a document-based system), and it's easier to test and reason about non-singletons than singletons. That said, probably 80% of my projects use a singleton model manager.
As a side note, that you appear to already understand: never store the model in the application delegate, and never use the application delegate as a "rendezvous point" to get to the model. That is, never have a sharedModel method on the application delegate. If you find yourself calling [[UIApplication sharedApplication] delegate] anywhere in your code, you're almost always doing something wrong. Hanging data on the application delegate makes code reuse extremely difficult.
Go with a singleton pattern, which has scope of application lifetime.
#interface DataManager ()
#end
#pragma mark -
#implementation DataManager
#pragma mark - Shared Instance
static DataManager* sharedInstance = nil;
#pragma mark - Singleton Methods
- (id)init
{
self = [super init];
if (self) {
// Initialization code here.
}
return self;
}
+ (DataManager*)sharedInstance
{
#synchronized([DataManager class])
{
if (!sharedInstance) {
//[[self alloc] init];
sharedInstance = [[DataManager alloc] init];
}
return sharedInstance;
}
return nil;
}
+ (id)alloc
{
#synchronized([DataManager class])
{
NSAssert(sharedInstance == nil, #"Attempted to allocate a second instance \
of a singleton.");
sharedInstance = [super alloc];
return sharedInstance;
}
return nil;
}
#end
Declare your properties in .h file and synthesize them here in .m file.
To use that property just call:
// set value
[[DataManager sharedInstance] setSharedProperty:#"ABC"]; // If its a string
// get Value
NSLog(#"value : %#", [[DataManager sharedInstance] sharedProperty]);
Hope this is what you required.
Enjoy Coding :)

Passing and retaining NSString to another view when popping current view

I have a method that will dismiss the current view in the navigation controller and replace it with another view. The code looks like this
-(void)didTransferRequest:(NSString *)_transferComments {
AddRequestViewController *ar = [[AddRequestViewController alloc]
initAsTransferForRequestID:requestID
withClosingComments: _transferComments]];
UINavigationController *currentNav = self.navigationController;
[[self retain] autorelease];
[currentNav popViewControllerAnimated:NO];
[currentNav pushViewController:ar animated:NO];
[ar release];
}
[AddRequestViewController.m]
-(AddRequestViewController *)initAsTransferForRequestID:(int)requestID
withClosingComments:(NSString *)closingComments{
self = [self initWithStyle: UITableViewStyleGrouped];
if (self) {
_requestID = requestID;
_closingComments = closingComments;
}
return self;
}
The problem is that once the new view is pushed onto the nav stack, it crashes when the view attempts to access the contents passed in by _transferComments. The pointer is pointing to something else which would make sense since the view gets popped.
I was successful in using withTransferComments: [_transferComments copy] but the Analyzer identified a memory leak with this.
Is using copy safe and should I ignore the leak message or is there a better way to send the string over?
Your AddRequestViewController isn't taking ownership of _transferComments.
You need to read Cocoa Core Competencies - Memory Management and Basic Memory Management Rules.
In the code snippet you posted (without the copy), I deduce that AddRequestViewController doesn't send retain to _transferComments. If it wants to make sure _transferComments stays around, it needs to send it the retain message to take ownership of the string. When AddRequestViewController is done with the string, it needs to send it release to relinquish ownership. You would probably do this in -[AddRequestViewController dealloc].
Basically, your initAsTransferForRequestID:withClosingComments: method should look something like this:
- (id)initAsTransferForRequestID:(int)requestId withClosingComments:(NSString *)transferComments {
if (!(self = [super init]))
return nil;
_requestId = requestId;
_transferComments = [transferComments retain];
return self;
}
(Note that I'm using the common convention of naming instance variables with a leading underscore.) Your dealloc method should look like this:
- (void)dealloc {
[_transferComments release];
[super dealloc];
}
When you changed your code to copy _transferRequest, you did create a memory leak. The copy message creates an "owning" reference to the copy, and something needs to take responsibility for relinquishing that ownership. You didn't change either of your objects to do that.

Fetching Core Data objects in the background: objects not faulted

I need some help in using objects from Core Data with GCD; I seem to get NSManagedObjects that are aren't faulted into the main thread, even when I access their properties. Would appreciate some help.
This is what I'm doing: on launch, I need to load a list of Persons from the Core Data DB, do some custom processing in the background, then reload the table to show the names. I am following the guidelines for Core Data multi-threading by only passing in the objectIDs into the GCD queues. But when I reload the tableview on the main thread, I never see the name (or other properties) displayed for the contacts, and on closer inspection, the NSManagedObjects turn out to be faults on the main thread, even though I access various properties in cellForRowAtIndexPath. The name property is visible in the background thread when I NSLog it; and it's also showing correctly on the main thread in NSLogs in cellForRowAtIndexPath. But they don't show in the tableView no matter what I do. I tried accessing the name property using the dot notation, as well as valueForKey, but neither worked.
Here's my code …. it's called from the FRC initializer:
- (NSFetchedResultsController *)fetchedResultsController
{
if (__fetchedResultsController != nil)
{
return __fetchedResultsController;
}
__fetchedResultsController = [self newFetchedResultsControllerWithSearch:nil]; // creates a new FRC
[self filterAllContactsIntoDictionary: __fetchedResultsController];
return [[__fetchedResultsController retain] autorelease];
}
- (void) filterAllContactsIntoDictionary: (NSFetchedResultsController *) frc
{
NSArray *fetchedIDs = [[frc fetchedObjects] valueForKey:#"objectID"];
NSArray *fetched = [frc fetchedObjects];
if (filterMainQueue == nil) {
filterMainQueue = dispatch_queue_create("com.queue.FilterMainQueue", NULL);
}
dispatch_async(self.filterMainQueue, ^{
NSManagedObjectContext *backgroundContext = [[[NSManagedObjectContext alloc] init] autorelease];
[backgroundContext setPersistentStoreCoordinator:[[self.fetchedResultsController managedObjectContext] persistentStoreCoordinator]];
NSMutableArray *backgroundObjects = [[NSMutableArray alloc] initWithCapacity: fetchedIDs.count];
// load the NSManagedObjects in this background context
for (NSManagedObjectID *personID in fetchedIDs)
{
Person *personInContext = (Person *) [backgroundContext objectWithID: personID];
[backgroundObjects addObject:personInContext];
}
[self internal_filterFetchedContacts: backgroundObjects]; // loads contacts into custom buckets
// done loading contacts into character buckets ... reload tableview on main thread before moving on
dispatch_async(dispatch_get_main_queue(), ^{
CGPoint savedOffset = [self.tableView contentOffset];
[self.tableView reloadData];
[self.tableView setContentOffset:savedOffset];
});
});
}
What am I doing wrong here? Is there any other way to explicitly make the Person objects fire their faults on the main thread? Or am I doing something wrong with GCD queues and Core Data that I'm not aware of?
Thanks.
Why not take the easy route, since you are not saving anything new ?
Instead of creating an extra context for the background thread and working with IDs, use the main managedObjectContext in the background thread after locking it.
for example:
- (void) filterAllContactsIntoDictionary: (NSFetchedResultsController *) frc
{
if (filterMainQueue == nil) {
filterMainQueue = dispatch_queue_create("com.queue.FilterMainQueue", NULL);
}
dispatch_async(self.filterMainQueue, ^{
NSManagedObjectContext *context = ... // get the main context.
[context lock]; // lock the context.
// do something with the context as if it were on the main thread.
[context unlock]; // unlock the context.
dispatch_async(dispatch_get_main_queue(), ^{
CGPoint savedOffset = [self.tableView contentOffset];
[self.tableView reloadData];
[self.tableView setContentOffset:savedOffset];
});
});
}
This works for me when I call a method with performSelectorInBackground, so I guess it should work for GCD dispatch too.
Well, mergeChangesFromContextDidSaveNotification: is your friend. You'll need to tell the MOC on the main thread that there have been changes elsewhere. This will do the trick.
Here's Apple's documentation. To quote from there:
This method refreshes any objects which have been updated in the other context, faults in any newly-inserted objects, and invokes deleteObject:: on those which have been deleted.
EDIT: original answer removed, OP is not fetching in the background
I looked closer at your code and it doesn't look like you are doing anything that will change data and/or affect the context on the main thread.
You have a fetchedResultsController on the main thread. Presumably, this is working and your table is populating with data. Is this true?
When filterAllContentsIntoDictionary is invoked, you pass an array of the fetchedResultsController's current objectIDs to a background thread and do some processing on them (presumably filtering them based on some criteria) but you are not changing data and saving backgroundContext.
internalFilterFetchedContents is a black box. Without knowing what you intend for it to do, hard to say why it's not working.
When this is done, you reload the table on the main thread.
You haven't made any changes to the store, the context, or the fetchedResultsController so of course, the table shows the same data it did before. The missing details to help further are:
Is your tableView showing correct data from the fetchedResultsController to begin with? If not, most likely your only problem is in handling the tableView delegate and dataSource methods and the rest of this isn't really relevant.
What do you intend to happen in filterAllContentsIntoDictionary and internalFilterFetchedContents?
If your intent is to filter the data as displayed by the fetchedResultsController not sure you need to do anything in the background. If you modify the fetchRequest and do performFetch again your table will reload based on the new results.
I you need more help, please answer my questions, add more relevant code to your post and let me know if I'm missing anything wrt the problem and what you're trying to accomplish.
good luck!

Thread safe lazy initialization on iOS

I have a view controller that I want to lazily initialize, and once initialized, use the same copy when possible (I don't use a singleton since I do want to remove it from memory eventually), I use the getter to do so, my code look like this:
#property (retain) UIViewController *myController
...
#synthesize myController = _myController;
...
- (UIViewController *)myController
{
if (!_myController) { // Evaluation
_myController = [[MyViewController alloc] init]; // Object Creation
}
return _myController;
}
This works, but it's not thread safe, and if more than one thread evaluate to true before the object is created, I'll have a memory leak. One solution I've tried is to #synchronized the code, but I'm not sure the correct way to do it.
This appears to work, (lockForMyController is a simple NSString) but it makes this section of code a lot slower:
- (UIViewController *)myController
{
#synchronized(self.lockForMyController){
if (!_myController) {
_myController = [[MyViewController alloc] init];
}
}
return _myController;
}
I was wondering if there is some other way to achieve a lazy initialized, thread safe, property?
This solution works
Note that this solution only works if myController is accessed on a background thread the first time. It will deadlock if called on the main thread.
You want to use gcd. The key is serialize the creation of the object, so that regardless of the threads starting the block, it will always only be created exactly once.
- (UIViewController *)myController
if (_myController == nil) {
dispatch_sync(dispatch_get_main_queue(), ^ { if (_myController == nil) _myController = [[MyViewController alloc] init]; });
}
return _myController;
}
Here, even if multiple threads execute the block, the execution of the block is serialized onto the main thread and only one MyViewController can ever be created.
You won't see a performance hit here unless the object is nil.
Since the property is implicitly atomic, that means that in the setter the value will be autoreleased. This should make it suitable for mingling with your custom getting, since it will autorelease any value changes to _myController.
http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocProperties.html#//apple_ref/doc/uid/TP30001163-CH17-SW2
However, you still may get into a race condition where you are setting the value on one thread but accessing it on another. Any time you set the value, you probably want to make sure and do something like this:
dispatch_sync(dispatch_get_main_queue(), ^ { self.myController = {newValueOrNil} });
This will make sure to serialize your setter methods calls without having to reinvent the wheel for atomic setters, which is very hard to get right.
This solution does not work
You want to use gcd.
http://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/c/func/dispatch_once
See this post about singletons. I know you don't want a singleton, but this demonstrates how to use the method. You can easily adapt it.
Create singleton using GCD's dispatch_once in Objective C

View controller / memory management

i'm a little bit confused with memory management in view controllers.
Lets say i have header file like this:
#interface MyController : UIViewController {
NSMutableArray *data;
}
#property (nonatomic, retain) NSMutableArray *data;
#end
and .m file looks like that:
#implementation MyController
#synthesize data;
- (void)dealloc
{
[self.data release];
[super dealloc];
}
- (void)viewDidLoad
{
[super viewDidLoad];
if (self.data == nil)
self.data = [[NSMutableArray alloc] init];
}
- (void)viewDidUnload
{
[super viewDidUnload];
[self.data release];
self.data = nil;
}
Is that ok from the correct memory management point of view? Will that work after dealloc via Memory Warning? How You do that in your apps?
Thanks for your answers ;)
While the alloc-retain calls balance out in viewDidLoad and viewDidUnload and should prove no problem memory-wise, it would be cleaner to take ownership only once and relinquishing it once rather than twice.
- (void)viewDidLoad
{
[super viewDidLoad];
if (self.data == nil)
self.data = [NSMutableArray array];
}
and
- (void)viewDidUnload
{
[super viewDidUnload];
self.data = nil;
}
You are not guaranteed that viewDidUnload will ever get called. Unlike init/dealloc, which get called in pairs, viewDidUnload is undeterministically called. viewDidUnload is only called if there is a low memory situation and your view is not the active view.
Depending on how your model is created and the implications of it remaining in memory, it may make more sense for you not to get rid of it. An example of this may be that recreating that data may involve an expensive web service call. It therefore would be a bad user experience to have to wait for that data to get recreated. If it must absolutely go, a better strategy may be to cache the data to disk so that you can easily reconstruct it.
viewDidUnload should only contain cleaning up your IBOutlets and flushing easily recreatable data.
These lines from -viewDidUnload both release data:
[self.data release];
self.data = nil;
Since you're using the property setter in the second line, and data is a retained property, the setter will release data. This is an over-release, and it'll cause a crash either right away or later, depending on whether other objects also retain that object. To fix, simply delete the first line and rely on the setter to do the right thing.
The -dealloc method, on the other hand, shouldn't use the setter as it does now. You should change:
[self.data release];
to:
[data release];
data = nil; // this line isn't strictly necessary, but often considered good form
The reasoning here is that it's conceivable that this class could be subclassed, and someone might override the property setter in such a way that it has some side effects that could cause problems when the object is being deallocated. You should access the ivar directly -- notice that I left off the "self." so that we're dealing with the ivar and not the property accessor. (-init and -dealloc are the only places where you have to worry about that; use the property accessors everywhere else.)

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