CODEWITHSUNDEEP
iphoneobjective-c
bigmac
bigmac

Reputation: 717

Stop lazy-loading images?

Here's the issue – I followed along with the Apple lazy-load image sample code to handle my graphical tables. It works great. However, my lazy-load image tables are being stacked within a navigation controller so that you can move in and out of menus. While all this works, I'm getting a persistent crash when I move into a table then move immediately back out of it using the "back" button. This appears to be a result of the network connections loading content not being closed properly, or calling back to their released delegates. Now, I've tried working through this and carefully setting all delegates to nil and calling close on all open network connections before releasing a menu. However, I'm still getting the error. Also – short posting my entire application code into this post, I can't really post a specific code snippet that illustrates this situation.

I wonder if anyone has ideas for tasks that I may be missing? Do I need to do anything to close a network connection other than closing it and setting it to nil? Also, I'm new to debugging tools – can anyone suggest a good tool to use to watch network connections and see what's causing them to fail?

Thanks!

Upvotes: 0

Views: 826

Answers (3)

bigmac
bigmac

Reputation: 717

Ah ha... after a large zombie hunt (thanks, Matt Long), I discovered that the issue stems from an error in Apple's LazyTableImages sample code. That example provides the following implementation for canceling all image loads, which I turned into a general-purpose stopAllImageLoads method...

From RootViewController.m in LazyTableImages sample code:

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];

    // terminate all pending download connections
    NSArray *allDownloads = [self.imageDownloadsInProgress allValues];
    [allDownloads performSelector:@selector(cancelDownload)];
}

There is in error in the last line of the above method where performSelector is called on an array of objects. The above implementation calls the selector on the array itself, rather that on each object in the array. Therefore, that last line should be this:

[allDownloads makeObjectsPerformSelector:@selector(cancelDownload)];

Once that line was changed, everything else fell into place. It turns out I wasn't calling my stopAllImageLoads method where I meant to – I had disabled it at one point because it was causing an error. Once that was back in place, the memory issues cleared up because image loads were successfully canceled before the table delegate was released.

Thanks all for your help.

Upvotes: 1

Dan Ray
Dan Ray

Reputation: 21893

If you're doing ANY asynchronous function (network requests, Core Location updates, etc), you run the risk that your view controller that is the delegate of that action is deallocated by the time the async function returns. i.e. you back out of the view and take the delegate target away from the background process. I've dealt with this several times.

Here's what you do. Use the ASIHTTPRequest library (which you should be doing anyway--it's brilliant). Create a synthesized property to hold your request. Then in viewWillDisappear, call -cancel on your request. To be safe, I also set its delegate to nil, but that should be unnecessary.

Here's a sketch of what you want to do. Note I typed this right here, haven't syntax-checked it or anything.

@implementation MyViewController

@synthesize req //this is an ASIHTTPRequest *req.

-(void)viewDidLoad 
{
  //make an NSURL object called myURL
  self.req = [ASIHTTPRequest requestWithURL:myURL];
  self.req.delegate = self;
  [self.req startAsynchronous];
}

-(void)viewWillDisappear
{
  [self.req cancel];
}

-(void)requestFinished:(ASIHTTPRequest *)request
{
  NSString *string = [request responseString];
}

Upvotes: 0

Matt Long
Matt Long

Reputation: 24466

Have you run it through the debugger (Cmd-Y)? Does it stop at the place where the crash is happening? That should show you in code where the issue is happening. I'm betting the issue has to do with over-releasing something rather than cleaning up connections. Are you getting EXC_BAD_ACCESS? Check any delegates and make sure they are nil when -viewWillDisappear gets called. That way, if anything tries to call back to a delegate, it will just be a no-op.

You may also want to try enabling zombies (NSZombieEnabled) which will tell you when an object that has been released is being accessed again. It's very helpful in finding over-released objects.

Upvotes: 2

Related Questions