UICollectionView update a single cell

Question

I am attempting to update a single cell inside a UICollectionView, specifically I am just trying to update an image in that specific cell detonated by cell.imageView. [self.collectionView reloadData] is not really an option because it makes the whole collection view flash. [self.collectionView beginUpdates] is not a thing in a UICollectionViewit seems.

I understand I may be able to use:

[self.collectionView performBatchUpdates:^{
    //do something
} completion:nil];

I am not exactly sure what to put inside a completion block to update that certain cell's imageView. This is all being done inside of didSelectItemAtIndexPath.Also I am not using NSFetchedResultsController. Any ideas?

how about invalidateLayout method in your collectionViewLayout? — Dinesh Raja, Nov 30 '13 at 05:46
@DineshRaja invalidateLayout doesn't reload the cell's content because the "cellForItemAt indexPath" isn't called. — Kappe, Mar 12 '18 at 11:31

score 79 · Accepted Answer · answered Nov 30 '13 at 05:48

79

- (void)reloadItemsAtIndexPaths:(NSArray *)indexPaths

Here it is a method to reload the specific indexPaths in your collectionView

answered Nov 30 '13 at 05:48

Dinesh Raja

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Thank you! that ended up working. Now let's say I have a collectionView of 50 cells, and i just in data for another 50 cells, how to i add those 50 extra cells and display the data in them without doing a reloadData? Doing a reloadData results in an unnecessary flash of all the images in the collectionView and I would like to avoid that – Paulius Dragunas Nov 30 '13 at 21:52
you have to call reloadData, if your array changes by any mean. because collection view don't have any idea, if you didn't call reloadData method. May be invalidateLayout method in collectionViewLayout works. But you don't have any other option. – Dinesh Raja Dec 02 '13 at 07:41
It can be a little tricky to implement correctly, but you could always use `-[UICollectionView insertItemsAtIndexPaths:]` to insert the cells in the collection view after you updated the data source. – Dillan Jun 19 '15 at 23:47

gavdotnet · Answer 2 · 2013-12-02T06:08:59.057

42

Dinesh's answer is spot-on. But to avoid unwanted animations when reloading (aka "flashing") use:

BOOL animationsEnabled = [UIView areAnimationsEnabled];
[UIView setAnimationsEnabled:NO];
[myCollectionView reloadItemsAtIndexPaths:myIndexPaths];
[UIView setAnimationsEnabled:animationsEnabled];

edited Dec 02 '13 at 06:08

answered Dec 01 '13 at 22:34

gavdotnet

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I doesn't seem that I can assign a UIVIew to a BOOL. – Paulius Dragunas Dec 02 '13 at 02:35
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@Unome It's a class method so no specific view. – gavdotnet Apr 15 '15 at 00:11

score 5 · Answer 3 · answered Nov 30 '13 at 05:47

See Inserting, Deleting, and Moving Sections and Items from the "Collection View Programming Guide for iOS":

To insert, delete, or move a single section or item, you must follow these steps:

Update the data in your data source object. Call the appropriate method of the collection view to insert or delete the section or item. It is critical that you update your data source before notifying the collection view of any changes. The collection view methods assume that your data source contains the currently correct data. If it does not, the collection view might receive the wrong set of items from your data source or ask for items that are not there and crash your app.

So in your case, you must add an image to the collection view data source first and then call insertItemsAtIndexPaths. The collection view will then ask the data source delegate function to provide the view for the inserted item.

UICollectionView update a single cell

3 Answers3

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