important question

[yunyun1989114]

viewForSupplementaryElementOfKind in collection view datasource function
but it is never be called
i want to know why

this is my source
override func viewDidLoad() {
super.viewDidLoad()
mCollectView.backgroundColor = UIColor.whiteColor()
mCollectView.dataSource = self
mCollectView.delegate = self
collectLayout.headerReferenceSize = CGSizeMake(mCollectView.frame.width, 100)
mCollectView.registerClass(UICollectionViewCell.classForCoder(), forCellWithReuseIdentifier: "cellID")
mCollectView.registerClass(YjiSectionHeader.classForCoder(), forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: "MyHeader")
mCollectView.collectionViewLayout = collectLayout
}

@IBAction func changeMode(sender: AnyObject) {
    mCollectView.setCollectionViewLayout(collectLayout2, animated: true)
}

// section number
func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
    return 2
}

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 2
}

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    let cellID = "cellID"
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier(cellID, forIndexPath: indexPath)
    cell.backgroundColor = UIColor.yellowColor()

    return cell
}


// ヘッダーを表示する
func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -> UICollectionReusableView {
    var header : YjiSectionHeader? = nil
    if (kind == UICollectionElementKindSectionHeader) {
        header = collectionView.dequeueReusableSupplementaryViewOfKind(kind, withReuseIdentifier: "MyHeader", forIndexPath: indexPath) as? YjiSectionHeader
        if header == nil {
            header = YjiSectionHeader(frame: CGRectMake(0, 0, collectionView.frame.width, 50))

        }
    }
    return header!
}
// ヘッダーのサイズを設定する
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
    return CGSize(width: collectionView.frame.width, height: 100)
}