'Exchange' is semantically wrong

[SteveFortune]

The implementation of exchange does not actually ever exchange. It drops.

We need to review 'append' vs 'exchange'.

[SteveFortune]

I propose the follow in replace of 'exchange' and 'append':

• First, the coordinator checks whether we're dropping on a specific index path, and if so, whether the data source implements [can]dropFrom:(NSIndexPath *)from inCollection:(UIView<I3Collection> *)fromCollection to:(NSIndexPath *)to inCollection(UIView<I3Collection> *)toCollection
  • If it does implement these methods and explicitly allows to drop on the given index path, we drop
  • If it does implement these methods and explicitly disallows to drop on the given index path, we snap back
  • Iff it does not implement these methods, then we check for [can]dropFrom:(NSIndexPath *)from inCollection:(UIView<I3Collection> *)fromCollection toPoint:(CGPoint)to inCollection(UIView<I3Collection> *)toCollection (this replaces append)

[SteveFortune]

To summarise this is the new alg:

• If data source says delete at this drop origin
  • Delete
• Else if stop drag on collection && if drag stopped on same collection as origin && valid item at drop origin && data source says rearrangeable
  • Rearrange
• Else if drag stopped on different collection
  • If data source implements drop on index selectors
    • If data source allows drop on index
      • Drops on index
    • Else
      • Reset
  • Else if data source implements drop on point selectors && can drop on point
    • Drops on point
  • Else
    • Reset
• Else
  • Reset