Scaling down the height of a UICollectionViewCell dynamically

Question

I am using UICollectionViewDelegateFlowLayout to display cells of dynamic height in my Collection View. The size of the cells are set in sizeForItemAt.

Goal: Each Collection View cell has a toggle function that should change the dynamically-calculated height to 50%. This is where I am running into problems.

In sizeForItemAt I am getting the dynamic height.

var dynamicHeight = (header + content + footer)
return CGSize(width: self.view.frame.width, height: dynamicHeight)

I'd like to update sizeForItemAt to something like the following, when a button on the cell gets pressed:

return CGSize(width: self.view.frame.width, height: (dynamicHeight / 2))

How do I change the height of a Collection View Cell while updating sizeForItemAt?

gcharita · Accepted Answer · 2021-03-25T21:07:41.407

In order to achieve what you want you need to:

Store the state of the cell in your datasource so that collectionView(_:layout:sizeForItemAt:) function can access it easily using the index path and
After changing the stored state of the cell you need to call reloadItems(at:) function of the collection view, passing the cell's index path, to update the cell's size.

You can see an example in the following code:

class TestCollectionViewCell: UICollectionViewCell {
    @IBOutlet weak var `switch`: UISwitch!
    var action: ((Bool) -> Void)?
    
    @IBAction func switchValueChanged(_ sender: UISwitch) {
        action?(sender.isOn)
    }
}

private let reuseIdentifier = "Cell"

class TestCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout {
    struct Item {
        let color: UIColor
        var scaledDown: Bool
    }

    var items: [Item] = [
        .init(color: .red, scaledDown: false),
        .init(color: .green, scaledDown: false),
        .init(color: .black, scaledDown: false),
        .init(color: .orange, scaledDown: false),
        .init(color: .yellow, scaledDown: false),
        .init(color: .gray, scaledDown: false),
        .init(color: .darkGray, scaledDown: false),
        .init(color: .lightGray, scaledDown: false),
    ]

    // MARK: UICollectionViewDataSource

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return items.count
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! TestCollectionViewCell
        let item = items[indexPath.item]
        
        cell.switch.isOn = item.scaledDown
        cell.backgroundColor = item.color
        cell.action = { [weak self] (isOn: Bool) in
            guard let self = self else { return }
            self.items[indexPath.item].scaledDown = isOn
            collectionView.reloadItems(at: [indexPath])
        }
    
        return cell
    }
    
    // MARK: - UICollectionViewDelegateFlowLayout
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let dynamicHeight: CGFloat = 200
        if items[indexPath.item].scaledDown {
            return CGSize(width: view.frame.width, height: (dynamicHeight / 2))
        }
        return CGSize(width: view.frame.width, height: dynamicHeight)
    }
}

Thanks a lot! Storing the state of the cell was definitely the missing piece I was looking for. I realize it's a bit out of scope, but is it possible to animate the size changing of the cells here? — Joe, Mar 27 '21 at 22:00
@Joe this actually does the default animation of the `UICollectionView`. You can see the difference if you put `collectionView.reloadItems(at: [indexPath])` line inside `UIView.performWithoutAnimation { ... }` block. Maybe answers of [this](https://stackoverflow.com/questions/13780153/uicollectionview-animate-cell-size-change-on-selection) question can serve your purpose better. — gcharita, Mar 28 '21 at 12:19

Scaling down the height of a UICollectionViewCell dynamically

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