Hi I have a nasty memory management issue under ARC and cannot figure out how to solve it. The problem stands like this, I have these objects:
ObjectManager - a singleton that does fetchRequests to core data and fetches one or more NSManagedObjects
UIViewControllerA - a view controller in which I have a button "PassManagedObject" and a property declared as below:
#property (strong, nonatomic) ManagedObject *objectForToday;
in viewDidLoad on UIViewControllerA, I call the method refreshDailyObject which does this:
self.objectForToday = nil;
self.objectForToday = [[ObjectManager sharedManager] getDailyObject];
if I tap the PassManagedObject button I create UIViewControllerB, pass the objectForToday to it and display it, see below
- (IBAction)passManagedObjectTapped:(id)sender {
UIViewControllerB *viewController = [[UIViewControllerB alloc] initWithNibName:#"UIViewControllerB"];
viewController.object = self.objectForToday;
[self.navigationController pushViewController:viewController animated:NO];
}
UIViewControllerB has a property declared like this:
#property (strong, nonatomic) ManagedObject *object;
and a button "Back" which does this:
- (IBAction)backAction:(id)sender {
self.object = nil;
[self.navigationController popViewControllerAnimated:NO];
}
Now the problem is this. If I tap continuously passManagedObjectTapped and then backAction, and again passManagedObjectTapped and again backAction and then again passManagedObjectTapped and automate this I'm getting eventually Received Memory Warning 1, and then crash.
The Instruments doesn't show any leaks but my memory allocation keeps slowly going up.
I am using ARC under iOS4.3 & iOS5. I've been struggling to figuring out what is wrong for a day now. Any help will be highly appreciated.
Thanks!
The self.object = nil; and self.objectForToday = nil; isn't necessary - ARC and the synthesized properties setters already take care of that.
It seems pretty likely that you have a circular reference somewhere. Do you have any situations where object A has a strong reference to object B and object B has a strong reference to object A?
If so, just change one of those references to weak instead (or assign if you want to support iOS 4.3).
Related
I wouldn't normally ask questions like this, but I really cant get my head around it. I have a class 'GetTableInfoFromDatabase', which connects to my database and downloads some JSON. This works great from the first screen of my tab-bar application. I can call the getNewData method as much as I want to effectively refresh the data.
My problem arises when I try and create an instance of the 'GetTableInfoFromDatabase' class and call the very same method from another tab. I get the following error:
*** -[GetTableInfoFromDatabase respondsToSelector:]: message sent to deallocated instance 0x1d89e830
The funny thing is, i'm using ARC. The culprit (in my opinion) is ASIHTTPRequest. I have had to enable -fno-objc-arc to get the project to compile. This library is used in the GetTableInfoFromDatabase class.
Here is the class:
- (void) getEventDataWithSender:(id)sender{
ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:[NSURL URLWithString:#"-------.com/getdata.php"]];
[request setDelegate:self];
NSLog(#"Running!");
[request startAsynchronous];
AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
self.managedObjectContext = appDelegate.managedObjectContext;
}
And this is how i'm calling it:
GetTableInfoFromDatabase *getInfo = [[GetTableInfoFromDatabase alloc]init];
[getInfo getEventDataWithSender:self];
I've even changed the order of the tabs around, so the first tab to be displayed purely just calls this method, nothing else. Not even before the 'GetTableInfoFromDatabase' has been previously initialised by the class that initialised it first last time. Still crashes.
Has anyone got any ideas? This is so frustrating!
You need to assign that variable to a property if you plan on exposing it to other view controllers. ARC will, and should, immediately deallocate getInfo after this code executes:
GetTableInfoFromDatabase *getInfo = [[GetTableInfoFromDatabase alloc]init];
[getInfo getEventDataWithSender:self];
So if this line is included in say viewDidLoad: and nothing else refers to getInfo in that method, it will be immediately released. Why, because you haven't told the compiler that it should retain it.
So in the view controller that's exposing this class, on whatever tab it might be a child of... you would do something like this:
ViewController.h
#class GetTableInfoFromDatabase; // forward declaration
#interface ViewController : UIViewController
#property (strong, nonatomic) GetTableInfoFromDatabase *getInfo;
ViewController.m
#implementation ViewController
#synthesize getInfo = _getInfo;
- (void) viewDidLoad {
[super viewDidLoad];
self.getInfo = [[GetTableInfoFromDatabase alloc]init]; // assign your value to a property
[self.getInfo getEventDataWithSender:self];
}
So when you declare your property as Strong in your header, it will maintain a strong reference to it. #Synthesize getInfo = _getInfo means that it will create a getter and setter for self.getInfo around an instance variable named _getInfo. If you didn't want to expose it as a property, just an instance variable... you could do this:
ViewController.h
#class GetTableInfoFromDatabase; // forward declaration
#interface ViewController : UIViewController {
GetTableInfoFromDatabase _getInfo;
}
ViewController.m
#implementation ViewController
- (void) viewDidLoad {
[super viewDidLoad];
_getInfo = [[GetTableInfoFromDatabase alloc]init]; // assign your value to a property
[_getInfo getEventDataWithSender:self];
}
By default, the compiler will maintain a strong reference to that instance variable unless otherwise specified. You can have weak references as well, and all of those options are pretty well documented. So with ARC, or plain old memory management in general, you need to make an instance variable or property if you want it to hang around for a while.
Honestly... all ARC is doing for you is keeping you from having to call retain and release. Before ARC, setting that property would look like this:
GetTableInfoFromDatabase getInfo = [[GetTableInfoFromDatabase alloc]init];
self.getInfo = getInfo;
[getInfo release];
Now with ARC, the compiler just writes that code for you ;) Hope this helps!
[self.getInfo getEventDataWithSender:self];
Your GetTableInfoFromDatabase object is being deallocated, almost certainly because nothing is holding a strong reference to it. In your code above, getInfo is a local variable, so I would expect it to be released very shortly after this code, unless you are storing it somewhere.
Almost certainly, your dealloc in GetTableInfoFromDatabase does not clear the request's delegate. You should be holding the request in an ivar of GetTableInfoFromDatabase so that it can remove itself as delegate when it is deallocated.
As a side note, avoid prefacing ObjC methods with get. That has a special meaning in KVC (it means that the first parameter is supposed to be updated by reference). Typically the kind of method you have here would be prefaced with "fetch."
I have a property declared in my .h file as
#property (weak, nonatomic) UIPickerView *levelPicker;
which is synthezised in my implementation file as:
#synthesize levelPicker = _levelPicker;
I then have a code block in the same implementation file which does the following:
if (self.levelPicker == nil) {
self.levelPicker = [[UIPickerView alloc] initWithFrame:CGRectZero];
self.levelPicker.delegate = self;
self.levelPicker.dataSource = self;
}
textField.inputView = self.levelPicker;
In this case, self._levelPicker is not set to the new UIPickerView. I.e. the self.levelPicker = blah assignment doesn't work.
However, if I change the property declaration to:
#property (strong, nonatomic) UIPickerView *levelPicker;
then everything works as expected and _levelPicker is set to the newly allocated UIPickerView.
Can someone please tell me why this is the case? I thought I was coming to an understanding of how references work, but I guess I still have more to learn. I read some of the other related SO posts, but it's still not entirely clear to me.
As #Inazfiger says, your objects need at least one strong (retaining) reference, otherwise they won't be retained.
In this case, you're assigning the picker view to a UITextField's inputView property. The text field will retain the picker view (I know this because the inputView property on UITextField is declared with the modifiers "readwrite, retain"), but only once you've made the assignment. So if you want to stick with a weak reference, you need to rearrange your code slightly - something like this:
// Declare a temporary UIPickerView reference. By default, this is
// a strong reference - so tempPicker will be retained until this
// variable goes out of scope.
UIPickerView *tempPicker = [[UIPickerView alloc] initWithFrame:frame];
// Configure the picker
tempPicker.delegate = self;
tempPicker.dataSource = self;
// Assign the picker view to the text field's inputView property. This
// will increase the picker's retain count. Now it'll no longer be
// released when tempPicker goes out of scope.
textField.inputView = tempPicker;
// Finally, assign the same object to self.levelPicker - it won't
// go out of scope as long as it remains assigned to textField's
// inputView property, and textField itself remains retained.
self.levelPicker = tempPicker;
Well, the short answer is that the assignment actually does work.
However, since it is a weak reference, it isn't retained since there is no (other) strong reference to your picker and it is automatically set to nil.
There has to be at least one strong reference to any object otherwise it isn't retained, which in this case there isn't.
For more information, see "ARC Introduces New Lifetime Qualifiers" in Apple's "Transitioning to ARC Release Notes".
Ray Wenderlich created a great tutorial on this here.
The "strong" qualifier creates an owner relationship that stops the object being deallocated, which is equivalent what you would do in the non-ARC world previously:
#property(retain) NSObject *obj;
While "weak" qualifier doesn't create the owner relationship so the object will be deallocated as you would do before:
#property(assign) NSObject *obj;
In your case, you need the first relationship because you need your instance variable (_levelPicker) to hold on to the newly created UIPickerView instance. The weak assignment you did actually work but it was deallocated shortly after.
I've done all sorts of research today on best practices with regards to declaring IBOutlets and instance variables, managing them, using the correct accessors and properly releasing them. I'm pretty much there, but I've got some niche questions that I hope somebody will be able to advise the best practice on. I'll format them as code and comment the questions so as to make it easier to understand. I've excluded some obvious parts that I didn't think were relevant and can be safely assumed to work (like pre-processor stuff, #end, required implementation methods etc).
MyViewController.h
#class OtherViewController;
#interface MyViewController : UIViewController {
NSString *_myString;
BOOL _myBOOL;
}
// The first two properties aren't declared in the interface
// above as per best practices when compiling with LLVM 2.0
#property (nonatomic, retain) OtherViewController *otherViewController;
#property (nonatomic, retain) UIButton *myButton;
#property (nonatomic, copy) NSString *myString;
#property (readwrite) BOOL myBOOL;
MyViewController.m
#implementation MyViewController
// Synthesizing IBOutlets on iOS will cause them to be
// retained when they are created by the nib
#synthesize otherViewController;
#synthesize myButton;
// Assign instance variables so as to force compiler
// warnings when not using self.variable
#synthesize myString = _myString;
#synthesize myBOOL = _myBOOL;
- (void)viewDidLoad {
// QUESTIONS:
// 1. Ignoring convenience methods, can you still alloc and init in dot notation
// even when it's being properly synthesized?
self.myString = [[NSString alloc] initWithString:#"myString"];
self.myString = existingNSStringObject;
// 2. Should you always call methods for IBOutlets and instance variables using dot notation?
// Is there any difference seeing as these aren't directly invoking setters/getters?
[self.myButton setText:self.myString];
[myButton setText:self.myString];
[self.otherViewController.view addSubview:mySubview];
[otherViewController.view addSubview:mySubview];
[self.myButton setAlpha:0.1f];
[myButton setAlpha:0.1f];
self.myButton.alpha = 0.1f;
myButton.alpha = 0.1f;
// 3. How fussy are scalar variables in terms of getters and setters,
// given that there is a #synthesize declaration for them?
self.myBOOL = YES;
myBOOL = NO;
if(self.myBOOL) { ... }
if(myBOOL) { ... }
// 4. On instantiation of new view controllers from NIBs, should you use
// dot notation? (I haven't been doing this previously).
otherViewController = [[OtherViewController alloc] initWithNibName:#"OtherView" bundle:nil];
self.otherViewController = [[OtherViewController alloc] ... ]
}
- (void)viewDidUnload {
// 5. Best practice states that you nil-value retained IBOutlets in viewDidUnload
// Should you also nil-value the other instance variables in here?
self.otherViewController = nil;
self.myButton = nil;
self.myString = nil;
}
- (void)dealloc {
[otherViewController release];
[myButton release];
[_myString release];
}
I always declare and explicitly set a property's underlying instance variable. It's a little more work up front, but in my mind it's worth it to explicitly differentiate variables and properties and see at a glance what instance variables a class has. I also prefix instance variable names, so the compiler complains if I accidentally type property instead of object.property.
Calling alloc / init creates an object with a retain count of 1. Your synthesized property will also retain the object, causing a memory leak when it's released (unless you release your property right after, but that's bad form). Better to alloc / and release the object on a separate line.
Dot notation is effectively the same as calling [self setObject:obj]. Not using dot notation accesses the underlying instance variable directly. In init and dealloc, always access the instance variable directly as the accessor methods can include extra operations (such as key value observing notifications) that are not valid when the object is being created or destroyed. All other times use the synthesized accessor methods. Even if you're not doing anything special now, you might later override these methods later to change what happens when the variable is set.
Scalars work the same way, only you don't have to worry so much about memory.
One accesses the synthesized accessor methods, the other accesses the instance variable directly. See questions one and two again, and be careful about memory leaks!
The view controller may be pushed onto the screen again, in which case your viewDidLoad method will be called a second time. If you're setting initial values in viewDidLoad, go ahead and set your properties to nil here. This makes sense for properties that use a lot of memory and aren't going to affect the state of the view. On the other hand if you want the property to persist until you're sure it's no longer needed, create it in your init method and don't release it until dealloc.
1) You've slightly misunderstood #synthesize. #synthesize does nothing with the object. It only tells the compiler to generate the getter and setter methods according to the options used in your #property declaration
// Synthesizing IBOutlets on iOS will
cause them to be
// retained when they
are created by the nib
The outlets aren't retained (outlets are just notices to interface builder and don't affect the code), the objects are retained when the setter generated by #synthesize is used. When the nib is loaded, the loading system calls your generated setter.
2) Deciding whether to use accessors in objective C is no different from deciding to use accessors in any other object oriented language. It is a choice of style, need and robustness. That the accessor is serving as an IBOutlet makes no difference.
But in objective C I would suggest you should NOT use accessors in two places: dealloc and within the var's accessor method itself.
And if you ARE using the accessors in init then you need to be careful about your retain counts.
self.myString = [[NSString alloc] initWithString:#"myString"];
This line leaks memory. Using your copy accessor retains the object, so you should release it here after creating it.
3) Not sure what you mean by fussy. Possibly see answer to 2)
4) See 2) and be careful about memory management. If you call alloc/init you are now responsible for releasing the object - this is entirely independent of the retains/releases used by accessors and dealloc.
5) No, you should not nil other instance variables in viewDidUnload. Your controller is expected to maintain its state even if the view goes away. viewDidUnload is only for cleaning up potentially memory-heavy view objects when the controller's view is not currently on screen.
Consider a navigation controller. View controller 1 is on the stack and then view controller 2 is pushed and is now visible. If memory conditions get low, the system could attempt to unload view controller 1's view and will then call viewDidUnload.
Then popping view controller 2 will not create the view controller 1 object again, but it WILL load view controller 1's view and call viewDidLoad.
Re comments
2) That's exactly right - you can use a convenience constructor or release immediately after your alloc/init and assignment, or release before the block exits, or autorelease. Which you choose is mostly a matter of style (though some would argue against autorelease - but not me!)
3) There are accessors for scalars - you have created some in your code
#property (readwrite) BOOL myBOOL;
This creates methods myBOOL and setMyBOOL on your class.
Remember that there is nothing special about dot notation. It is only a convenience and when the code is compiled myObject.property is exactly equivalent to [myObject property] and myObject.property = x is exactly equivalent to [myObject setProperty:x]. Using dot notation is purely a style choice.
Dot notation and brackets notation are pretty much the same.
By self.myVariable you are accessing the getter of the property of the instance variable myVariable and by myVariable you are accessing the local variable. They're not the same thing.
You can customize the setters and the getters by overriding the methods and specific some certain conditions for them.
See first answer ( brackets are preferred - better understanding of the code )
Better make a separate method.
Like:
- (void) releaseOutlets {
self.firstOutlet = nil;
self.mySecondOutlet = nil;
……………………
self.myLastOutlet = nil;
}
and then call this method both in viewDidUnload and in dealloc methods.
Hope it helps !
Early warning - code sample a little long...
I have a singleton NSMutableArray that can be accessed from anywhere within my application. I want to be able to reference the NSMutableArray from multiple NIB files but bind to UI elements via NSArrayController objects. Initial creation is not a problem. I can reference the singleton NSMutableArray when the NIB gets loaded and everything appears fine.
However, changing the NSMutableArray by adding or removing objects does not kick off KVO to update the NSArrayController instances. I realize that "changing behind the controller's back" is considered a no-go part of Cocoa-land, but I don't see any other way of programmatically updating the NSMutableArray and letting every NSArrayController be notified (except it doesn't work of course...).
I have simplified classes below to explain.
Simplified singleton class header:
#interface MyGlobals : NSObject {
NSMutableArray * globalArray;
}
#property (nonatomic, retain) NSMutableArray * globalArray;
Simplified singleton method:
static MyGlobals *sharedMyGlobals = nil;
#implementation MyGlobals
#synthesize globalArray;
+(MyGlobals*)sharedDataManager {
#synchronized(self) {
if (sharedMyGlobals == nil)
[[[self alloc] init] autorelease];
}
return sharedMyGlobals;
}
-(id) init {
if(self = [super init]) {
self.globals = [[NSMutableArray alloc] init];
}
return self
}
// ---- allocWithZone, copyWithZone etc clipped from example ----
In this simplified example the header and model for objects in the array:
Header file:
#interface MyModel : NSObject {
NSInteger myId;
NSString * myName;
}
#property (readwrite) NSInteger myId;
#property (readwrite, copy) NSString * myName;
-(id)initWithObjectId:(NSInteger)newId objectName:(NSString *)newName;
#end
Method file:
#implementation MyModel
#synthesize myId;
#synthesize myName;
-(id)init {
[super init];
myName = #"New Object Name";
myId = 0;
return self;
}
#end
Now imagine two NIB files with appropriate NSArrayController instances. We'll call them myArrayControllerInNibOne and myArrayControllerInNib2. Each array controller in the init of the NIB controller sets the content of the array:
// In NIB one init
[myArrayControllerInNibOne setContent: [[MyGlobals sharedMyGlobals].globalArray];
// In NIB two init
[myArrayControllerInNibTwo setContent: [[MyGlobals sharedMyGlobals].globalArray];
When each NIB initializes the NSArrayController binds correctly to the shared array and I can see the array content in the UI as you would expect. I have a separate background thread that updates the global array when content changes based on an external event. When objects need to be added in this background thread, I simply add them to the array as follows:
[[[MyGlobals sharedMyGlobals].globalArray] addObject:theNewObject];
This is where things fall apart. I can't call a willChangeValueForKey and didChangeValueForKey on the global array because the shared instance doesn't have a key value (should I be adding this in the singleton class?)
I could fire off an NSNotification and catch that in the NIB controller and either do a [myArrayControllerInNibOne rearrangeObjects]; or set the content to nil and reassign the content to the array - but both of these seems like hacks and. moreover, setting the NSArrayController to nil and then back to the global array causes a visual flash within the UI as the content is cleared and re-populated.
I know I could add directly to the NSArrayController and the array gets updated, but I don't see a) how the other NSArrayController instances would be updated and b) I don't want to tie my background thread class explicitly to a NIB instance (nor should I have to).
I think the correct approach is to either fire off the KVO notification somehow around the addObject in the background thread, or add something to the object that is being stored in the global array. But I'm at a loss.
As a point of note I am NOT using Core Data.
Any help or assistance would be very much appreciated.
Early warning - answer a little long…
Use objects that model your domain. You have no need for singletons or globals, you need a regular instance of a regular class. What Objects are your storing in your global array? Create a class that represents that part of your model.
If you use an NSMutableArray as storage it should be internal to your class and not visible to outside objects. eg if you are modelling a zoo, don't do
[[[MyGlobals sharedMyGlobals].globalArray] addObject:tomTheZebra];
do do
[doc addAnimal:tomTheZebra];
Dont try to observe a mutable array - you want to observe a to-many property of your object. eg. instead of
[[[MyGlobals sharedMyGlobals].globalArray] addObserver:_controller]
you want
[doc addObserver:_controller forKeyPath:#"animals" options:0 context:nil];
where doc is kvo compliant for the to-many property 'anaimals'.
To make doc kvo compliant you would need to implement these methods (Note - you don't need all these. Some are optional but better for performance)
- (NSArray *)animals;
- (NSUInteger)countOfAnimals;
- (id)objectInAnimalsAtIndex:(NSUInteger)i;
- (id)AnimalsAtIndexes:(NSIndexSet *)ix;
- (void)insertObject:(id)val inAnimalsAtIndex:(NSUInteger)i;
- (void)insertAnimals:atIndexes:(NSIndexSet *)ix;
- (void)removeObjectFromAnimalsAtIndex:(NSUInteger)i;
- (void)removeAnimalsAtIndexes:(NSIndexSet *)ix;
- (void)replaceObjectInAnimalsAtIndex:(NSUInteger)i withObject:(id)val;
- (void)replaceAnimalsAtIndexes:(NSIndexSet *)ix withAnimals:(NSArray *)vals;
Ok, that looks pretty scary but it's not that bad, like i said you don't need them all. See here. These methods dont need to be part of the interface to your model, you could just add:-
- (void)addAnimal:(id)val;
- (void)removeAnimal:(id)val;
and write them in terms of the kvc accessors. The key point is it's not the array that sends notifications when it is changed, the array is just the storage behind the scenes, it is your model class that send the notifications that objects have been added or removed.
You may need to restructure your app. You may need to forget about NSArrayController altogether.
Aaaaaannnnnyyywaaayyy… all this gets you nothing if you do this
[[[MyGlobals sharedMyGlobals].globalArray] addObject:theNewObject];
or this
[doc addAnimal:tomTheZebra];
from a background thread. You can't do this. NSMutableArray isn't thread safe. If it seems to work then the best that will happen is that the kvo/binding notification is delivered on the background as well, meaning that you will try to update your GUI on the background, which you absolutely cannot do. Making the array static does not help in any way i'm afraid - you must come up with a strategy for this.. the simplest way is performSelectorOnMainThread but beyond that is another question entirely. Threading is hard.
And about that static array - just stop using static, you don't need it. Not because you have 2 nibs, 2 windows or anything. You have an instance that represents your model and pass a pointer to that to you viewControllers, windowControllers, whatever. Not having singletons/static variables helps enormously with testing, which of course you should be doing.
First of all, i have never seen so many memory issues in my app since i started placing "self" everywhere after reading an article about how memory behaves in obj-C. Now, im getting all kinds of issues (reveals the sloppiness of my coding). Granted I am a newbie at Objective-C, i'll admit i have never had so much issues with memory management before in my life. But i reckon it takes practice to get used to this.
Now, on to my question.
I have a class interface property (self.todoCreate) that holds a reference to the above controller. This controller is navigated to by pressing a button.
#property (nonatomic, retain) TodoTaskCreateController *todoCreate;
The code below are the snippets that cause the navigation view change:
TodoTaskCreateController *viewController = [[TodoTaskCreateController alloc]
initWithNibName:#"TodoTaskCreateController"
bundle:[NSBundle mainBundle]];
self.todoCreate = viewController;
[viewController release];
// slide-in todoCreate controller.
if (self.navigationController != nil && self.todoCreate != nil) {
[self.navigationController pushViewController:self.todoCreate animated:YES];
}
So here is my problem:
The first time i run this it works.
Once I'm on the second view screen, I navigate back to the main view.
And if i try to navigate again a 2nd time, then the app crashes, right where self.todoCreate is being assigned viewController.
Note that within the main view's viewDidAppear method, I call [self.todoCreate release].
Can anyone explain this?
PS - No wonder so many iPhone apps randomly crash.
todoCreate is a property, which means when you assign a value to it, it invokes a method called setTodoCreate which looks something like:
- (void) setTodoCreate:(Foo*) newVal
{
[todoCreate release]; // release the previous object
todoCreate = [newVal retain]; // point to new object, and also retain it
}
Now your viewDidAppear method is releasing self.todoCreate at which point the retain count of todoCreate is 0. When you create a new TodoTaskCreateController and assign it to self.todoCreate another release is performed, but this time on an object with retain count of 0.
Instead of [self.todoCreate release], you should be using self.todoCreate = nil.
You probably shouldn't be continually destroying and creating your TodoTaskCreateController either.
Your comment regarding application crashing is most likely due to developers not testing their application to see if it handls memory warnings properly. In the simulator there is a menu option to simulate this, and your application should "survive" the warning in all of its views.
I'm not going to speak to soon, but it APPEARS that i have resolved the crash by simply adding the viewController to the autorelease pool, and then removing all manual occurances of its release.
Now does it matter if my #property for createTodo is defined as (nonatomic, retain) as opposed to (nonatomic, assign)?