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Radar: UITableView cell index inconsistent state after editing

on iOS radar bugreport

When working on UITableView I came across a bug in iOS, and have just got around to filing my first ever apple bug report. Under certain conditions it’s possible to get the index paths of cells in the table to become inconsistent with the content.

Filed as rdar://13846681 and on OpenRadar.

Summary

It’s possible to get a UITableView with differing cell heights into a state where the cell index paths are inconsistent with the datasource.

Steps to Reproduce

  1. Create a UITableView in which the topmost cell is over twice the height of the one immediately below it.
  2. Enable editing (specifically we want row reordering).
  3. Enter edit mode and scroll the table down so that you can only just see the reordering grabber on the top cell.
  4. Tap the grabber. This will switch cells 0 and 1 (due to the height difference).

The problem occurs only when the row switch causes a row to disappear off screen (hence why we need the top row to be over double the height of the one below it).

Expected Results

The tableView:moveRowAtIndexPath:toIndexPath: datasource method should be called with the correct index paths, and the index paths of the cells in the table should be updated appropriately.

Actual Results

The datasource method is called correctly, but the cells in the table are left with inconsistent index paths.

Regression

Tested in iOS 4, 5 & 6 with the same results.

Notes

I have attached a sample project which demonstrates this. To see the effects:

  1. Open the project
  2. Click the edit button
  3. Scroll the table down so that the grabber icon is only just visible for the top cell (cell 0).
  4. Single tap the reordering grabber.

The table now has an inconsistent index path state. The log status button shows the index path (row) for each of the cells currently visible table, and the corresponding index of the content of that cell in the backing array. These should always match. Pressing it after the above actions will reveal that this isn’t the case, and if you scroll back to the top of the table and log the status again you’ll see that some of the cell content is repeated. Further scrolling of the table will cause blank rows to appear etc.

Sample project

The sample project which I uploaded is available on GitHub at github.com/sammyd/UITableView-RadarSample.