Mysterious borders during MKMapView animation

I have a UIView in which I flip between a UITableView and a MKMapView. The MKMapView is created at runtime and animated in the following function:

[UIView beginAnimations:nil context:NULL];
[UIView setAnimationDuration:0.5];
[UIView setAnimationTransition:(self.mapView == nil ? UIViewAnimationTransitionFlipFromRight : UIViewAnimationTransitionFlipFromLeft)
                       forView:self.topView cache:YES];
// create new map view if necc.
if (self.mapView == nil)
{
    self.mapView = [[VTMapView alloc] initWithFrame:self.topView.bounds];
    mapView.mapType = MKMapTypeStandard;
    mapView.delegate = self;
    mapView.scrollEnabled = YES;
    mapView.zoomEnabled = YES;
    mapView.showsUserLocation = YES;
    }
// map visible? show or hide it
if (![self isMapVisible])
{
    tableView.hidden = YES;
    [self.topView insertSubview:mapView aboveSubview:self.tableView];
    mapLoadCount = 0;
} else {
    tableView.hidden = NO;
    [mapView removeFromSuperview];
}
[UIView commitAnimations];

The first time it works fine, but in future runs there are horizontal bars on the top and bottom of the map view during the animation. It开发者_运维问答 looks something like this:

I've tried playing with the cache setting, removing other views, etc. It doesn't happen in the simulator but it happens on OS 4.1.x and 3.1.x.

If I don't hide the tableView, I see bits of the table instead of the grey bars, so I think the mapview is being resized incorrectly during the flip.

Change this line:

UIView setAnimationTransition:(self.mapView == nil ? UIViewAnimationTransitionFlipFromRight : UIViewAnimationTransitionFlipFromLeft)
                   forView:self.topView cache:YES];

To this line:

UIView setAnimationTransition:(self.mapView == nil ? UIViewAnimationTransitionFlipFromRight : UIViewAnimationTransitionFlipFromLeft)
                   forView:self.topView cache:NO];

And that is the answer.