Parallax scrolling is a big UI design trend these days. It is a technique where background images move by the camera slower than foreground images, creating an illusion of depth in a 2D scene and adding to the immersion.
In this example we are using UICollectionView which is subclass of a UIScrollView and two images per CollectionView cell UICollectionViewCell. Each image is contained inside an UIImageView within the cell:
- Background image which we are going to use for the parallax scrolling effect. The image here can be an image of some scenery
- Foreground alpha image which remains static on top of the background image. The foreground can be a transparent image with some text
override public func collectionView(collectionView: UICollectionView,
cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
let cell:ParallaxCollectionViewCell =
collectionView.dequeueReusableCellWithReuseIdentifier(
"parallaxCellId", forIndexPath: indexPath) as! ParallaxCollectionViewCell
cell.backgroundImage = UIImage(named: "\(images[indexPath.row])_background.png")!
cell.foregroundImage = UIImage(named: "\(images[indexPath.row])_alpha.png")!
return cell
}
The parallax scrolling is achieved by overriding UIScrollView delegate method scrollViewDidScroll where we change
the Y axis offset of the background image view in relation to the parent cell:
override public func scrollViewDidScroll(scrollView: UIScrollView) {
for cell in collectionView!.visibleCells() {
var cell:ParallaxCollectionViewCell = cell as! ParallaxCollectionViewCell
let yOffset:CGFloat = ((collectionView!.contentOffset.y - cell.frame.origin.y) /
cell.backgroundImageView.bounds.height) * ParallaxConstants.OffsetSpeed
cell.setBackgroundImageOffset(CGPointMake(0, yOffset))
}
}
We are using delegate collectionView:layout:sizeForItemAtIndexPath: from UICollectionViewDelegateFlowLayout to control the height of cells in our UICollectionView. In other words,
how many cells should be visible to user at once:
public func collectionView(collectionView: UICollectionView,
layout collectionViewLayout: UICollectionViewLayout,
sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
return CGSizeMake(self.view.bounds.width,
self.view.bounds.height / ParallaxConstants.CellsPerCollectionView)
}