I am currently storing an instance of a class in an NSMutableArray by using NSValue with "valueWithPointer". The problem is within this class I have another NSMutableArray, whenever I convert the instance of the class back from NSValue the NSMutableArray contained within it is of length 0, all of its elements have been removed. Does anyone know why this happens and how to solve it?
Any help would be much appreciated.
Unless there's a very good reason that you specifically need NSValue, you should not add a class instance to an NSMutableArray that way. Just do:
[myMutableArray addObject:myClassInstance];
This does all the right things with regards to memory management (and avoids ugly pointer value stuff), and should let you get at your class instance's array objects after retrieving the object. See the NSMutableArray docs for a quick starter on how to use the class properly.
Related
Sorry if this question is too simple but it is something that I have always wondered and I would like to hear the opinion of an expert.
I have always used NSLog(#"%#",object); to check the string of a NSString, or to see what a NSDictionary has inside. So I am really curious about if you can use this technique to NSLog all objects in objective C.
According to my experience, a NSLog to an object who doesn't have text like content will simply give you the instance description, which will be the object class and instance number I think (like an UIView or a UIButton).
Is there any situation in where I would get an EXE_BAD_ACCESS for logging an object (not a structure/primitive variable)? Or can I keep doing this as I have so far?
Thanks.
You can safely NSLog any object that inherits from NSObject. The description method will be called to produce the output. The default implementation returns the name of the class, but you are free to override description on your own objects to produce any text that you think may be of help to you during debugging.
I have a controller class that store a list of contact and I need to share this list to objects that will implement KVO on such array.
The most obvious solution is to use a NSArray instead of NSMutableArray, in order that everyone can use the same instance variable of the controller and it's not possible modify the content of the array.
Now, let's image that this list can change every 3 minutes and only the controller can update the content of the array.
If I dealloc and re-init the contact list array, all my KVO objects will lost the reference to my array.
Finally:
NSArray PRO: No one can alter the content of the array
NSArray CON: The controller can not modify the array without a dealloc, so other objects lost the reference.
NSMutableArray PRO: Controller can modify array content, all the object can easily share the same reference of the array
NSMutableArray CON: Everyone can modify the array.
Any suggestion?
NSArray in my opinion, given that it isn't being updated very often.
The object that owns and modifies the array exposes it through a property of type NSArray*. The observers observe this property. Whenever the object that owns the array recreates it, it sets the property to the new NSArray which then triggers KVO on all of the observers.
Alternatively, you can implement the indexed accessor patterns for the property.
I have class Item and class List (which has an NSMutableArray).
Every time class Item is instantiated (and destroyed) it posts a notification, which is listened-to by class List. When class List receives the notification is adds the instance of class Item to its list.
I'm trying to have class Item also post a notification that its about to be dealloc'd. The problem is that class List's NSMutableArray retains the instance of class Item.
What's the most appropriate means of handling this situation? If I decrement the count when adding it to List's array, then an exception will be thrown when class List attempts to call removeObject (since it'll try to dealloc the object.)
Basically, I want a "monitor" class List that contains a list of all "live" instances of Item. But, I also need the ability to release/dealloc the instances and have them report they're being dealloc'd so List can remove them from its NSMutableArray.
Thanks for your help.
If I understand correctly, you want an array that maintains weak references to its items, as opposed to strong references?
I don't know of a way to do this with anything "built-in" in Cocoa. The only way I'd know of to do this is to make the array yourself, and have the storage be __weak id[]. That would automatically zero-out the place in the array when the object deallocates. If you're under the retain-release model, you could use something like MAZeroingWeakRef to get the same behavior.
This is definitely an interesting question, and I don't know of an easier answer. I'd love to be proven wrong!
Ha, I love being wrong!
There's a class called NSPointerArray that looks like it can do what you're looking for. However, it's only available on the Mac, and it only auto-zeros when you're using garbage collection.
I'll keep thinking about this. This is an interesting problem! :)
So I kept thinking about this, and came up with a solution. It uses two unconventional things:
A subclass of NSMutableArray (egads!)
Using an associated object to determine object deallocation
For the first bit, I had to to subclass NSMutableArray so that I could inject some custom logic into addObject: (and related methods). I didn't want to do this via swizzling, since NSArray and friends are a class cluster, and swizzling into/out of clusters is fraught with peril. So, a subclass. This is fine, but we're going to lose some of the awesome features we get from "pure" NSArray instances, like how they do weird things when they get big. Oh well, such is life.
As for the second bit, I needed a way for any arbitrary object to notify that it is about to or just finished deallocating. I thought of dynamically subclassing the object's class, injecting my own dealloc/finalize method, calling super, and then smashing the isa of the object, but that just seemed a little too crazy.
So, I decided to take advantage of a fun little thing called associated objects. These are to ivars what categories are to classes: they allow you to dynamically add and remove pseudo-instance variables at runtime. They also have the awesome side effect of getting automatically cleaned up with the object deallocates. So what I did is just created a little throw away object that posts a notification when it is deallocated, and then attached it to the regular object. That way when the regular object is deallocated, the throw away object will be as well, resulting in a notification being posted, which I then listen for in the NSMutableArray subclass. The notification contains a (stale) pointer to the object that is in the process of getting destroyed, but since I only care about the pointer and not the object, that's OK.
The upshot of all of this is that you can do:
DDAutozeroingArray *array = [DDAutozeroingArray array];
NSObject *o = [[NSObject alloc] init];
[array addObject:o];
NSLog(#"%ld", [array count]); //logs "1"
[o release];
NSLog(#"%ld", [array count]); //logs "0"
The source is on github, and it should (theoretically) work just as well on iOS as Mac OS X (regardless of GC mode): https://github.com/davedelong/Demos
Cheers!
... and I just thought of a way to do this without a custom subclass, but I'm tired and will post the updated answer tomorrow.
the next morning...
I've just updated the project on Github with an NSMutableArray category that allows you to create a true NSMutableArray that auto-zeroes its objects as they're deallocated. The trick was to create a CFMutableArrayRef with a custom retain callback that sets up the proper observation, and then just cast that CFMutableArrayRef to an NSMutableArray and use that (ah, the magic of Toll-Free Bridging).
This means you can now do:
NSMutableArray *array = [NSMutableArray autozeroingArray];
I added a typedef to define these as NSAutozeroingMutableArray, just to make it explicitly clear that while this is an NSMutableArray, it doesn't retain its objects like a normal NSMutableArray. However, since it's just a typedef and not a subclass, you can use them interchangeably.
I haven’t tested this, so comments are welcome.
You could use an NSPointerArray for the list (in a retain property):
self.array = [NSPointerArray pointerArrayWithWeakObjects];
When an Item object is created, it would post a notification that’s listened by your List class. Upon receiving the notification, List adds the object to the pointer array:
[array addPointer:pointerToTheObject];
In this setting, the pointer array doesn’t keep a strong reference to its elements — in particular, it doesn’t retain them. This applies to both garbage-collected and non-garbage-collected builds.
In a garbage-collected build, if an element is garbage collected then the garbage collector automatically assigns NULL to the position in the array where the object was stored.
In a non-garbage-collected build, you’ll need to manually remove the element or assign NULL to the position in the array where it was stored. You can do this by overriding -[Item dealloc] and posting a notification that the object is being deallocated. Your List class, upon receiving the notification, would act upon it.
Note that, since objects are not owned by the pointer array, you must keep a strong reference to (or retain) them if you want to keep them alive.
I have a pretty standard setup where I have an array in my controller that I'm binding to an NSTableView. When I add an object to this array with the UI, I add elements by copying the array to a mutable array, adding the contents, then copying it back. I saw this idiom somewhere, but I'm not really sure where, or whether this is actually a sane thing to do. I t works fine on Snow Leopard, but it crashes really badly on Leopard. GDB tells me it happens right at the marked line, where I copy the new array back.
#interface MyAppDelegate : NSObject {
NSArray * urls;
//other declarations
}
#property (nonatomic, retain) NSArray *urls;
#implementation MyAppDelegate
#synthesize urls;
- (void)addThing:(id)sender {
NSMutableArray *newUrls = [NSMutableArray arrayWithArray: [self urls]];
[newUrls addObject: newurlimadding];
[self setUrls: [NSArray arrayWithArray: newUrl]];
}
I'm pretty sure I must be doing something boneheaded here. Is there a better way to do this? Why am I crashing?
NSMutableArray *newUrls = [NSMutableArray arrayWithArray: [self urls]];
[newUrls addObject: newurlimadding];
[self setUrls: [NSArray arrayWithArray: newUrl]];
What did you create newUrls for if not to set it as the new value of urls?
Besides that, there are a couple of things you're doing wrong:
No model objects. Bindings hates this. Unless your table view exists solely to display the different parts of the URL (scheme, host, path, etc.), each in one column, you're not giving Bindings what it wants.
Pay attention to the fields in the Bindings Inspector. Note that there are two: Controller Key and Model Key Path. They're exactly what they say on their tins: The Controller Key is the key for a property of the controller, which should provide model objects. The Model Key Path is the key path for one or more properties in the model, which usually should provide value objects, such as strings and numbers.
You probably need to create a model class and make the URL a property of that class. I'd guess that you have other objects, perhaps in parallel arrays, that you can move to properties of the model class. Do that, so that you're working with Bindings instead of against it.
Not using array accessors. You're just setting the whole array at once, which is inefficient and may cause display problems (such as selection highlights disappearing). Implement and use array accessors for this array property. Accessorizer can help you here.
I actually have no idea whether this will fix your crash because you haven't told us anything about it. You should edit your question to include any exception messages or other crash-describing output, which you'll find in the Run Log/Debugger Console.
Also, since the type of urls can be mutable, you should set it to copy, not retain.
NSArray * things; - since this can be modified you better represent using NSMutableArray instead NSArray.
When ever you need to add some new element to the list just use 'addObject' and insert element to the list.
Your binding will make sure that UI is updated using KVO and KVC.
It looks like the problem was that I had NSURLs as my object type. Once I changed the object in my array to a custom-made KVC compliant object, I wasn't getting any more crashes.
Maybe NSURL isn't KVC-compliant on 10.5, but it is on 10.6?
I'm getting sublayers of a CALayer with this property accessor:
// #property(copy) NSArray *sublayers
NSArray* layer = mylayer.layer.sublayers;
Since this property uses "copy", every time I simply execute:
mylayer.layer.sublayers
is an entire copy of the sublayers array being made for me? If so that might be bad, because I could have one hundred+ layers, and definitely don't want to create a huge copy of them. I'm just looking to get a count and iterate through the existing layers, just a pointer to the existing layers,
Thank you
When you use the 'copy' declaration and you #synthesize the property, then copy is used when the property is set.
In your example, you are only accessing the value which just gives you a pointer to the sublayers array.
Here's a link to the ADC documentation on this point.
Update
IIRC, the runtime is smart enough to know if the object being set is mutable. If an immutable object is being passed in to the property it is retained and not copied.
In some cases, if you are passing in a mutable object that you want to be able to modify, then you should write your own setter that calls mutableCopy on the object. This is shown in the documentation link that I provided.
I'm not sure I understand your answer Abizern so let me say this:
If you use (copy) on a property you will be making a whole new copy of that object so would be wasting a lot of memory. I'm not sure why they are doing that, they could just use (readonly) instead if they want to protect the values from change. Plus remember most value classes are immutable in Obj-C so they can't change the value anyway.