Which of the following code section is correct?
Definition of correct for "me":
It should not have a retain cycle and so should not leak.
It must guarantee the running of method2 and method3. So MyObject variable in block must never ever be nil.(which may occur in __weak definitions...) (I do not want to check if it is nil to prevent crash. I always want it to be non-nil)
Update: (Additional Info)
Instruments tool did show strange leaks until I replace __block with __weak. However, after that I remembered that __weak references may disappear anytime. I have to be sure that it doesn't disappear and leak too. I don't have a timer. This someMethod is called on main thread when it observes a specific NSNotification.
#implementation MyObject...
-(void)someMethod{
AnotherObject *abc=[[AnotherObject alloc]init];
__weak MyObject *weakSelf=self;
abc.onSuccess=^{
__strong MyObject * strongSelf = weakSelf;
[strongSelf method2];
[strongSelf method3];
}
}
OR
#implementation MyObject...
-(void)someMethod{
AnotherObject *abc=[[AnotherObject alloc]init];
__block MyObject *blockSelf=self;
abc.onSuccess=^{
[blockSelf method2];
[blockSelf method3];
blockSelf=nil;
}
}
Update 2: Actual Code which always leaks if I don't use __weak:
__block RButton *_self=self;
_aimageView.onSuccess=^(void){
[_self.headerLabel setText:[_self.book title]];
_self = nil;
};
A block can mention self without causing any leak. So the first thing to do is specify the circumstances. Why do you think your block leaks? Have you checked? There's no point worrying if there is no reason to worry.
If the block's mentioning self does cause a leak, then the steps you can take depend upon the reason why that is happening. But you haven't shown us that. For example, a dispatch timer block that mentions self might cause a retain cycle, but when you are done with the timer you can break the cycle, so then there's no leak.
Thus it is better to understand a particular retain cycle than to program every block defensively when you don't have to.
EDIT (in response to your comment) This code from my book demonstrates various ways of handling memory management in an NSNotification observer situation: click here
I have looked at your actual code in "Update 2", but it's missing a lot of information. For example, is _aimageView an instance variable? local variable? or what? And what causes this onSuccess to be called?
If _aimageView is a local variable then I don't see evidence of a retain cycle.
My first thought is I don't understand why you want to guarantee that the code inside the body of the block runs in this example. It looks like the code inside is just updating some UI elements. Well, if the UI is no longer displaying (as when the current object has no references to it other than perhaps by this block), what's the point of updating the UI elements?
It's important to know what causes onSuccess to be called because different types of callbacks require different memory management architectures. For example, if the block is fired in response to a touch or some kind of event like that, then chances are that the object pointed to by self (which is probably some kind of view or view controller) must be still alive in order that the event occurs. If that's the case, then __weak will do what you want.
Basically, by the naming of these variables, it is reasonable to conclude that _aimageView is probably an image view that is conceptually "owned" by the current object, and the onSuccess is a completion block that is "owned" by the image view. Unless some other object has a strong reference to it, the "owned" object's lifetime is limited to its "owning" object's lifetime. Thus the block will not outlive the image view, which will not outlive the current object.
The only way that what you are afraid of (when the block runs, the object pointed to by self is deallocated) can happen, is if some other object stores a strong reference to _aimageView, or to the block. For the block, it's very unlikely that a "success block" of one object will be stored with other objects. For the image view, it's similarly unlikely if the image view "belongs" to the current object. (It's conceivable that it may be stored by the autorelease pool or something; but the autorelease pool will not call stuff on it except release, so that's not a problem.)
The only exception I can think of is if the image view is kept by a pending network operation or something, which when it is done calls the onSucess block. But if that were the case, I would say it's bad design to have a single object serve as both a view and a network operation. Rather, in that case, one should have a dedicated network operation object, which is a local variable, and set a completion block on it (e.g. store image in image view, set labels, etc.), start the operation, and not need to store the operation. Then the block can refer to self strongly, but there is no retain cycle.
To summarize, the block (and the image view that owns it) should fall into one of two categories, based on who keeps it alive (i.e. who retains it, who maintains a strong reference to it):
If self keeps it alive: e.g. A view controller keeps its views and subviews alive. In this case, it is safe to use __weak because the block does not exist outside of the life of self.
If someone else keeps it alive: e.g. an alert view -- you just create it and show it; the system maintains it on screen afterwards. In this case, self should not (and does not need to) have a reference to it, and it's okay to reference self strongly in the block; it won't cause a retain cycle.
Try to avoid situations where both self and someone else keep it alive.
As you require to guarantee that method2 and method3 are executed then you have a requirement that the object referenced by self stays alive. To do this you follow the standard pattern: keep a strong reference for as long as you need it. Your second code fragment does that.
Reference cycles are not inherently bad, it is unmanaged ones that are. Here you make a cycle and then break it, so there is no issue.
(Note as you can send a message to nil the first code fragment will not crash if strongSelf is nil - it just won't do anything.)
-(void)someMethod{
AnotherObject *abc=[[AnotherObject alloc]init];
abc.onSuccess=^{
[self method2];
[self method3];
}
}
This will make. It won't cause a retain cycle.
Related
Using self. in blocks causes retain cycles, so I need to create a reference to weakSelf. I understand this
BUT!
If from my block I call a method which uses self", does this too cause a retain cycle? For instance, if I reload a UITableView from a block and in some of my UITableView delegate methods I call self., am I causing a retain cycle? Does that mean I have to pass around this weak reference everywhere? Seems hokey.
I might be misreading your question, but your wording:
If from my block I call a method which uses "self.", does this too cause a retain cycle? For instance if I reload a UITableView from a block and in some of my UITableView delegates I call "self.", I'm causing a retain cycle? That means I have to pass around this weakReference everywhere?
suggests you are misunderstanding what self is. Hopefully if so the following will help and not hinder your understanding...
What is self?
The identifier self is just the name of one of the parameters to your method, it is just passed implicitly rather then explicitly like the other parameters. For example if you have a class:
#implementation MyClass
- (void) someMethod:(NSInteger)value
{
... self ... value
}
#end
then the method is effectively (i.e. bending the facts just a little for clarity):
- (void) someMethod:(NSInteger)value withObject:(MyClass *)self
{
... self ... value
}
When an instance method is called the value passed for the self parameter is a reference to the instance the method should operate on, e.g. the call
MyClass *someInstance = ...
[someInstance someMethod:42];
is effectively a call to:
someMethod:42 withObject:someInstance
Strong reference cycles
An object - which includes both instances of classes and blocks - is kept alive as long as there exists a strong reference to the object.
If an object A holds a strong references, e.g. in an instance variable or property, to an object B, then B will be kept alive at least (there could be other strong references to B) as long as A is alive.
If an object A holds a strong reference to B and B holds one to A then you have a strong reference cycle - each object is keeping the other alive and neither will ever be collected. This could lead to a memory leak - where unused memory is never collected - unless both A and B are meant to live from creation till the end of the program.
Further, you do not create a strong reference cycle simply by having references stored in the local variables and parameters of methods. By their nature such variables, and therefore their contents, are transient and are destroyed when the method returns.
Using self in blocks
using "self." in blocks causes retain cycles so I need to create a reference to weakSelf.
Not quite. When you use self in a block, either directly or indirectly by referencing an instance variable, then the compiler will warn you that you may create a reference cycle.
(Note: There are other ways to create reference cycles, both with and without using blocks, and the compiler will not warn you at all. Managing cycles is just something you need to be aware of.)
You will only actually create a cycle if you store a reference to the block in the object referenced by self. However this is not bad in itself, as long as at some point you break the cycle manually - say by storing nil in the variable referencing the block - the cycle need not be problematic at all.
Finally...
You have nothing per se to worry about with your:
UITableView delegates I call "self."
as that self is just a local parameter to the delegate whose initial value, at some point going back up the call chain, came from you evaluating your weakSelf reference and determining that it was not nil and then calling methods on it.
HTH
First of all: self does NOT cause a retain cycle. This is an urban legend. The incorrectness is obvious: A single reference can never cause a cycle. The usage of self inside a block causes a retain cycle, if the block is directly or indirectly referred by self, too, for example via a property.
To your Q:
If you "call" a method inside the block, the message probably has the receiver self, so you have a usage of self inside the block. For this reason it is captured and retained.
If you really have no usage of self inside the block by neither using self nor using a property of self you do not have a usage of self and it is not captured, therefore not retained. But in this case you can have a dangling pointer or a nil'd reference.
You don't need to worry about references while the block is executing - eventually it finishes doing whatever it does, and all these references go away.
What you need to worry about are the references that are captured when the block is created. These references stay until the block goes away. So if your block has a reference to "self", that reference is there just because the block exists. And if you store that block in a property of self, you have a cycle.
So if you store a block as a property in self, then the block shouldn't capture self. That's easily done by letting it access and capture a weak copy of self. Remember that when the block is executing, the weak copy of self may be nil. Which means the self object has already left our world, and your block might not need to do anything.
Short answer: no, in this situation self is not retained.
Long answer
First of all, retaining self and a reference cycle are not the same thing. Reference cycle is a cycle of strong references between a number of objects: A->B->C->A is a retain cycle.
The general idea is, you want to always guarantee that if you are referencing self in a block, you don't reference this block strongly, and don't reference it through a chain of strong references. In reality, retain cycles can be used purposefully if you are making sure you're breaking the retain cycle under certain conditions. Not that I personally recommend this.
Take a look at documentation on Apple's website. It clearly states that values are captured in blocks, and capturing an object reference retains this object in block.
Basically what this means is that referencing an object in a block increments its retainCount by 1, and when this block gets deallocated, retainCount is decremented by 1.
However, when using a __weak pointer in a block, the retain count is untouched.
Here's an example:
- (void) doSomething {
NSLog(#"%#", self);
}
- (void) callBlock {
__weak typeof(self) weakSelf = self;
dispatch_block_t block = ^{
[weakSelf doSomething];
};
}
When you write [obj method:params] this actually translates into following call:
objc_msgSend(obj, #selector(method:), params). One of the features of Objective-C is that if you call a method on nil pointer, it returns nil. This is guaranteed by the fact that objc_msgSend(nil, #selector(anyselector), ...) always return nil. Note that SEL is just a const char[] under the covers, so it doesn't affect retain counts by any means.
Hence when the block will be executed, if your object was deallocated, the weak weakSelf variable will be nullified, and the block's body will translate into objc_msgSending to zero, which does nothing except of wasting few CPU cycles.
To sum it up, the Objective-C messaging system is implemented in such a way that calling a method does not retain this object or this method or this method's implementation, as it's a simple function call.
Using self. in blocks causes retain cycles, so I need to create a reference to weakSelf. I understand this
BUT!
If from my block I call a method which uses self", does this too cause a retain cycle? For instance, if I reload a UITableView from a block and in some of my UITableView delegate methods I call self., am I causing a retain cycle? Does that mean I have to pass around this weak reference everywhere? Seems hokey.
I might be misreading your question, but your wording:
If from my block I call a method which uses "self.", does this too cause a retain cycle? For instance if I reload a UITableView from a block and in some of my UITableView delegates I call "self.", I'm causing a retain cycle? That means I have to pass around this weakReference everywhere?
suggests you are misunderstanding what self is. Hopefully if so the following will help and not hinder your understanding...
What is self?
The identifier self is just the name of one of the parameters to your method, it is just passed implicitly rather then explicitly like the other parameters. For example if you have a class:
#implementation MyClass
- (void) someMethod:(NSInteger)value
{
... self ... value
}
#end
then the method is effectively (i.e. bending the facts just a little for clarity):
- (void) someMethod:(NSInteger)value withObject:(MyClass *)self
{
... self ... value
}
When an instance method is called the value passed for the self parameter is a reference to the instance the method should operate on, e.g. the call
MyClass *someInstance = ...
[someInstance someMethod:42];
is effectively a call to:
someMethod:42 withObject:someInstance
Strong reference cycles
An object - which includes both instances of classes and blocks - is kept alive as long as there exists a strong reference to the object.
If an object A holds a strong references, e.g. in an instance variable or property, to an object B, then B will be kept alive at least (there could be other strong references to B) as long as A is alive.
If an object A holds a strong reference to B and B holds one to A then you have a strong reference cycle - each object is keeping the other alive and neither will ever be collected. This could lead to a memory leak - where unused memory is never collected - unless both A and B are meant to live from creation till the end of the program.
Further, you do not create a strong reference cycle simply by having references stored in the local variables and parameters of methods. By their nature such variables, and therefore their contents, are transient and are destroyed when the method returns.
Using self in blocks
using "self." in blocks causes retain cycles so I need to create a reference to weakSelf.
Not quite. When you use self in a block, either directly or indirectly by referencing an instance variable, then the compiler will warn you that you may create a reference cycle.
(Note: There are other ways to create reference cycles, both with and without using blocks, and the compiler will not warn you at all. Managing cycles is just something you need to be aware of.)
You will only actually create a cycle if you store a reference to the block in the object referenced by self. However this is not bad in itself, as long as at some point you break the cycle manually - say by storing nil in the variable referencing the block - the cycle need not be problematic at all.
Finally...
You have nothing per se to worry about with your:
UITableView delegates I call "self."
as that self is just a local parameter to the delegate whose initial value, at some point going back up the call chain, came from you evaluating your weakSelf reference and determining that it was not nil and then calling methods on it.
HTH
First of all: self does NOT cause a retain cycle. This is an urban legend. The incorrectness is obvious: A single reference can never cause a cycle. The usage of self inside a block causes a retain cycle, if the block is directly or indirectly referred by self, too, for example via a property.
To your Q:
If you "call" a method inside the block, the message probably has the receiver self, so you have a usage of self inside the block. For this reason it is captured and retained.
If you really have no usage of self inside the block by neither using self nor using a property of self you do not have a usage of self and it is not captured, therefore not retained. But in this case you can have a dangling pointer or a nil'd reference.
You don't need to worry about references while the block is executing - eventually it finishes doing whatever it does, and all these references go away.
What you need to worry about are the references that are captured when the block is created. These references stay until the block goes away. So if your block has a reference to "self", that reference is there just because the block exists. And if you store that block in a property of self, you have a cycle.
So if you store a block as a property in self, then the block shouldn't capture self. That's easily done by letting it access and capture a weak copy of self. Remember that when the block is executing, the weak copy of self may be nil. Which means the self object has already left our world, and your block might not need to do anything.
Short answer: no, in this situation self is not retained.
Long answer
First of all, retaining self and a reference cycle are not the same thing. Reference cycle is a cycle of strong references between a number of objects: A->B->C->A is a retain cycle.
The general idea is, you want to always guarantee that if you are referencing self in a block, you don't reference this block strongly, and don't reference it through a chain of strong references. In reality, retain cycles can be used purposefully if you are making sure you're breaking the retain cycle under certain conditions. Not that I personally recommend this.
Take a look at documentation on Apple's website. It clearly states that values are captured in blocks, and capturing an object reference retains this object in block.
Basically what this means is that referencing an object in a block increments its retainCount by 1, and when this block gets deallocated, retainCount is decremented by 1.
However, when using a __weak pointer in a block, the retain count is untouched.
Here's an example:
- (void) doSomething {
NSLog(#"%#", self);
}
- (void) callBlock {
__weak typeof(self) weakSelf = self;
dispatch_block_t block = ^{
[weakSelf doSomething];
};
}
When you write [obj method:params] this actually translates into following call:
objc_msgSend(obj, #selector(method:), params). One of the features of Objective-C is that if you call a method on nil pointer, it returns nil. This is guaranteed by the fact that objc_msgSend(nil, #selector(anyselector), ...) always return nil. Note that SEL is just a const char[] under the covers, so it doesn't affect retain counts by any means.
Hence when the block will be executed, if your object was deallocated, the weak weakSelf variable will be nullified, and the block's body will translate into objc_msgSending to zero, which does nothing except of wasting few CPU cycles.
To sum it up, the Objective-C messaging system is implemented in such a way that calling a method does not retain this object or this method or this method's implementation, as it's a simple function call.
I have been using blocks and familiar with the memory management when using self inside blocks in
Non-ARC environment
But I have two specific questions:
1) I understand I can use __block to avoid the retain cycle a retained block which in turn using self can create, like below:
__block MyClass *blockSelf = self;
self.myBlock = ^{
blockSelf.someProperty = abc;
[blockSelf someMethod];
};
This will avoid the retain cycle for sure but I by doing this I have created a scope for self to be released and eventually deallocated by someone else. So when this happen self is gone and blockSelf is pointing to a garbage value. There can be conditions when the block is executed after the self is deallocated, then the block will crash as it is trying to use deallocated instance. How do we can avoid this condition? How do I check if blockSelf is valid when block executes or stop block from executing when self is deallocated.
2) On the similar lines suppose I use block like below:
__block MyClass *blockSelf = self;
self.myBlock = ^{
[blockSelf someMethod:blockSelf.someProperty];
};
// I am taking someProperty in an method argument
-(void) someMethod:(datatype*)myClassProperty
{
myClassProperty = abc;
}
Now there can be situations where self is not released but someProperty is released before someMethod's execution starts (This could happen when there are multiple threads). Even if I do self.someProperty = nil; when it is release, myClassProperty is not nil and pointing to some garbage, hence when someMethod is executed the first line will lead to crash. How do I avoid this?
This is the same issue as non-zeroing weak references everywhere else in non-ARC code, e.g. delegates etc. (MRC doesn't have zeroing weak references; so these are the only kind of weak reference in MRC. Yet people were still able to write safe code in the pre-ARC days.)
Basically, the solution is that you need a clear map of ownership. Either self is responsible for keeping the block alive; or some other object is responsible for keeping the block alive.
For example, with delegates, usually, a "parent" object is the delegate of a "child" object; in this case the "parent" object is responsible for keeping the "child" object alive, so the "parent" object will outlive the "child" object, thus the back reference can be weak and is safe (because the child object's methods could only possibly be called by the parent object while the parent object is alive).
On the other hand, if you have an asynchronous operation, and the block is given to the operation as a callback, then usually the operation is responsible for holding onto the block. In this case, self would not hold onto the block. And the block would hold a strong reference to self, so that when the operation is done, it can still safely do whatever it is that it needs to do on self. In fact, whatever object self is, it doesn't even need to be retained by whoever uses it, since it is indirectly retained by the asynchronous operation -- it can just be a create, fire, and forget kind of thing.
If you have something where the block is sometimes kept alive by self, and sometimes kept alive by something else, then you should re-think your design. You say "There can be conditions when the block is executed after the self is deallocated"; well, you should describe your whole design and how this can happen. Because usually, for something that is executed asynchronously, self need not hold onto the block.
Your code is really confusing and doesn't make sense. For example, why would you assign to a parameter (myClassProperty)? What's the point of passing an argument when the parameter is going to be overwritten anyway? Why would you name a local variable "myClassProperty"?
What I think you are asking about is accessing a property that can be changed on different threads, and how to deal with the memory management of that. (This question is unrelated to blocks or ARC/MRC. It is equally an issue in ARC.)
The answer is that you need an atomic property. You can make a synchronized property atomic, or implement an atomic property manually if you know how. What an atomic property of object pointer type needs to do is its getter needs to not just return the underlying variable, but also retain and autorelease it, and return the result of that. The retrieval and retaining in the getter occurs in a critical section that is synchronized with a critical section in the setter that includes the release of the old value and retaining of the new. Basically, this synchronization guarantees that the value will not be released in the setter in between retrieving the value and retaining it in the getter. And the value returned from the getter is retained and autoreleased, so it is guaranteed to be alive for the duration of the scope.
Solution for 2)
__block MyClass *blockSelf = self;
self.myBlock = ^{
datatype* p = [blockSelf.someProperty retain];
[blockSelf someMethod:p];
[p release];
};
// I am taking someProperty in an method argument
-(void) someMethod:(datatype*)myClassProperty
{
myClassProperty = abc;
}
For 1) don't know how you use myBlock. If it's used only by self, then all will be fine. And if it's used by some other object, then it's also should have retained reference at self and then there is also all will be fine.
Imagine the following scenario using Manual Memory Management (aka non-ARC):
I have a VC that passes a block to a class method. Before the block being executed, the VC is popped up out of a UINavigationController. A weak reference in the form of __block MyVC *weakSelf = self is passed to the block which is then converted to MyVC *strongSelf = weakSelf (aka weak/strong dance). The block is never retained by any of the intervenients.
In this scenario, what I am seeing in my code is:
The dealloc of the VC is called when it is popped out.
The block is eventually is called.
The app crashes because I am accessing garbage (the strongSelf is pointing to it).
My question is: I don't want my VC to stay alive until the block eventually executes, but once the block is executed I want to confirm that strongSelf is valid.
I have checked this (non-trivial example by Apple) which doesn't work because it's designed with ARC in mind. So how can I have the same behaviour with MMM? Ideally I want to have what __weak does: if the retainCount reaches zero, I want its references to point to nil and not to garbage.
After reading this from Apple:
In some cases you can use __unsafe_unretained if the class isn’t
__weak compatible. This can, however, become impractical for nontrivial cycles because it can be hard or impossible to validate
that the __unsafe_unretained pointer is still valid and still points
to the same object in question.
Since I don't have access to __weak what I want to do is even possible?
I battled with this exact issue back in the iOS 4.x days. It's not easy without weak pointers giving you a hand!
If you are guaranteed that block is executed on the main thread at a later point (i.e. where strongSelf is assigned from weakSelf) then you basically need a place to store a "isDead" flag, which you set when the VC is dealloced. You then check this flag before doing anything with weakSelf/strongSelf. One solution is this:
You need a class who's only job is to store this "isDead" flag in a member variable. Something like a NSMutableBoolean. I won't write one out, it's simple enough, it just needs a single BOOL property.
In your VC's -[NSObject init] method you create an instance of this flag object, which should initially be set to false.
You then pass this object through to any block you queue for later execution, such that it is automatically retained/released by the block (i.e. without going through the weak/strong dance). Inside the block, you check if the flag is still NO before doing anything with weakSelf.
The key of course is to set the flag to YES inside the VC's -[NSObject dealloc] method, and then release it. If any blocks are still pending execution, they will have already retained the flag object themselves, and when they are later executed they will query the flag discover that the VC is now dead.
This sounds cumbersome (and it is, a bit) but the idea is that the "isDead" flag lives outside the scope of the VC and is therefore not tied to it's lifetime. As for thread safety, as long as you only set/query the flag inside the VC's init/dealloc method and when the block is executed (on the main thread, not on a background thread) it will be thread safe.
I don't want my VC to stay alive until the block eventually executes
But why does it matter? The memory of one VC shouldn't be that much, and if the block performs any UI actions on the VC, that's okay too, since the VC isn't displayed anyway.
Basically, the block should not capture a weak reference to the VC if the VC doesn't retain the block. Using non-zeroing weak reference means you guarantee that the object will stay alive, which is not the case here. I would suggest you not do this weak thing.
As I was walking through some line of codes I stumbled upon this problem a couple of days ago,
- (void)dealloc {
...
[self.postOfficeService deregister:self];
...
}
Where the de-registration from the Post Office Service is an asynchronous operation, even if it's not self evident from the interface as there's no block or function passed to the postOfficeService.
The internal implementation of postOfficeService's -deregister method is something like that
// -deregister:(id)formerSubscriber implementation
//some trivial checks here
// deregister former subscriber
dispatch_asynch(_serialQueue, ^{
[self.subcribers removeObject:formerSubscriber];
});
...
The container, self.subscribers, does perfectly its job and contains only weak references. I.e. it is a NSHashTable.
As long as the deregistration method got called within the dealloc method, I keep on getting a crash while postOfficeService is trying to remove the former subscribers from its list inside that asynch block, which is used for thread safety purposes I guess.
Adding a breakpoint on [self.subscribers removeObject:formerSubscriber], it's possible to notice that the formerSubscriber object is always a NSZombieObject. That's the reason for crashes.
I know that it's possible to get thread safety for deregister method without incurring in this problem - I figure it should be enough use the dispatch_synch in lieu of the dispatch_asynch version
I think this is one of the reason why asynchronous methods shouldn't be called within dealloc methods.
But the question is how's possible to constantly get NSZombie objects even if we are in an ARC environment and the container objects is a NSHashTable (so it should be working I guess)?
The rule is: When dealloc is called, the object will be gone once dealloc returns to its caller (whoever called release when the reference count was 0), and nothing is going to prevent this.
Before ARC, you might have tried to retain an object inside dealloc - doesn't help; once dealloc is called the object will go (and dealloc will be called only once for it, in case you do a retain / release inside dealloc). ARC does the same, just automatically.
Using ARC doesn't means all your memory problem magically disappeared.
What happened is
[obj release] called by ARC
[obj dealloc]
[obj.postOfficeService deregister:obj]
[obj retain] - sorry you can't cancel the deallocation process
dispatch_async
free(obj) - from this point, obj is a zombie
GCD scheduling tasks
dispatch_async execute task
use obj - crash
The correct solution is use dispatch_sync to make sure you not trying to use object after it is deallocated. (be careful about dead lock)
Don't call asynchronous cleanup methods from dealloc. It's just not a good idea. Your -deregister should be synchronous.
NSHashTable stores pointers - it's the equivalent of __unsafe_unretained or assign - UNLESS it was created using +weakObjectsHashTable or the equivalent set of options (NSHashTableZeroingWeakMemory and NSPointerFunctionsObjectPersonality). If it was not created that way, it is quite likely you will have values pointing to zombie objects.
The question of "why am I getting zombies" is best answered by profiling your application with the Zombies template in Instruments and stimulating the required behavior.
I agree with the others that you should probably avoid asynchronous cleanup in your -dealloc method. However, it may be possible to fix this by making the parameter to -deregister: __unsafe_unretained. That method would then have to treat the pointer purely as a opaque value. It must not dereference it or message it. Unfortunately, you don't control the implementation of NSHashTable and can't guarantee that. Even if NSHashTable could be relied upon, the interface of -removeObject: takes an implicitly strong object pointer, so ARC might retain the pointer when it's copied from the unsafe unretained pointer.
You might use the C function API for hash tables (e.g. NSHashRemove()) as suggested in the overview for the NSHashTable class.