UIKit state preservation not restoring scroll offset

Question

I have an app that is using UIKit state preservation in iOS 6. I am able to save/restore the state of the view controllers, ie which tab is selected and navigation controller hierarchy, however I cannot get my table view to restore it's offset. I have a restoration identifier in my storyboard for the view as well as the view controller and the view controller (the table's data source) implements UIDataSourceModelAssociation as follows:

- (NSString *)modelIdentifierForElementAtIndexPath:(NSIndexPath *)indexPath inView:(UIView *)view
{
    TSStatus *status = [self._fetchedResultsController objectAtIndexPath:indexPath];

    return status.objectID.URIRepresentation.absoluteString;
}

- (NSIndexPath *)indexPathForElementWithModelIdentifier:(NSString *)identifier inView:(UIView *)view
{
    NSURL *statusURL = [NSURL URLWithString:identifier];
    NSManagedObjectID *statusID = [[TSDataController sharedController].persistentStoreCoordinator managedObjectIDForURIRepresentation:statusURL];
    TSStatus *status = (TSStatus *)[[TSDataController sharedController].mainContext objectWithID:statusID];

    return [__fetchedResultsController indexPathForObject:status];
}

modelIdentifierForElementAtIndexPath:inView: is getting called when the app goes into the background, however modelIdentifierForElementAtIndexPath:inView: never gets called.

Answer 1

This isn't a real answer to your question, but I've not been able to get a table view restore its contentOffset, either.

I guess this is a bug in iOS 6, because the documentation clearly states that a UITableView restores its contentOffset, when 1) it has a restorationIdentifier 2) the view controller the view belongs to has a restorationIdentifier and 3) the data source conforms to the UIDataSourceModelAssociation protocol.

You can restore the contentOffset and the selected item manually in your view controller, though:

- (void)encodeRestorableStateWithCoder:(NSCoder *)coder
{
    [super encodeRestorableStateWithCoder:coder];

    [coder encodeObject:[NSValue valueWithCGPoint:self.tableView.contentOffset] forKey:@"tableView.contentOffset"];

    NSIndexPath *indexPath = [self.tableView indexPathForSelectedRow];
    if (indexPath != nil) {
        NSString *modelIdentifier = [self modelIdentifierForElementAtIndexPath:indexPath inView:self.tableView];
        [coder encodeObject:modelIdentifier forKey:@"tableView.selectedModelIdentifier"];
    }
}

- (void)decodeRestorableStateWithCoder:(NSCoder *)coder
{
    [super decodeRestorableStateWithCoder:coder];

    CGPoint contentOffset = [[coder decodeObjectForKey:@"tableView.contentOffset"] CGPointValue];
    self.tableView.contentOffset = contentOffset;

    NSString *modelIdentifier = [coder decodeObjectForKey:@"tableView.selectedModelIdentifier"];
    if (modelIdentifier != nil) {
        NSIndexPath *indexPath = [self indexPathForElementWithModelIdentifier:modelIdentifier inView:self.tableView];
        if (indexPath != nil) {
            [self.tableView selectRowAtIndexPath:indexPath animated:NO scrollPosition:UITableViewScrollPositionNone];
        }
    }
}

I have no idea why UITableView doesn't do that automatically, even though the documentation says it does. If someone knows the answer, please comment.