UICollectionView how to deselect all

问题

I have a FollowVC and FollowCell Setup with collection View. I can display all the datas correctly into my uIcollection view cell using the following code with no problem.

    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

    if let cell = collectionView.dequeueReusableCellWithReuseIdentifier("FollowCell", forIndexPath: indexPath) as? FollowCell {

        let post = posts[indexPath.row]

        cell.configureCell(post, img: img)

        if cell.selected == true {
            cell.checkImg.hidden = false
        } else {
            cell.checkImg.hidden = true
        }
        return cell
    }
}

Note that I could also select and deselect multiple images using the following code

    func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {

    if deletePressed == true {
        let cell = collectionView.cellForItemAtIndexPath(indexPath) as! FollowCell
        cell.checkImg.hidden = false
    } else {
        let post = posts[indexPath.row]
        performSegueWithIdentifier(SEGUE_FOLLOW_TO_COMMENTVC, sender: post)
    }
}

func collectionView(collectionView: UICollectionView, didDeselectItemAtIndexPath indexPath: NSIndexPath) {
    let cell = collectionView.cellForItemAtIndexPath(indexPath) as! FollowCell
    cell.checkImg.hidden = true
}

When In "Select" mode, I can perform the selction of each cell and a check mark will be displayed on the cell. However, what I want to do is to have a cancel buttom to disable all the selected cell and removing the checkImg.

I have tried

    func clearSelection() {
    print("ClearSelection posts.count = \(posts.count)")

    for item in 0...posts.count - 1 {
        let indexP = NSIndexPath(forItem: item, inSection: 0)
        followCollectionView.deselectItemAtIndexPath(indexP, animated: true)
        let cell = followCollectionView.cellForItemAtIndexPath(indexP) as! FollowCell
        cell.checkImg.hidden = true
    }
}

The program crashes here giving me a fatal error: Unexpectedly found nil while unwrapping an optional error at

let cell = followCollectionView.cellForItemAtIndexPath(indexP) as! FollowCell

I dont know why it is having trouble unwrapping the cell to be my FollowCell which contains an instance of the checkImg. I already used it before in a similar situation in didSelectItemAtIndexPath and it seems to work?

Thanks,

回答1:

Not all of the selected cells may be on screen at the point when you are clearing the selection status, so collectionView.cellForItemAtIndexPath(indexPath) may return nil. Since you have a force downcast you will get an exception in this case.

You need to modify your code to handle the potential nil condition but you can also make your code more efficient by using the indexPathsForSelectedItems property of UICollectionView

 let selectedItems = followCollectionView.indexPathsForSelectedItems
 for (indexPath in selectedItems) {
     followCollectionView.deselectItemAtIndexPath(indexPath, animated:true)
     if let cell = followCollectionView.cellForItemAtIndexPath(indexPath) as? FollowCell {
        cell.checkImg.hidden = true
     }
 }

回答2:

Using Extension in Swift 4

extension UICollectionView {

    func deselectAllItems(animated: Bool) {
        guard let selectedItems = indexPathsForSelectedItems else { return }
        for indexPath in selectedItems { deselectItem(at: indexPath, animated: animated) }
    }
}

回答3:

To simplify further, you could just do

followCollectionView.allowsSelection = false
followCollectionView.allowsSelection = true

This will in fact correctly clear your followCollectionView.indexPathsForSelectedItems even though it feels very wrong.

回答4:

This answer may be useful in swift 4.2

 let selectedItems = followCollectionView.indexPathsForSelectedItems

 for (value in selectedItems) {
     followCollectionView.deselectItemAtIndexPath(value, animated:true)
     if let cell = followCollectionView.cellForItemAtIndexPath(value) as? FollowCell {
        cell.checkImg.hidden = true
     }
 }

来源：https://stackoverflow.com/questions/38430668/uicollectionview-how-to-deselect-all

标签

ios

objective-c

swift

uicollectionview

uicollectionviewcell