Why UICollectionView.cellForItemAtIndexPath always calls UICollectionViewDatasource.cellForItemAtIndexPath?

Question

Scenario: I want to check selected state of specific visible UICollectionViewCells with indexPath for which I am calling cellForItemAtIndexPath on the UICollectionView reference.

Problem: Calling UICollectionView.cellForItemAtIndexPath always calls UICollectionViewDatasource.cellForItemAtIndexPath which returns a new cell without the selection state.

Question: Why UICollectionView.cellForItemAtIndexPath always calls UICollectionViewDatasource.cellForItemAtIndexPath ?

Apple documentation says the return value is "The cell object at the corresponding index path or nil if the cell is not visible or indexPath is out of range."

Am I missing something or is my implementation of cellForItemAtIndexPath in datasource incorrect ?

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

SudoCell *cell = [collView dequeueReusableCellWithReuseIdentifier:CELL_ID forIndexPath:indexPath];
[cell setValuesWithSection:indexPath.section item:indexPath.item modelObject:_model];
cell.backgroundColor =  [UIColor whiteColor];

return cell;

}

As a work around currently I am setting storing the section and item values as instance values of the cell. Looping through all visible cells to find matching cells with the section and item values and checking visible state. This becomes tedious when number of cells are huge.

Please advice.

Literphor Literphor · Accepted Answer · 2014-06-04T18:17:10

You're misunderstanding how protocols and delegates work. cellForItemAtIndexPath: is a delegate method that UICollectionView and UITableView call on its datasource in order to populate the Collection or Table View.

So lets say you have a CollectionView and you gave it a datasource. At some point when you run your application that CollectionView is going to call a method on the datasource numberOfSectionsInCollectionView: in order to get how many sections are needed for the CollectionView

Then it calls collectionView:numberOfItemsInSection in order to get the items for each section for the collection. This method is called for each individual section defined in the Collection View.

Finally it calls collectionView:cellForItemAtIndexPath: in order to get the Cells for each item of the collection. This method is called for each individual item defined in the Collection View. This would be where you could programmatically configure the cell to display the information you want, for instance, if you wanted to give the collection a Cell that had an image attached to it, this is where you would do it.

https://developer.apple.com/library/ios/documentation/uikit/reference/UICollectionViewDataSource_protocol/Reference/Reference.html

These are all datasource methods with the sole responsibility of providing data for the Collection View. If you want to respond to user interactions you need to use UICollectionViewDelegate Protocol and implement the methods

collectionView:didSelectItemAtIndexPath:

collectionView:didDeselectItemAtIndexPath:

As the signature implies, these methods are called in response to actions performed on the collection, along with an IndexPath describing the section and item number of the cell the action as performed on.

https://developer.apple.com/library/ios/documentation/uikit/reference/UICollectionViewDelegate_protocol/Reference/Reference.html

UICollectionDelegate Protocol reference above you can use to respond to different events that occur in that view

But none of that information above is any use to you if you don't have a basic understanding of protocols and delegates. I would recommend spending time reinforcing that understanding first before continuing

Why UICollectionView.cellForItemAtIndexPath always calls UICollectionViewDatasource.cellForItemAtIndexPath?

1 Answers