使用GCD将图像从文件加载到UICollectionView

Question

When i take a picture, I save the image file in to a folder i made in the NSDocuments Directory. (/Documents/Photos/....png)

Then I load all the photos in the photo's folder in the collection view. Since the files are big, I decided to use Grand Central dispatch to perform grabbing the image on the background thread. But when i scroll up and down the UICollectionView i can see images changing randomly before displaying the correct image. I assume that whenever i grab the image I just set it on the main thread, but I really don't know how to deal with this.

     - (UICollectionViewCell *)collectionView:(UICollectionView *)cv cellForItemAtIndexPath:(NSIndexPath *)indexPath {

    NSUInteger rowNumber = [indexPath row];

    //NSString *imagePath = [self.photoPathArray objectAtIndex:rowNumber];

    NSArray *directoryContent = [[NSFileManager defaultManager] contentsOfDirectoryAtPath:[[NSHomeDirectory() stringByAppendingPathComponent:@"Documents"] stringByAppendingPathComponent:@"Photos"] error:NULL];

    NSString *imagePath = [NSHomeDirectory() stringByAppendingFormat:@"/Documents/Photos/%@",[directoryContent objectAtIndex:rowNumber]];

    NSLog(@"the image path is %@", imagePath);

        imageCell *cell = [cv dequeueReusableCellWithReuseIdentifier:@"imageCell" forIndexPath:indexPath];

     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

         UIImage *originalImage = [UIImage imageWithContentsOfFile:imagePath];
         UIImage *thumbNail = [self shrinkImage:originalImage withSize:CGSizeMake(200, 200)];


         dispatch_sync(dispatch_get_main_queue(), ^{
             cell.imageView.image = thumbNail;
         });

     });

    return cell;

}

Answer 1

Check out Apple's sample app: LazyTableImages. I just incorporated it successfully into a collection view.

http://developer.apple.com/library/ios/#samplecode/LazyTableImages/Introduction/Intro.html