Reusing detailed view controller in UISplitViewController

Question

Basically when I implement a split view like the one presented in the Apple example 'MultipleDetailsViews', everything works fine, it allocates a new detailed view each time a row is selected. Here is the relevant code from the example:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {    
    UIViewController <SubstitutableDetailViewController> *detailViewController = nil;

    if (row == 0) {
        FirstDetailViewController *newDetailViewController = [[FirstDetailViewController alloc] initWithNibName:@"FirstDetailView" bundle:nil];
        detailViewController = newDetailViewController;
    }

    // ...

    NSArray *viewControllers = [[NSArray alloc] initWithObjects:self.navigationController, detailViewController, nil];
    splitViewController.viewControllers = viewControllers;

    // ...
}

But what I'm looking for is reusing detailed view controllers, that is lazily allocating a view controller when it is selected, and keeping a reference on it in my object. That way when another row is selected the view controller is not deallocated and when it is selected again it would be reused instead of allocating a new one. Here is the relevant code:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {    
    UIViewController <SubstitutableDetailViewController> *detailViewController = nil;

    if (row == 0) {
        if (self.firstDetailViewController == nil) {
           FirstDetailViewController *newDetailViewController = [[FirstDetailViewController alloc] initWithNibName:@"FirstDetailView" bundle:nil];
           self.firstDetailViewController = newDetailViewController;
           [newDetailViewController release];
        }
        detailViewController = self.firstDetailViewController;
    }

    // ...

    NSArray *viewControllers = [[NSArray alloc] initWithObjects:self.navigationController, detailViewController, nil];
    splitViewController.viewControllers = viewControllers;

    // ...
}

self.firstDetailViewController is instanciated the first time the first row is selected then it is reused.

When I'm doing that it works well in landscape mode but in portrait mode after a few clicks in the popover menu it raises an exception: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Popovers cannot be presented from a view which does not have a window.'

So why I care? and why I don't want to reallocate view controllers? because in some of them I want to perform tasks that wouldn't be interrupted (killed) if the user navigate in a new detailed view while the task isn't completed yet.

Is there someone with an idea of what happens or with a working implementation of what I'm trying to achieve?

Answer 1

The view controllers are designed to be created and thrown away, if you need something to run in the background for longer then best move it into the master view controller or a separate object.

If you do however want to experiment with reusing a view controller this can be achieved by setting the viewControllers property on the new navigation controller with the previous detail controller saved in the viewDidLoad:

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    if ([[segue identifier] isEqualToString:@"showDetail"]) {
        NSIndexPath *indexPath = [self.tableView indexPathForSelectedRow];
        NSManagedObject *object = [[self fetchedResultsController] objectAtIndexPath:indexPath];
        UINavigationController* navigationController = (UINavigationController*)[segue destinationViewController];

        // reuse existing controller
        navigationController.viewControllers = @[self.detailViewController];

        // update the detail controller as normal.
        [controller setDetailItem:object];
        controller.navigationItem.leftBarButtonItem = self.splitViewController.displayModeButtonItem;
        controller.navigationItem.leftItemsSupplementBackButton = YES;
    }
}