展开可选值时，UiCollectionView单元意外发现为零

Question

In this function I'm calling didSelectItemAtIndexPath to notify the collection view that I want certain cells selected:

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

//some code here
....
//cell!.contentView.addSubview(pulseView)
        if (strings_chosen[indexPath.row] as! Bool) {
            self.collectionView(self.collectionView!, didSelectItemAtIndexPath: indexPath)
            cell!.backgroundColor = MaterialColor.grey.lighten1;
        } else {
            cell!.backgroundColor = MaterialColor.white
            self.collectionView(self.collectionView!, didDeselectItemAtIndexPath: indexPath)
        }
...
}

However I'm getting nil here (for cell ). Specifically the error is fatal error: unexpectedly found nil while unwrapping an Optional value :

override func collectionView(collectionView: UICollectionView, didDeselectItemAtIndexPath indexPath: NSIndexPath) {
    let cell:UICollectionViewCell? = collectionView.cellForItemAtIndexPath(indexPath)
    cell!.selected = false; //THIS LINE IS GIVING ME AN ERROR
    strings_chosen[indexPath.row] = false;
    cell!.backgroundColor = MaterialColor.white
    updateDeleteButtonTitle()
}

I don't understand why cell is nil, I'm pretty sure I'm grabbing the cell at the correct indexPath. Any reason why cell could be nil?

Answer 1

当该单元格在屏幕上不可见时， cellForItemAtIndexPath返回nil 。

Answer 2

It will be calling cellForItemAtIndexPath inside of didDeselectItemAtIndexPath before the cell is returned inside of cellForItemAtIndexPath, so the cell is nil.

Instead of calling the functions directly try calling selectItemAtIndexPath and deselectItemAtIndexPath.