Lazy load images in UITableView

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I have some 50 custom cells in my UITableView. I want to display an image and a label in the cells where I get the images from URLs.

I want to do a lazy load of images so the UI does not freeze up while the images are being loaded. I tried getting the images in separate threads but I have to load each image every time a cell becomes visible again (Otherwise reuse of cells shows old images). Can someone please tell me how to duplicate this behavior.

objective-c uitableview iphone ios
8 Answers

I'd suggest going for an NSOperation and doing whatever you need on another thread.

This is a class I wrote for image loading:

- (id)initWithTarget:(id)trgt selector:(SEL)sel withImgURL:(NSString *)url {
    if(self = [super init]) {
        if(url == nil || [url isEqualToString:@""])
            return nil;
        target = trgt;
        action = sel;
        imgURL = [[NSURL alloc] initWithString: url];
    }
    return self;
}

- (void)main {
    [NSThread detachNewThreadSelector:@selector(loadImage) toTarget:self withObject:nil];
}

- (void)loadImage {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    UIImage *img = [UIImage imageNamed: @"default_user.png"];
    if(![[imgURL absoluteString] isEqualToString: @"0"]) {
        NSData *imgData = [NSData dataWithContentsOfURL: imgURL];
        img = [UIImage imageWithData: imgData];
    }
    if([target respondsToSelector: action])
        [target performSelectorOnMainThread: action withObject: img waitUntilDone: YES];
    [pool release];
}

- (void)dealloc {
    [imgURL release];
    [super dealloc];
}

Hope that helps!

user_1263920

I am not aware of any built in way to accomplish this but it sounds like you have something working with another thread already.

Assuming that your only remaining problem now is invalidating the contents of a cell when it comes back into view you should look at:

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath

By implementing this delegate method you could check the contents of the cell before it draws to see if it needs new contents. If it does then you could empty the cells contents and trigger your threaded loading again.

You might also consider posting some code.

user_34930
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