The didSelectRowAtIndexPath method doesn't work in my app. I don't get printed any output with NSLog:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    PhotoCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    PFObject *imageObject = [self.imageFilesArray objectAtIndex:indexPath.row];
    PFFile *imageFile = [imageObject objectForKey:@"file"];
    [imageFile getDataInBackgroundWithBlock:^(NSData *data, NSError *error) {
        if (!error) {
            cell.imageView.image = [UIImage imageWithData:data];

        }
    }];

    return cell;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{

        PhotoCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
NSLog(@"didSelectRowAtIndexPath");
    cell.imageView.image = self.image;
    return cell;
}

I would like that when I tap on a cell, that take the image selected, and give the image to self.image.

Can we make that in segue? Because my didSelectRowAtIndexPath method doesn't work at all.

-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"showPhotoDetail"]) {
        NSLog(@"SEGUE showPhotoDetail");
    UICollectionViewCell *cell = sender;
    NSIndexPath *indexPath = [self.collectionView indexPathForCell:cell];

    PhotoDetailViewController *photoDetailViewController = (PhotoDetailViewController *)segue.destinationViewController;
        photoDetailViewController.truckImage = self.image;

    //[UIImage imageNamed:[self.imageFilesArray objectAtIndex:indexPath.row] ];
        NSLog(@"%@", photoDetailViewController.truckImage);
    }
}

Your method signature of selecting Item is wrong, the correct method signature is this

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath

More Info here

Edit

try this code to get the selected image.

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
    PhotoCell *cell = [collectionView cellForItemAtIndexPath:indexPath];
    self.image = cell.imageView.image;
}

Edit 2

-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"showPhotoDetail"]) {
         // get index path of selected cell
         NSIndexPath *indexPath = [collectionView.indexPathsForSelectedItems objectAtIndex:0];
         // get the cell object
         PhotoCell *cell = [collectionView cellForItemAtIndexPath:indexPath];
         // get image from selected cell
         self.image = cell.imageView.image;
    }
}