Dynamically setting layout on UICollectionView causes inexplicable contentOffset change

Question

According to Apple\'s documentation (and touted at WWDC 2012), it is possible to set the layout on UICollectionView dynamically and even animate the changes:

Answer 1

2019 actual solution

Say you have a number of layouts for your "Cars" view.

Let's say you have three.

CarsLayout1: UICollectionViewLayout { ...
CarsLayout2: UICollectionViewLayout { ...
CarsLayout3: UICollectionViewLayout { ...

It will jump when you animate between layouts.

It's just an undeniable mistake by Apple. It jumps when you animate, without question.

The fix is this:

You must have a global float, and, the following base class:

var avoidAppleMessupCarsLayouts: CGPoint? = nil

class FixerForCarsLayouts: UICollectionViewLayout {
    
    override func prepareForTransition(from oldLayout: UICollectionViewLayout) {
        avoidAppleMessupCarsLayouts = collectionView?.contentOffset
    }
    
    override func targetContentOffset(
        forProposedContentOffset proposedContentOffset: CGPoint) -> CGPoint {
        if avoidAppleMessupCarsLayouts != nil {
            return avoidAppleMessupCarsLayouts!
        }
        return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
    }
}

So here are the three layouts for your "Cars" screen:

CarsLayout1: FixerForCarsLayouts { ...
CarsLayout2: FixerForCarsLayouts { ...
CarsLayout3: FixerForCarsLayouts { ...

That's it. It now works.

---

1. Incredibly obscurely, you could have different "sets" of layouts (for Cars, Dogs, Houses, etc.), which could (conceivably) collide. For this reason, have a global and a base class as above for each "set".

2. This was invented by passing user @Isaacliu, above, many years ago.

3. A detail, FWIW in Isaacliu's code fragment, finalizeLayoutTransition is added. In fact it's not necessary logically.

The fact is, until Apple change how it works, every time you animate between collection view layouts, you do have to do this. That's life!