iOS（Swift）：对于不可见的单元格，集合视图indexPath（for：cell）为nil

Question

I have a UIViewController that populates a UICollectionView with an array of type Object :

class MyViewController: UIViewController {

    let objects: [Object]!

    weak var collectionView: UICollectionView!

    func object(for cell: MyCell) {
        guard let indexPath = self.collectionView.indexPath(for: cell) else { fatalError("No cell at index path") }
    }

}

The collectionView cells are represented by:

class MyCell: UICollectionViewCell {

    weak var delegate: MyCellDelegate?

    override func apply(_ layoutAttributes: UICollectionViewLayoutAttributes) {
        super.apply(layoutAttributes)

        let obj = self.delegate?.object(for: self)
        // configure various subviews with obj ...

    }
}

which has a delegate that talks to MyViewController to get the Object instance that is representing:

protocol MyCellDelegate: class {
    func object(for cell: MyCell) -> Object
}

This works fine when I scroll down between cells however, it crashes at guard let indexPath = self.collectionView.indexPath(for: cell) ... for cells that are off the screen when the apply(_ layoutAttributes: UICollectionViewLayoutAttributes) method is called.

Is there a way to fix this? The cells need to know which instance of Object they are representing so that the view controller can tweak the model.

Thanks a lot for any help!

Answer 1

Always assume that if a cell is offscreen, then the collection view has already recycled it. The collection view constantly does this to keep memory usage low and performance high.

To perform some action offscreen, what I often do is hold a separate object (view model, in MVVM) that a visible cell can attach to. This object is owned and retained by my objects and is never recycled. If I need to do any offscreen action, this object does it.

The cell's only jobs are:

1. to display data and
2. receive user input

Both of those require it to be visible.

When I create cells, I use prepareForReuse to detach it from the previous view model and do necessary clean up before attaching the cell to the next one.

Alternate Solution

If all you need is the index path to get your special object, you could simply use the index path from the layout attributes.

protocol MyCellDelegate: class {
    func object(for indexPath: IndexPath) -> Object
}

class MyCell: UICollectionViewCell {

    weak var delegate: MyCellDelegate?

    override func apply(_ layoutAttributes: UICollectionViewLayoutAttributes) {
        super.apply(layoutAttributes)

        let obj = self.delegate?.object(for: layoutAttributes.indexPath)
        // configure various subviews with obj ...

    }
}