I am new to iphone and I want to know when does -(void) dealloc method call. I know it is used to release objects but can we release any kind of object using dealloc method? And is it necessry to add this method and why we write [suped dealloc] in dealooc method?
share your views.
Here is the explanation straight from APPLE's Docs
The NSObject class defines a method, dealloc, that is invoked
automatically when an object has no owners and its memory is
reclaimed—in Cocoa terminology it is “freed” or “deallocated.”.
Important
Never invoke another object’s dealloc method directly.
Here is the answer why we call [super dealloc]?
You must invoke the superclass’s implementation at the end of your
implementation. You should not tie management of system resources to
object lifetimes
When an application terminates, objects may not be sent a dealloc
message. Because the process’s memory is automatically cleared on
exit, it is more efficient simply to allow the operating system to
clean up resources than to invoke all the memory management methods.
You Can release object those which is allocted, copy, or retain by you this is simple to understand. You can not release object those are not allocating by you means autorelease object, which ios detect for no more use in memory ,that time autorelease pool drain that time those object released from memory automatically.
And object dealloc method called when object retain count becomes zero.
You never send a dealloc message directly. Instead, an object’s dealloc method is invoked indirectly through the release NSObject protocol method (if the release message results in the receiver's retain count becoming 0). See Memory Management Programming Guide for more details on the use of these methods.
Subclasses must implement their own versions of dealloc to allow the release of any additional memory consumed by the object—such as dynamically allocated storage for data or object instance variables owned by the deallocated object. After performing the class-specific deallocation, the subclass method should incorporate superclass versions of dealloc through a message to super:
Refer this Link
Related
According to the rules of memory management in a non garbage collected world, one is not supposed to retain a the calling object in a delegate. Scenario goes like this:
I have a class that inherits from UITableViewController and contains a search bar. I run expensive search operations in a secondary thread. This is all done with an NSOperationQueue and subclasses NSOperation instances. I pass the controller as a delegate that adheres to a callback protocol into the NSOperation.
There are edge cases when the application crashes because once an item is selected from the UITableViewController, I dismiss it and thus its retain count goes to 0 and dealloc gets invoked on it. The delegate didn't get to send its message in time as the results are being passed at about the same time the dealloc happens.
Should I design this differently? Should I call retain on my controller from the delegate to ensure it exists until the NSOperation itself is dealloc'd? Will this cause a memory leak? Right now if I put a retain on the controller, the crashes goes away. I don't want to leak memory though and need to understand if there are cases where retaining the delegate makes sense.
Just to recap.
UITableViewController creates an NSOperationQueue and NSOperation that gets embedded into the queue. The UITableViewController passes itself as a delegate to NSOperation. NSOperation calls a method on UITableViewController when it's ready. If I retain the UITableViewController, I guarantee it's there, but I'm not sure if I'm leaking memory. If I only use an assign property, edge cases occur where the UITableViewController gets dealloc'd and objc_msgSend() gets called on an object that doesn't exist in memory and a crash is imminent.
In the general case, a delegate owns the objects it has set itself as the delegate to, or at least retains references to them directly or indirectly. In the dealloc method, a delegate should either release all objects that it is the delegate of in such a way that prevents future callbacks, like NSTimer invalidate, or clear the delegate member of those objects that may persist.
While it is only convention that prevents retaining a delegate, it is convention based on good design. In your case, wouldn't the results be discarded anyway since the delegate is being disposed?
You can make the delegate property of your NSOperation atomic by not setting the nonatomic flag and synthesizing the getter and setter. Or you can use performSelectorOnMainThread before using the delegate member.
To recap, there is usually a better solution than retaining the delegate.
I really wonder though if the "don't retain your delegate" rules still apply specifically to multi-threaded object/delegate relationships, especially the one you're writing about. I found my way to this question because I'm in exactly the same situation: I'm running a finite but unpredictable length asynchronous network operation (it will finish and self-terminate "sometime soon") in an NSOperation, and using a delegate stored in the NSOperation to notify completion of the operation back to the original requestor object.
Retaining the delegate in this case makes perfect sense to me. The operation will complete eventually, it will call the "done" method on the delegate, and then will release the delegate and self-terminate. If the calling object would have been dealloc'd while the NSOperation was running, it just sticks around a little longer until the operation completes.
Note that marking the delegate property atomic as drawnonward suggests is in itself not sufficient to protect against race conditions! Setting a property as atomic only means that a reader or writer of the property will write or read an entire whole value at a time, nothing else. The following sequence would result in a crash:
(NSOperation thread): NSOperation has finished and is preparing to notify the delegate object . The code atomically reads the value of the delegate property preparing to call its "done" method.
----> Thread switch to main
(main thread) the requesting object is dealloc'd, and in -(void)dealloc (atomically!) sets the NSOperation's delegate property to nil, dealloc finishes, and the object is now gone
-----> Thread switch to NSOperation thread
(NSOperation thread) calls [delegate allDone] and the program crashes (if you're lucky) because the delegate object is gone. If you're lucky. Maybe something else was allocated in that space in the meantime and now you have unpredictable corruption, what fun!
The key thing here is that the retain cycle is temporary by its very nature -- the NSOperation will complete and clean up all on it own. It's not like a UITextField and a UIViewController holding retained references to each other that will never go away and thus leak the memory.
It seems to me that retaining the delegate in the case of an asynchronous, limited, self-terminating operation is the cleanest and most robust implementation.
EDIT:
if you absolutely must not retain the delegate because it causes memory problems, then instead the delegate object and the operation object must both use an explicit locking mechanism to synchronize access to the delegate pointer stored in the operation. "atomic" properties do not provide a thread-safe protocol to marking the delegate pointer nil.
But I think this gets really complicated and full of race conditions. I think at the very least that you must in the delegate object's dealloc, run a locking protocol to make sure that the operation's delegate pointer is safely set to nil so that arbitrary thread interleaving cannot under any circumstances call to a dealloc'd delegate object.
Slavish adherence to rules sometimes makes things way more complicated than they need to be. It's best to understand what the rules are, why they are there, and so you know when it makes sense (like I believe it does in this very particular scenario) not to follow them, and what the advantages/disadvantages are of doing so.
I approach this a little differently, and I do not retain delegates.
If I need to lose the view, I cancel the NSOperation. I believe that its good design that if a thread's return value has nowhere to go, then the thread should stop.
I also have seen the edge case where the thread cannot be stopped. In this case, I nil the unretained delegate, just to be sure, and check that the delegate is not nil before making the callback.
Holding on to an object, even temporarily, when its no longer needed, especially on the iPhone chews up memory and, in my opinion, is bad design.
As per #Bogatyr's answer, a simple check for nil before calling [delegate allDone] is much cleaner.
You could retain the UITableViewController at the beginning of the NSOperation and release it at its end.
Alternatively, you could set it to nil after it is released so the dangling call from the NSOperation won't crash your program.
Depending on how the NSOperation is executed you could also autorelease the UITableViewController instead of releasing it, so the NSOperation can still use it.
However, all these things really only cure the symptoms, not the illness. The correct way to do it is outlined by drawnonward.
I found that the iphone have viewDidUnload, and dealloc. I want to release the object. Which method should I use to release the object? What's the different between them?
Send release or autorelease to release an object. You shouldn't send dealloc; the Obj-C runtime will do that.
If you're asking where you should release an owned object, read: "When should I release objects in -(void)viewDidUnload rather than in -dealloc?"
Do not call dealloc. Use the retain-release model for memory management, and Objective-C will take care of deallocating memory for you.
See this link for a good explanation of how retain-release works.
The difference is that viewDidUnload is used to release "spare" objects in low memory situations while dealloc is used to release all objects when the view is no longer needed.
This means that you will almost always have a dealloc method but have a viewDidUnload method only where it makes sense.
When an application comes back from low memory conditions (ie there was low memory, things were freed and the app is now back to normal use scenario), what happens to the state of objects that were initialized and set up via
-(id)init
method?
When you receive low memory warnings, you persist all of the data and the viewDidUnload method is invoked. Eventually, the view maybe reloaded but the class's 'init' method is not called a second time.
So would you persist any state information you have initialized in the 'init' method and later manipulated during the course of the use of the application?
What I'm asking more specifically is whether classes & other related data created during the 'init' method would be reinstated when coming back from a low-memory condition.
If an object is dealloc'ed you got a memory alert, then you have to do the init again. It will not be done for you.
The system will typically not dealloc your objects unless you do so by releasing. You could do nothing in response to a memory alert or you could release some views and with them, some of their owned objects.
viewDidUnload is a way for you to know whether your view was unloaded in which case you should go through and free and cleanup the things that you did in viewDidLoad. When your view comes back up, viewDidLoad will get called again and you get a chance to redo all the initialization.
viewDidUnload is probably what you really need study:
viewDidUnload Called when the
controller’s view is released from
memory.
(void)viewDidUnload
Discussion This method is called as a
counterpart to the viewDidLoad method.
It is called during low-memory
conditions when the view controller
needs to release its view and any
objects associated with that view to
free up memory. Because view
controllers often store references to
views and other view-related objects,
you should use this method to
relinquish ownership in those objects
so that the memory for them can be
reclaimed. You should do this only for
objects that you can easily recreate
later, either in your viewDidLoad
method or from other parts of your
application. You should not use this
method to release user data or any
other information that cannot be
easily recreated.
Typically, a view controller stores
references to objects using an outlet,
which is a variable or property that
includes the IBOutlet keyword and is
configured using Interface Builder. A
view controller may also store
pointers to objects that it creates
programmatically, such as in the
viewDidLoad method. The preferred way
to relinquish ownership of any object
(including those in outlets) is to use
the corresponding accessor method to
set the value of the object to nil.
However, if you do not have an
accessor method for a given object,
you may have to release the object
explicitly. For more information about
memory management practices, see
Memory Management Programming Guide
for Cocoa.
By the time this method is called, the
view property is nil.
Special Considerations If your view
controller stores references to views
and other custom objects, it is also
responsible for relinquishing
ownership of those objects safely in
its dealloc method. If you implement
this method but are building your
application for iPhone OS 2.x, your
dealloc method should release each
object but should also set the
reference to that object to nil before
calling super.
I have an instance variable *TangramLevel(:UIView) currentLevel; in my viewController class, and I have an instance allocated at start (it's retainCount 1). I attached it to the view [self.view addSubview:currentLevel]; (retainCount 2).
When the level finishes, it notifies the controller, so controller removes it from the view [currentLevel removeFromSuperview]; (retainCount 1), and release the allocated instance - [currentLevel release]; -, to have it deallocated (retainCount 0 = invoke dealloc).
Then on the next line, controller wants to allocate/addSubview a new level instance with another level data, but the application crashes (EXEC BAD ACCESS).
If I don't release currentLevel after removeFromSuperview, the appliaction works, but I have an unused level instance left in memory then, which is also a problem (the main problem itself).
Is there any bug in the method I wrote above? Or the bug is elsewhere, maybe in the level class? I allocated some UIImageView in the level, but I release every allocated object in the levels dealloc method. Any ideas?
Post your code.
This is definitely a memory management issue. The question is "where is the problem created?" To answer that, we need to examine the following:
Your "currentLevel" iVar handling code (do you used synthesized properties, etc.). Post it.
How are you assigning the view to currentLevel?
Where are you releasing this, specifically?
How is your view's dealloc implemented (what do you release and how)?
Is there any other code that retains/releases this view or anything related to it?
The fact that you're calling release in your "I'm done with this level, let's swap in the next" code suggests an overall design issue. Make the memory management of each of a class's iVars the responsibility of its accessors and ONLY use the accessors to interact with it (even from within the class/instance). With synthesized properties, this makes it brain-dead-simple. That way you don't have to worry about where to retain/release iVars because it's always funneled through the accessors.
I am wondering what the difference is between release and dealloc? After reading, the memory management rules (See below)., I am thinking most of the time I will be using release. However, I wanted to know what to do for properties.
#property(retain)....
I have been using dealloc but after reading this article I am not sure that is correct.
You take ownership of an object if you create it using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message. You are responsible for relinquishing ownership of objects you own using release or autorelease. Any other time you receive an object, you must not release it.
You should never call dealloc on anything other than super.
The only time you will be calling dealloc is in the dealloc method of a custom inherited object, and it will be [super dealloc].
When the retain count of an object goes down to zero, dealloc will automatically be called for you, so in order to manage memory properly, you need to call retain and release when appropriate.
If this isn't clear to you or you'd like details about how memory is managed in Cocoa, you should read the Memory Management Programing Guide.
You never call dealloc directly. It is invoked by the system when the retainCount of the object goes to 0. Every time you do a retain, the retainCount is incremented with 1. Every time you do a release, it gets decremented. This way, by balancing your retains and releases, you ensure than when the retainCount gets to 0, dealloc will be automatically called, and your object freed.
As Ben S noted, the only time and place you would call dealloc is in in the dealloc method of an inherited object.
When you use #property(retain) and then #synthesize to generate the property code, you do not need to do any manual memory management on the property. The other answers are correct in that you shouldn't be using dealloc except in your own override of a parent class dealloc.