objective-c Embedded UICollectionViewController shows 0 items

Question

I'm trying to have a UICollectionViewController inside my UIViewController. Here's how I set it up

TagsCollectionViewController* tagsCollectionViewController = [[TagsCollectionViewController alloc] initWithCollectionViewLayout:[UICollectionViewLayout new]];
    UIView *tagsView = tagsCollectionViewController.view;
    [self.view addSubview:tagsView]; // setting up constraings too

And this is my TagsCollectionViewController

@interface TagsCollectionViewController () <UICollectionViewDelegateFlowLayout>

@end

@implementation TagsCollectionViewController

static NSString * const reuseIdentifier = @"Cell";

- (void)viewDidLoad {
    [super viewDidLoad];

    self.collectionView.backgroundColor = [UIColor yellowColor];

    [self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:reuseIdentifier];

}

#pragma mark <UICollectionViewDataSource>

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    return 20;
}

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

    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];
    cell.backgroundColor = [UIColor redColor];
    return cell;

}

#pragma mark <UICollectionViewDelegateFlowLayout>

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    return  CGSizeMake(50, 50);
}

@end

What's strange is that I get yellow view so my collection seems to be working, however I don't get to see any items. But when I present whole controller everything works fine. So the problem occurs only when I'm trying to embed it. What can be the issue?

EDIT: Related: Adding UICollectionViewController to UIViewController Not Working

Do you keep a valid reference to your TagsCollectionViewController around so it can act as the delegate/datasource? — Phillip Mills
– Phillip Mills, Commented Jan 5, 2020 at 22:14
@PhillipMills I made it a strong property now and initialized it in viewDidLoad. Is that what you meant? Still doesn't work though. — giann carlo
– giann carlo, Commented Jan 5, 2020 at 22:18
Does its methods get called, e.g. for number of items? (Maybe put in a breakpoint to see what you're returning.) — Phillip Mills
– Phillip Mills, Commented Jan 5, 2020 at 22:25
Yes, it gets called and it works just fine as a separate controller. It probably has to do something with the way I embed it. — giann carlo
– giann carlo, Commented Jan 5, 2020 at 22:28
That's OK but I suspect cellForItemAtIndexPath is not being called at all. — Phillip Mills
– Phillip Mills, Commented Jan 5, 2020 at 23:12

Phillip Mills · Accepted Answer · 2020-01-05 23:09:41Z

0

It looks as if it's a problem with estimating the size of items in the collection. I was able to get results by using the following code after creating a similar example and noticing that cells weren't being requested.

@interface ViewController ()
@property (nonatomic, strong) TagsCollectionViewController *tags;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    UICollectionViewFlowLayout *layout = [UICollectionViewFlowLayout new];
    layout.itemSize = CGSizeMake(50.0, 50.0);
    self.tags = [[TagsCollectionViewController alloc] initWithCollectionViewLayout:layout];
    UIView *tagsView = self.tags.view;
    [self.view addSubview:tagsView];
}

answered Jan 5, 2020 at 23:09

Phillip Mills

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5 Comments

giann carlo Over a year ago

Thanks a lot! I honestly not quite sure how, but it worked!!

Phillip Mills Over a year ago

Not sure either. Perhaps when you present the controller directly it applies some default sizing and, without that, decides not to ask for cells because it doesn't know how to display them. (Just guessing.)

giann carlo Over a year ago

It actually works even without itemSize, I'm almost sure that I tried the exact version but it didn't work back then, huh

giann carlo Over a year ago

I was initializing it with UICollectionViewLayout instead of UICollectionViewFlowLayout...

Phillip Mills Over a year ago

Yes, I think I gave you the right answer but with the wrong reasoning. :)

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