swift fatal error: unexpectedly found nil while unwrapping an Optional value - ios

Question

I'm new to swift and I can't quite figure out how to address this error.
I'm creating a collection view and this is my code:
import UIKit
class FlashViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
#IBOutlet weak var collectionView: UICollectionView!
override func viewDidLoad() {
super.viewDidLoad()
// Move on ...
let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
layout.sectionInset = UIEdgeInsets(top: 20, left: 10, bottom: 10, right: 10)
layout.itemSize = CGSize(width: 90, height: 90)
collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
self.collectionView.dataSource = self
self.collectionView.delegate = self
collectionView.registerClass(CollectionViewCell.self, forCellWithReuseIdentifier: "CollectionViewCell")
collectionView.backgroundColor = UIColor.whiteColor()
self.view.addSubview(collectionView!)
}
func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
return 1
}
func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return 20
}
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCellWithReuseIdentifier("CollectionViewCell", forIndexPath: indexPath) as CollectionViewCell
cell.backgroundColor = UIColor.blackColor()
cell.textLabel?.text = "\(indexPath.section):\(indexPath.row)"
cell.imageView?.image = UIImage(named: "circle")
return cell
}
}
every time I run it the self.collectionView.dataSource = self line gets highlighted and I get the above mentioned error.

Answer 1

As you use weak reference your collection view released before you call it.
So you have to make it strong by removing "weak" keyword.
#IBOutlet var collectionView: UICollectionView!
Or make it stay in memory in another way.

Answer 2

...
collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
// because collectionView is a weak variable, it will be released here
self.collectionView.dataSource = self // error, collectionView is nil
...
As #Vitaliy1 said, you can make collectionView a strong reference, or use a local variable to hole it before you add it to the view hierarchy.
...
let collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
collectionView.dataSource = self
...
view.addSubview(collectionView)
// view establishes a strong reference to collectionView,
// so you can reference it until it is removed from the view hierarchy.
self.collectionView = collectionView
or, why not just use a subclass of UICollectionViewController