Question

I have a small iPhone app which has a button on the first view. When I select this button I load up my new view which has an image on it. I’m currently using the following code to load the image from an online source on a separate thread, whilst allowing the user to continue controlling the application:

- (void) loadImageTest
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSURL *url = [[NSURL alloc] init];
    url = [NSURL URLWithString:logoPath];

    NSData *data = [[NSData alloc] init];
    data = [NSData dataWithContentsOfURL:url];

    loadingImage = [UIImage imageWithData:data];
    titleLogoImage.image = loadingImage;

    //[pool drain];
    [pool release];
}

This is called from this line of code in the new view’s init:

[NSThread detachNewThreadSelector:@selector(loadImageTest) toTarget:self withObject:nil];

Now this works fine (ish), but if I close the new view and then load a new one up again in quick succession (or just after-wards in some cases) it will bomb out with the classic EXC_BAD_ACCESS. I’m sure that this is due to the code within the thread pool, but can anyone see why this is happening?

Thanks

Answer 1

Yours:

// This is ok, you might try using NSURLConnections asynchronous methods instead of making
// your own thread.
[NSThread detachNewThreadSelector:@selector(loadImageTest) toTarget:self withObject:nil];

- (void)loadImageTest
{
    // This is fine
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    // you're making and then abandoning this url object so it will leak
    NSURL *url = [[NSURL alloc] init];
    // this is fine
    url = [NSURL URLWithString:logoPath];
    // again making and abandoning an object
    NSData *data = [[NSData alloc] init];
    data = [NSData dataWithContentsOfURL:url];
    // this works, but is not thread safe,
    // can't operate on UIKit objects off the
    // main thread
    loadingImage = [UIImage imageWithData:data];
    titleLogoImage.image = loadingImage;
    // this is fine
    //[pool drain];
    [pool release];
}

Cleaned up to make things thread safe, etc. Should help:

// I'm assuming you have a iVar for logoPath but we don't want to
// make threaded calls to an iVar (it's not mutable, so you could do it, but it's just bad form)
// If i'm wrong about logoPath being an iVar don't sweat copying it.
- (void)whateverMethodYouWant
{
    NSString *aLogoPath = [[logoPath copy] autorelease];
    [NSThread detachNewThreadSelector:@selector(loadImageForPath:) toTarget:self withObject:aLogoPath];
}
- (void)loadImageForPath:(NSString *)aLogoPath
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:aLogoPath]];
    // the performSelector will retain the data until the method can be performed
    [self performSelectorOnMainThread:@selector(setImageForTitleLogo:) withObject:imgData waitUntilDone:NO];

    [pool release];
}
- (void)setImageForTitleLogo:(NSData *)imgData
{
    // This part is not strictly necessary
    // but it's a nice way to wrap a method that needs to happen on the main thread.
    if ([NSThread isMainThread])
    {
        // make the image (i'm assuming you meant loadingImage to be a local scope variable)
        UIImage *loadingImage = [UIImage imageWithData:imgData];
        // make sure the button still exists 
        // also make sure you're setting any references to this button to nil when you're releasing and making new views
        // so that you're not addressing a button that's been released
        // (assigning the image to the button will cause the button to retain it for you)
        if (titleLogoImage != nil)
            titleLogoImage.image = loadingImage;
    }
    else
    {
        [self performSelectorOnMainThread:@selector(setImageForTitleLogo:) withObject:imgData waitUntilDone:NO];
    }
}