UICollectionView's cellForItemAtIndexPath is n

问题:

Only my second time using UICollectionView's and perhaps I have bitten off more than I can chew but nevertheless:

I am implementing a UICollectionView (myCollectionView) that uses custom UICollectionViewCell's that I have subclassed. The subclassed cells (FullReceiptCell) contain UITableView's and are the size of the viewcontroller. I am trying to allow for horizontal scrolling between the FullReceiptCells.

The subclassed UICollectionViewController that contains myCollectionView is being pushed on to a nav controller stack. Currently, myCollectionView loas and horizontal scrolling is enabled. However, no cells are visible. I have confirmed that

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section

has run and is returning an integer greater than 0. I have also confirmed that myCollectionView's delegate and datasource are properly set in IB to the subclassed UICollectionViewController.

The method where the cells are to be loaded:

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

is not being called.

Here is where I push the UICollectionViewController and my viewDidLoad method within that controller (NOTE: initWithBill is an override of the normal initializer):

In the prior ViewControllers .m file:

FullReceiptViewController *test = [[FullReceiptViewController alloc] initWithBill:currentBill];
test.title = @"Review";
[self.navigationController pushViewController:test animated:YES];

In FullReceiptViewController.m:

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.
    [self.myCollectionView registerClass:[FullReceiptCell class] forCellWithReuseIdentifier:@"FullReceiptCellIdentifier"];
    self.myCollectionView.pagingEnabled = YES;
    // Setup flowlayout

    self.myCollectionViewFlowLayout = [[UICollectionViewFlowLayout alloc] init];
    [self.myCollectionViewFlowLayout setItemSize:CGSizeMake(320, 548)];
    [self.myCollectionViewFlowLayout setSectionInset:UIEdgeInsetsMake(0, 0, 0, 0)];
    [self.myCollectionViewFlowLayout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
    self.myCollectionViewFlowLayout.minimumLineSpacing = 0;
    self.myCollectionViewFlowLayout.minimumInteritemSpacing = 0;
    [self.myCollectionView setCollectionViewLayout:myCollectionViewFlowLayout];
    //testing to see if the collection view is loading
    self.myCollectionView.backgroundColor = [UIColor colorWithWhite:0.25f alpha:1.0f];

Any clue as to why it is not being called?

回答1:

For those who stumble here later.... the reason:

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

was not being called was because of the itemSize for the collectionViewFlowLayout's height was too big.

[self.myCollectionViewFlowLayout setItemSize:CGSizeMake(320, 548)];

If I change the height to 410, it will execute cellForItemAtIndexPath.

回答2:

In my case, it was because my layout class incorrectly subclassed from UICollectionViewLayout instead of UICollectionViewFlowLayout

回答3:

The cellForItemAtIndexPath will not get called if you do not provide the content size information to collection view.

1. If you are using Flow layout: You need to set the item sizes properly.
2. If you have a custom layout subclassed from UICollectionViewLayout: Ensure you are returning a proper size from the collectionViewContentSize method.

In case of the latter, you will also observe that your layoutAttributesForElementsRect is also not called. The reason is that you have not specified what is your content size and by default the size will be CGSizeZero. This basically tell collection view that you don't have any content to paint so it does not bother asking you for attributes or cells.

So, just override collectionViewContentSize and provide a proper size there, it should solve your problem.

回答4:

Maybe I'm just overlooking it, but it appears your missing your delegate and data source. In your header file, make sure you have added these:

<UICollectionViewDelegate, UICollectionViewDataSource>

and in your viewDidLoad method add this:

self.myCollectionView.delegate = self;
self.myCollectionView.dataSource = self;

Also, if you are loading it via an .xib, make sure you are have connected the IBOutlet to the UICollectionView.

回答5:

For a more complicated view hierachy please check this blog. It saved my life!

self.automaticallyAdjustsScrollViewInsets = NO;

回答6:

If your class is subclass from UICollectionviewController and you are creating collectionView programmatically then use

let layout = UICollectionViewFlowLayout()
    layout.scrollDirection = .Vertical
    layout.itemSize = CGSizeMake(50, 50)

    collectionView?.setCollectionViewLayout(layout, animated: false)

回答7:

In my case, what I had very stupidly forgotten to do was implement the UICollectionViewDataSource protocol method numberOfItemsInSection, so that is another thing to check.

回答8:

It was happening for me when item height == 0 by fixing that cellForItem method was called.

回答9:

Ten minutes ago I also encountered this problem, I made a silly mistake.

My intention is to use UICollectionViewFlowLayout,

But because of mistakes, I used UICollectionViewLayout.

回答10:

In my case, changing UINavigationBar's translucent to NO solved the problem.

回答11:

In Xcode 7.0.1, we got this problem when we copied a storyboard and accompanying code from another project. The collection view had a custom flow layout set in the storyboard. Solution was to:

• Remove the flow layout in IB
• Compile/run the app
• Set the flow layout back

Now it worked :)

回答12:

My issue was that I had 2 collection views within the same view, and both were sharing a computed UICollectionViewFlowLayout instance. Once I created separate instances of the same flow layout, it worked fine.

回答13:

I was using autolayout, and in my case height constraint was missing

回答14:

Make sure numberOfSectionsInCollectionView returns a positive integer as well.

There has to be at least one section in the collection.

回答15:

Make sure -(NSInteger) collectionView:numberOfItemsInSection: is returning a positive (read, greater than zero) value. It may be a matter of placing [self.collectionView reloadData]; in the completion handler.

回答16:

I forgot to set the autoresizing mask to false on the collection view:

collectionView.translatesAutoresizingMaskIntoConstraints = false

回答17:

It can be realated layout problems. Check layout warnings from console, if exist.

回答18:

Same happened to me. I had 2 UICollectionViews and I removed once since I didn't need that. After that I realised that the CellForItemAtIndexPath was not getting called but the other required methods. I tried all of the above but then I did the standard magic. Removed the collection view from storyboard and added again. Then it started working. Not sure why and I have no explanation but maybe some connection issues.

回答19:

If you're using collection view protocols you must connect the CollectionViewDataSource and CollectionViewDataSource protocols to your ViewController.

回答20:

In my case, set collectionView.prefetchingEnabled = NO; solved my problem. Only works on iOS 10.

回答21:

When using UICollectionViewDelegateFlowLayout be careful on what this function returns. Both width and height should be greater than 0. I used window as frame reference but due to complicated view hierarchy window was nil at runtime. If you need adjust the width of the element based on screen width use UIScreen.main.bounds.

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: 
           UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {}

回答22:

In my case the collection view was not fully visible in its parent view due to wrong auto layout constraints. So fixing the constraints made the collection view all visible and cellForItemAtIndexPath was called.

回答23:

In storyboard/xib UICollectionView property cellSize MUST not be {0, 0}

回答24:

Also, make sure that reloadData is not called while collection view is updating with animation (insert, delete, update or performBatchUpdates).

回答25:

For me, I update the height of self.view (the collectionView's superview) via autolayout, and MUST call layoutIfNeeded after that.

[self.view mas_updateConstraints:^(MASConstraintMaker *make) {
    make.height.equalTo(@(height));
}];
[self.view layoutIfNeeded];

回答26:

Just resolved this issue, for a somewhat specific situation.

In my case, I was manually initializing the UICollectionViewController in a parent ViewController, and then adding the UICollectionViewController's view as a subview.

I resolved the issue by calling 'setNeedsLayout' and/or 'setNeedsDisplay' on the collectionView in the parent's viewDidAppear:

- (void)viewDidAppear:(BOOL)animated {
   [super viewDidAppear:animated];
   [_collection.view setNeedsLayout];
   [_collection.view setNeedsDisplay];
}

回答27:

I sunk a bit of time with this issue coming op with a CollectionView in a Contained view controller loaded from a storyboard. I ended up just deleting the contained view controller, and recreating it in the storyboard, and this seemed to get rid of the issue. My guess is there was some kind of storyboard corruption that had taken place.

回答28:

Setting collectionView frame inside custom cell's layoutSubViews() worked for me:

override func layoutSubviews() {
        super.layoutSubviews()
        let frame = self.contentView.bounds
        self.CollectionView.frame = frame
    }