I have a UIViewController subclass that loads a bunch of images for each cell in a tableview asynchronously which is handled by a separate download class. I keep a list of all of these download requests in a dictionary which is keyed to the index of the cell that is requesting the image.
My question is i where should i put the code that cancels the image download if the viewcontroller is popped off the navcontroller? I need to do this because if the user hits back while there are still images being downloaded (which could take a while) then when they are finished downloading the viewcontroller has already been released.
I cant put it in the viewWillDisappear method because i do not want to stop the download if the user clicks on a separate tab and only when the hit the back button. For now i put this code in the viewcontrollers dealloc method which works fine although it doesnt seem right for some reason. I thought of using the viewDidUnload method but it seems this is only called when there is a low memory warning?
Any ideas?
dealloc is the perfect place for this as the view controller gets deallocated and you are responsible to clean your stuff up.
You may want to consider putting the code in viewWillDisappear: anyway, after all if the screen you are navigating to needs anything loaded it will be slowed by the background image load...
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Most of the iOS apps I use are very responsive, when I tap on an element it goes to the next view right away. In my app, some of my view controllers take 0.5-1.0 second to load.
My code is all in the viewDidLoad method and I'm pretty sure that's the problem but I can't move anything out since I need every single element that I instantiate.
A solution I thought is to move all the work I do in viewDidLoad in a thread then call the main thread when I'm ready to call addSubview, would that work even if UIKit is not thread safe? Or is there something else I'm missing?
Try to move some code you might have in viewDidLoad to viewdidAppear. viewDidAppear is being called once the view is presented. If you have to make some hard work, do it there and maybe show aa spinner somewhere while you do that.
What are you exactly doing in viewDidLoad? Btw remember that a view is only loaded when you need it, if you want to switch between views faster I can suggest you to create an initializion phase where you call -view on all the view controller you want to show, maybe helped with a spinner or a progress bar. but pay attention this would work only with intensive loading task and not memory consuming tasks. It sounds very strange your request, so is better the you try to explain better why your viewDidLoad is so slow, maybe there is something wrong.
Define your UI elements in Xcode as part of designing the interface. That way, Xcode can compile your storeyboard or xib files into the rapidly loading binary form.
I'm doing an app that uses a TabBarController and each Tab uses its own navigation controller.
The app has dynamic content and I use viewDidDisappear viewDidAppear methods to create or destroy the objects that I need each time I enter or exit into the ViewController.
My problem is when I start to sail very fast and I don't give time to load the Threads that I use for uploading content such as XML peta app or destroy objects when I leave the ViewController.
How I could control the tabs of the navigationbar or tabbarviewcontroller for not respond until the viewcontroller has loaded all contents?
Excuse me if I'm not well expressed. Thanks!
No matter you use synchronous request or asynchronous request, just show an UIAlertView while loading the data. This will both serve as a notification to the user that something is being loaded, and the it will block the interactions with all the other views on the screen.
As others have suggested in comments, I believe that what you want to do is rearrange the order in which things are triggered. Perhaps something like this:
On viewWillAppear:, clear (or disable or whatever is appropriate) your objects that are no longer valid and begin the load-new-content thread. Perhaps display a UIActivityIndicator or similar.
On viewWillDisappear:, tell the load-new-content thread that it can stop, its results are no longer needed. If you put up an activity indicator, take it down.
At the end of the load-new-content thread, take down any activity indicator, update the UI with the new contents and activate.
I don't really see any way around this -- if the UI is not valid until the new content is loaded, then you have to wait for it.
Another solution might be to cache the contents from the previous fetch, and always display those on viewDidLoad. Then, at the end of your new-content-thread, cache the new contents, and update the UI.
I have created a window based application with the following
a TableViewController (Without a XIB file)
a ViewController (With a XIB file) <-- to be used as modal view
a CoreData model to store some data
I managed to load the application and populate the TableView with the data from the Entity, and I was able to scroll through all of the cells of the TableView, without any issues.
I added a UIBarButton item (rightBarButton) that causes a Modal View to appear for the user to input some data. The model view has a SAVE and CANCEL buttons.
The problem is once I press the Cancel Button, I go back to the TableView but if I try to scroll throgh the items in the tableview, the app crashes.
After 4 hours of searching Google and StackOverflow, I was not able to see why my app crash. I did however notice by the debugger that the ManagedObjectContext is set to NIL the second time I scroll the tableview (after the modalview is dismissed), although no data is changed and no insertion/deletion occured.
I tried using a timer to call reloadData as I found some answers on StackOverflow, but that did not work. I tried setting the ManagedObjectContext as a property with retain and removed all occurences of [myManagedObjectContext release] to avoid releasing it earlier than needed, but that did not help.
It seems that I am doing an obvious mistake, but I am not sure where.
Please help.
ivars do not become nil just because they're released somewhere else (at least not in iOS 4.3). So an over-release is not the specific cause of myManagedObjectContext becoming nil. Assuming you're using accessors to reference your ivars (and you should be), hand-implement setManagedObjectContext: and put a breakpoint in there to see who's calling it. Alternately, you can add a gdb watchpoint to myManagedObjectContext to see when the memory is changed.
You haven't indicated what the crash stack is when you crash. You should be focused on what memory you're accessing at the point of the crash, and ensuring that the crash is due to a memory violation rather than an exception. Check your debugger output. Often it will tell you what's happening.
I have a tableview based app that downloads rss feeds from the web. So i have view1, i click a row in the table and the results show on view2. This works great but if i change back to view1 while the data is being downloaded. It crashes!
Any ideas what i need to change (its nsxmlparser by the way)?
Thanks
Some more information about the problem would be useful to understand what's going on, but based on what you posted my guess would be that you are setting a delegate for something (the NSXMLParser or the url connection, or maybe something else) and when you go back to view1, view2 gets destroyed, destroying the delegate object and leaving whoever it was with a dangling delegate pointer.
I'm developing an app to browse online photos, and sometimes received memory warning(level 1), after that, if I go back to last UIViewController(in a NavigationControll), I found that some custom UIView lost, I cannot get them back, but I can create new such UIView
does anyone know the problem?
The memory warning prompts the os to dump any unneeded views. If you create them in IB, or in the viewDidLoad or loadView method in your view controller they'll be re-created when your view re-loads. You could also have your view controller retain them specifically, but that'll have a larger memory footprint than just re-creating them whenever they're needed.
I should say, loadView is only called if you don't use a nib for creating your view, so viewDidLoad is probably where you want to put them.
It's expected behavior. You can override didReceiveMemoryWarning (i.e. do nothing and don't call on super); but what you really want is to handle those situations correctly, i.e. set the views up again if necessary.