Why I have this error when I try to use my own CollectionViewCell class?

1.I'm putting a UICollectionView into a UIView.

2.I'm using my own UICollectionViewCell class.

3.The error is "Type 'TimeLineViewController' does not conform to protocol UICollectionViewDataSource"

4.If I change the return type of func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) to UICollectionViewCell, there would not be error.

Here are my codes:

import UIKit

class TimeLineViewController: UIViewController, UICollectionViewDataSource,UICollectionViewDelegate {

@IBOutlet weak var TimeLineColleciontView: UICollectionView!

// TODO TODO set cell size permeantly
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
    return CGSize(width: collectionView.frame.width-20,
        height: (collectionView.frame.width-20) * 1.2 )
}

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 3
}

func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
    return 1
}


func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> TimeLineCollectionViewCell {
    let id = "TimeLineCell"
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier(id, forIndexPath: indexPath) as! TimeLineCollectionViewCell
    return cell
}

override func preferredStatusBarStyle() -> UIStatusBarStyle {
    return UIStatusBarStyle.LightContent
}

override func viewDidLoad() {
    super.viewDidLoad()
    self.TimeLineColleciontView.backgroundColor = UIColor(white: 0, alpha: 0)

    TimeLineColleciontView.dataSource = self
    TimeLineColleciontView.delegate = self

    // Do any additional setup after loading the view, typically from a nib.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}


}

And my cell class is simple:

import UIKit

class TimeLineCollectionViewCell: UICollectionViewCell {

var cover : UIImageView = UIImageView()
var date : UILabel = UILabel()

override func awakeFromNib() {
    cover.frame = CGRect(x: 0, y: 0, width: self.frame.width, height: self.frame.width)
    date.frame = CGRect(x: 0, y: 0, width: 300, height: 300)
    self.insertSubview(cover, atIndex: 0)
    self.insertSubview(date, atIndex: 2)
}
}