UICollectionViewFlowLayout movement has incorrect sizing behavior

Number:rdar://25235882 Date Originated:18-Mar-2016 09:26 AM
Status:Open Resolved:
Product:iOS Product Version:9.2.1
Classification:UI/Usability Reproducible:Always
This is a duplicate of rdar://25235459

When interactively moving cells around, the collection view flow layout keeps asking the delegate for `collectionView:layout:sizeForItemAtIndexPath:` with the index paths for the current UI representation. This results in cells jumping in size while one cell is being moved.

Steps to Reproduce:
- Open the sample project
- Run the sample project
- Tap and hold on one cell for 0.5 secs
- Drag that cell around for reordering.

Expected Results:
All cells remain in their initial size and move along the screen.

Actual Results:
Cells jump in size whenever there are cells of different size involved.

This can be fixed by implementing `collectionView:targetIndexPathForMoveFromItemAtIndexPath:toProposedIndexPath:` and keeping track of the current movement. With that information one can do a mapping inside the `collectionView:sizeForItemAtIndexPath:` method and return the correct size. However other delegate methods like `collectionView:cellForItemAtIndexPath:` do not require this mapping as new cells enter the screen while moving objects around. So that definitely does not look like the intended approach.

Sample project: https://github.com/michaelochs/radars/tree/master/25235459%20-%20CollectionViewMovementSizing


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