Detect when a UITableView is being reordered

Question

I have a UITableView backed by an NSFetchedResultsController which may trigger updates at any time. If the user is currently reordering rows, applying these updates will cause an exception because the table view has temporarily taken over and you get an error like

Invalid update: invalid number of rows in section [...]

How can I detect when the user has started moving a cell so I can delay updates caused by the fetched results controller? There don't seem to be any table view delegate methods to detect this. This delegate method:

- (NSIndexPath *)tableView:(UITableView *)tableView targetIndexPathForMoveFromRowAtIndexPath:(NSIndexPath *)sourceIndexPath toProposedIndexPath:(NSIndexPath *)proposedDestinationIndexPath {

Doesn't get called when the user initially detaches the first cell, only when they actually move it somewhere else.

One solution is to check isEditing in the fetched results callbacks and just do a bulk reloadData rather than dynamically inserting/deleting rows, but I'm wondering if there is a way to check specifically for the 'reordering' mode.

Answer 1

Before a table moves a row it sends...

– tableView:canMoveRowAtIndexPath:

... to its data source. You can put a edit lock there which will take effect before the cell actually moves.

You might want to rethink your design some. It sounds like you have a data-model which can be modified by two different sources simultaneously. If so, then you need to set up some kind of concurrency lock within the data-model itself so the two modifications can't actually occur at the same time. Simultaneous modifications won't just cause problems with the UI but with anything that uses the data-model.