I have a view that will be displaying downloaded images and text. I'd like to handle all the downloading asynchronously using ASIHTTPRequest, but I'm not sure how to go about notifying the view when downloads are finished...
If I pass my view controller as the delegate of the ASIHTTPRequest, and then my view is destroyed (user navigates away) will it fail gracefully when it tries to message my view controller because the delegate is now nil?
i.e. if i do this:
UIViewController *myvc = [[UIViewController alloc] init];
request.delegate = myvc;
[myvc release];
Do myvc, and request.delegate now == a pointer to nil?
This is the problem with being self-taught... I'm kinda fuzzy on some basic concepts.
Other ideas of how to handle this are welcome.
update: Looking at the source code for ASIHTTPRequest, it does not retain its delegate, so your code will either have to ensure that the delegate has not been released, or set the request's delegate property to nil before releasing your controller.
If you are going to have several asynchronous HTTP requests running, it may be a good idea to create an HTTPRequestManager class to act as delegate for all of your HTTP requests. This HTTPRequestManager class would remain in memory for the entire lifetime of your application, and it could take care of failing gracefully if/when your view controllers are released.
I would like to add a clarification about Cocoa's reference-counted memory management. It is important to remember that a released object is not automatically set to nil. Sending a message to nil is perfectly legal (and simply does nothing), but sending a message to a deleted object is undefined behaviour. Keeping a pointer to a deleted object is asking for trouble, and such living/dead pointers are referred to as zombies.
Related
I'm familiar with the delegate pattern and nilling my delegates, especially when doing asynchronous calls which are still in progress when my view controllers disappear. I nil the delegate, and the callback successfully returns on a nil object.
I'm now experimenting with using completion blocks to make my code a little easier to read.
I call a network service from my view controller, and pass a block which updates my UITableView. Under normal circumstances it works fine. However, if I leave the view before it completes, the completion handler block is executed - but the UITableView is now a zombie.
Whats the usual pattern for handling this?
UPDATE WITH CODE SAMPLE
This is an iPad app, I have two view controllers on screen at once, like a split view. One is the detail, and the other is a grid of images. I click an image and it tell the detail to load the info. However, if i click the images too fast before they have chance to do the network call - I have the problems. On changing images the code below is called which counts the favourites of a image....
So here is my dilemma, if I use the code below - it works fine but it leaks in instruments if you switch images before the network responds.
If I remove the __block and pass in self, then it crashes with zombies.
I can't win... I'm sure i'm missing something fundamental about using blocks.
__block UITableView *theTable = [self.table retain];
__block IndexedDictionary *tableData = [self.descriptionKeyValues retain];
FavouritesController *favourites = [Container controllerWithClass:FavouritesController.class];
[favourites countFavouritesForPhoto:self.photo
completion:^(int favesCount) {
[tableData insertObject:[NSString stringWithFormat:#"%i", favesCount]
forKey:#"Favourites:" atIndex:1];
[theTable reloadData];
[tableData release];
[theTable release];
}];
Any tips? Thanks
SECOND UPDATE
I changed the way I loaded the favourites. Instead of the favourites being a singleton, I create an instance on each photo change. By replacing this and killing the old one - the block has nowhere to callback (i guess it doesn't even exist) and my code now just looks like the below, and it appear to be working:
[self.favourites countFavouritesForPhoto:self.photo
completion:^(int favesCount) {
[self.descriptionKeyValues insertObject:[NSString stringWithFormat:#"%i", favesCount]
forKey:#"Favourites:" atIndex:1];
[self.table reloadData];
}];
It doesn't leak, and doesn't appear to be crashing either.
I recommend you test that the tableview is not nil at the start of the block. It sounds like the tableview is properly discarded when its parent view goes off-screen, so after that point, no tableview operations are valid.
Retaining the UITableView within the block is a bad idea, because datasource/tableview updates can result in implicit method calls and notifications that will not be relevant if the tableview is not on-screen.
Block will retain any object that it references, except for those annotated with __block. If you want not to execute completion blocks at all, just make some property like isCancelled and check whether it is YES before calling completion block.
So you have a background operation which has to call back another object after it finishes and the object can be destroyed in the meantime. The crashes you describe happen when you have non retained references. The problem as you see is that the referred object goes away and the pointer is invalid. Usually, what you do is unregister the delegate inside the dealloc method so that the background task continues, and whenever it is ready to communicate the results back it says "Shoot, my callback object is nil", and at least it doesn't crash.
Still, handling manually weak references is tedious and error prone. You can forget to nil a delegate inside a dealloc method and it may go without notice for months before you encounter a situation where the code crashes.
If you are targeting iOS 5.0 I would read up upon ARC and the weak references it provides. If you don't want to use ARC, or need to target pre 5.x devices, I would recommend using zeroing weak reference libraries like MAZeroingWeakRef which work also for 3.x devices.
With either ARC's weak references or MAZeroingWeakRef, you would implement the background task with one of these fancy weak reference objects pointing back to your table. Now if the pointed object goes away, the weak pointer will nil itself and your background task won't crash.
I am working on an iPad (only) app and I stumbled across a weird problem. The app gets terminated after a memory warning on iPad 1 but works fine on iPad 2.
I am using ARC and targeting iOS 5. I use nibs and most of my assets are displayed using UIImageViews. I also have a few hundred buttons and plenty of gesture recognizers... I re-watched the WWDC11 videos (sessions 323 and 322) on ARC and I don't seem to be doing anything special.
The app is UIImage intensive, I am doing lots of animations using UIImage. I am however using the initWithContentsOfFile constructor rather than the imageNamed call. I'm trying to prevent the images from being cached by the system.
I'm also using GCD to schedule sound effects and to animate views. I'm always doing this on the main thread.
The app uses a UINavigationController that never has more than 1 UIViewController on it's stack. I can confirm that this is true because the didReceiveMemoryWarning only gets called on the current view controller (I'm logging the call).
The thing I don't understand is why Instruments is reporting high numbers (as if the view controllers don't get deallocated) in both the Allocations and VM Tracker instruments. The Allocations instrument shows a small drop when I navigate from one view controller to another (which is what I expect) but the VM Tracker Instrument shows that the Dirty Size is not dropping when I do the same thing. Eventually the app uses too much memory and gets terminated (on iPad 1). When I get memory warnings on the iPad 2 the app does NOT get terminated though...
It feels as if my images, sounds or views don't get destroyed and the memory does not get reclaimed... My object hierarchy is very basic and there should not be any retain cycles of any sort. I don't even have simple delegates...
Do you have any suggestions? I really don't want to release this app only for the iPad 2 or newer... It's an app for kids and it would be a pitty... I'd be so much happier to learn that I'm doing something wrong, as I really want to make sure this app is the best it can be...
Cheers,
Nick
There are ways to say, 'optimise' your objects by setting their properties to nil when certain things aren't needed -- so while you can't write a dealloc method anymore, you can do self.object = nil (when pertinent) which ends up doing something like this in a non-ARC world for an 'retain' (i.e., strong) property:
- (void)setObject:(id)newObject
{
[object release]; // send release message to current object
object = newObject; // set reference to newObject
[object retain]; // send retain message to newObject
}
Now while in ARC you don't/can't write retain/release yourself in your code, the compiler inserts these calls for you, meaning that in practise, setting a property to nil would do this in the above example:
[object release]; // send release message to current object
object = nil; // set reference to nil
[object retain]; // send retain message to nil (no effect)
Moreover, this is just the tip of the iceberg -- you should make sure that there are no retain cycles in your code, which could be resulting in objects leaking without recourse to their destruction. This means, that there may be places where you're using strong references to a property (i.e., an object), when you should be using a weak property. The difference being, that strong references are retained, and weak references are assigned, the former having its retainCount incremented and the latter resulting in a property assignment that looks like this if handwritten:
- (void)setObject:(id)newObject
{
object = newObject;
}
I don't like answering my own question but I figured it could be helpful to future googlers. I implemented my own UIImage based animation and I no longer use the animationImages property. Turns out my memory issues are now gone as I no longer need to store all the images in memory and I load them as they are required using a timer.
I actually think that rolling out a custom animation is beneficial since it allows for callbacks and more powerful customisation options.
Once I'm happy with it and I feel like it's ready to be shared I will post the class(es) on GitHub.
I have a very simple code to show a modal controller (nextController is a class member):
nextController = [[InstructionsScreen alloc] initWithNibName:#"InstructionsScreen" bundle:nil];
[self presentModalViewController:nextController animated:YES];
[nextController release];
And then when the controller should hide:
[self dismissModalViewControllerAnimated:YES];
nextController = nil;
All works good as expected, but when I run instrument Object Allocations it shows that after dismissing the modal controller the memory it allocated is not freed. This becomes a problem because when I show several controllers the memory is over ...
Can anybody give me some clues ? Clang doesn't see any problems, so I'm stuck hitting the memory limit, because the memory of the dismissed controllers won't get released.
EDIT: What I discovered up to now is that it seems to be a leak somewhere in Apple's stuff. Way to reproduce: XCode -> create new project with the template "Utility application". Don't write any code yourself. Just create a new utility application and run it with "Object allocations", choose to see "Created & Still living". Now flip the modal controller few times - you'll see the allocated memory only grows and grows every time the modal controller is appearing and when it's disappearing too ...
There is no leak in the code you show as far as I can see. There could be a leak in InstructionsScreen that would prevent it being deallocated.
I think it's worth running the Static Analyser to see if it finds a leak.
The leak in the Apple template code is interesting. It could be that there is a leak. It seems unlikely but obviously it's not impossible. I would say that it's more likely that it's a false-positive in Instruments, which is why I'd suggest using the Static Analyser.
(You might want to raise a bug report about the leak.)
Modal views are not subviews of the calling view but are instead subview of the apps window and are retained by the window itself. You generally you do not retain a reference to them in the controller that calls them. Instead, evoke the modal view and then have it communicate with the controller by defining the controller as the modal view's delegate.
I think that if you use synthesize to create the accessor for a nextController property defined with retain, then the accessor will retain any object assigned to the property. Simply setting the value to nil will not release the object unless the accessor is set up to do that and I don't think the autogenerated ones do.
You will expressly have to call release before setting to nil.
If this doesn't work, post the code for your definition of the nextController property.
I am trying to implement the delegate Pattern in Objective-C, however I am experiencing a Bad Access exception when invoking the delegate sometimes. It seems this is caused by the delegate being released. Apple does not recommend to retain delegates.
How can I check my delegate if is still valid before trying to send it a message?
If there's a chance that the delegate will get released by the setter, then there's something wrong with your design. You should only set delegates on objects that have a shorter lifespan than the delegate itself. For example, setting a delegate on a subview/controller is fine, because the subview/controller has a shorter lifespan than the caller.
AFAIK, there is no reliable way to detect if an object has been released already.
What Apple means about not retaining delegates is that objects should not retain their delegates because they don't own them. These are only objects that handle messages.
That doesn't mean that you shouldn't retain delegates at all. The object that creates the delegate needs to own it. In the context of non-GC apps this means it should handle the retain and release cycle, and for GC apps, it means that the controller object keeps hold of a pointer to the delegate in an iVar.
without seeing some code or the error message, it is hard to find the root of this problem.
In a photoviewer application I'm using asynchronous http to load images; it happens that the user often dismisses the current view (referenced by my async http object through a delegate) before the http download completed causing a BAD_ACCESS when calling the view controller delegate method. I solved this by setting the .delegate to nil inside the dealloc block of the view controller
I'd like to share my experience also, which is very similar to Nico's one.
I've been working with a modified example of LazyTablesCode, wich is an example that comes direcly from Apple and loads images in a UITableView asynchronously. Communication between the downloader and the view it's made via delegates.
In my code, I had the problem that sometimes the load of the image finishes when the form that should be called through the delegate has been released. I've been forced to add this piece of code inside the code of the viewController (dealloc method):
if (self.nsDictionaryWithObjectsDownloading != nil) {
for (id theKey in self.nsDictionaryWithObjectsDownloading) {
Myobj *downloader = [self.nsDictionaryWithObjectsDownloading objectForKey:theKey];
downloader.delegate = nil;
}
}
It seems that these lines are solving the problem. Anyway It would be very appreciated opinions about if it's a good solution or not or even about memory issues when doing downloader.delegate = nil;
Thanks and greetings,
I'm working on an iPhone app that performs some communication with the server to be initialized the first time it's run. I have a class ServerCommunication with methods for each of the basic tasks to be performed (authenticate user, get activation key, download updates, etc.), and a new instance of ServerCommunication is created for each task; I'm not reusing them, although it would be more efficient.
When the user completes the first initialization screen, ServerCommunication gets created four times. I keep track of it with NSLog(#"Initializing ServerCommunication instance %p", self); in its -init method. The second initialization screen also calls ServerCommunication a few times when the user taps the "Next" button, but on its last instantiation, the app hangs with the message -[ServerCommunication insertDataIntoLocalDB:]: message sent to deallocated instance 0xec75b0 in the console. The thing is, 0xec75b0 is the address of the first instance of ServerCommunication I created way back at the first screen.
Why would it be sending messages to that instance? I'm not retaining them anywhere; they're mostly autoreleased. If it helps, all of the methods in that class perform asynchronous downloading of XML data with NSURLConnection and then parse it with NSXMLParser. The parser's delegate method -(void)parserDidEndDocument:(NSXMLParser *)parser then sends off NSNotifications that are received by methods in my view controllers so they know whether to proceed to the next screen or stay there and display an error message.
Help is much appreciated!
The first thing I would do is turn on NSZombies, which should let you break at the point where your zombie is being messaged.
A common cause of problems like this is when you have objects with weak references to each other that are not allocated and deallocated at the same time. So (hypothetically), some other object stores a pointer to your ServerCommunication object as a delegate or owner. When ServerCommunication is deallocated, it doesn't unregister, and then some time down the road the object holding the weak reference tries to message you.
If I had to completely guess (and I do!) I bet you add your ServerCommunication objects as an NSNotification observer, but never remove them. Try making sure that you do this:
[[NSNotificationCenter defaultCenter] removeObserver:self];
sometime before deallocation. (It's also possible that there's a more circuitous path involving NSNotification here -- such as a pointer to the ServerCommunication object being passed as data to the view controller, which is then trying to message it.)