Extension for the UICollectionView and UICollectionViewLayout that allows a user to move items with drag and drop.

• Include UICollectionView+Draggable catagory (and related files).
• Include QuartzCoreFramework.
• Set draggable to true on collection view.
• Set collectionViewLayout to a layout that implements the UICollectionViewLayout_Warpable protocol (will fallback if layout does not). For the default Flow layout (grid) use DraggableCollectionViewFlowLayout - you can set this in Interface Builder.
• Implement the UICollectionViewDataSource_Draggable extended protocol.

It works just like UITableView. The extended protocol contains similarly named methods related to drag and drop found in the UITableViewDataSource protocol. The moveItemAtIndexPath:toIndexPath method is only called once - when the user lifts their finger. This is acomplished by "warping" the cells by modifying the output from the layoutAttributesForElementsInRect method. This allows you to physically move the cells around without touching the data source.

This extension can work with most custom layouts - just implement the UICollectionViewLayout_Warpable protocol. An easy way to do this is subclass your layout and feed the output from layoutAttributesForElementsInRect through LSCollectionViewLayoutHelper. For an example how to do this see DraggableCollectionViewFlowLayout - you can pretty much copy and paste from there. DraggableCollectionViewFlowLayout is included for Apple's default UICollectionViewFlowLayout (see FlowLayoutDemo). There is also CircleLayoutDemo for Apple's CircleLayout example from WWDC 2012.

DraggableCollectionView is available under the MIT license.