Question

What’s the role of registerClass:forCellWithReuseIdentifier: method?
According to Apple’s developer documentation it’s supposed to

“Register a class for use in creating new collection view cells.”

When I try to use it my project I get a black collection view. When I delete it everything works fine.

#define cellId @"cellId"
#import "ViewController.h"
#import "Cell.h"
@interface ViewController ()
@property (weak, nonatomic) IBOutlet UICollectionView *collectionView;
@property(strong, nonatomic)NSMutableArray * photoArray;


@end

@implementation ViewController


- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSLog(@"%@",_photoArray);
    _photoArray = [[NSMutableArray alloc]initWithCapacity:0];
    [_collectionView registerClass:[Cell class] forCellWithReuseIdentifier:cellId];
    for(int i=1;i<=12;i++)
    {
        NSString * imgName = [NSString stringWithFormat:@"%d.png",i];
        UIImage *img  = [UIImage imageNamed:imgName];
        [_photoArray addObject:img];
    }
}


- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
    return _photoArray.count;
}

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView{
    return 1;
}

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

    Cell* cell = [_collectionView  dequeueReusableCellWithReuseIdentifier:cellId forIndexPath:indexPath];
    cell.cellImage.image = [_photoArray objectAtIndex:indexPath.row];
    return cell;
}

Answer 1

If you’ve already created your UICollectionView in Storyboard, connected your dataSource and delegate, and you have added all of the required methods:

• numberOfItemsInSection
• numberOfSectionsInCollectionView (not a required method – refer to this comment)
• cellForItemAtIndexPath

Then the registerClass / registerCell method isn’t required. However, if you need to reuse a view, data, or cells then you should use those methods so that iOS can populate your UICollectionView as needed. This can also be done in your Storyboard by setting the Prototype Cell (the same principle as the registerClass method).

Also, if you’re looking for a good explanation on what registerCell does and how to use it check out this link and scroll to the bottom section titled "Cell and View Reuse."