viewDidAppear後にUICollectionReusableView/Headerの縦サイズを変更する

stackoverflowにも書いちゃったけど自己解決。
https://stackoverflow.com/questions/45747953/when-uicollectionviewflowlayout-is-used-can-i-change-the-vertical-size-of-the-h

func collectionView(_ collectionView: UICollectionView,layout collectionViewLayout: UICollectionViewLayout,referenceSizeForHeaderInSection section: Int) -> CGSize {
が最初によばれるでしょ。
困るわー

storyboardでUICollectionViewにtag=1しておくこと

class testo: UIViewController,UICollectionViewDataSource,UICollectionViewDelegate,UICollectionViewDelegateFlowLayout {
var xxx:CGFloat=1000
.
.
override func viewWillAppear(_ animated: Bool) {
}
override func viewDidAppear(_ animated: Bool) {
    xxx=2000
    let aa = self.view.viewWithTag(1) as! UICollectionView
    aa.reloadData()
}

func collectionView(_ collectionView: UICollectionView,
                    layout collectionViewLayout: UICollectionViewLayout,
                    referenceSizeForHeaderInSection section: Int) -> CGSize {
    return CGSize(width:self.view.frame.width, height:xxx)
}