Objective-C: Problems accessing objects in other UIViewControllers

Ignore the fact that view controllers are even involved. What you have are OBJECTS, connected together in a certain pattern. Accessing data in another view controller is no different from accessing data in any other object. There's no "magic" with view controllers, other than they have a few standardized connections to other objects.

IMHO, this is a poor design. For starters, your cell shouldn't need a reference to the view controller that the table it's in is in (read that twice, it barely makes sense just because the very idea of it is confusing). You have a strong reference to this view controller. So what happens when the OS tries to deallocate your view controller? It will never be able to, because the table view cell as a strong reference to it, keeping its retain count at 1. The same situation holds true for the cell. You risk running into a retain cycle here. Generally, child views should have weak references to their parents.

But this isn't even really a true parents/child relationship. I would suggest instead an approach like this, which all occurs in your view controller that contains the table view:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    // Assuming you set a reuse identifier "cellId" in the nib for your table view cell...
    MyCell *cell = (MyCell *)[tableView dequeueReusableCellWithIdentifier:@"cellId"];
    if (!cell) {
        // If you didn't get a valid cell reference back, unload a cell from the nib
        NSArray *nibArray = [[NSBundle mainBundle] loadNibNamed:@"MyCell" owner:nil options:nil];
        for (id obj in nibArray) {
            if ([obj isMemberOfClass:[MyCell class]]) {
                // Assign cell to obj, and add a target action for the checkmark
                cell = (MyCell *)obj;
                [cell.checkMarkButton addTarget:self action:@selector(checkPressed:) forControlEvents:whateverEventYouWant];
                break;
            }
        }
     }
     // Set the tag of the cell here, since we may get a different cell back from the reuse queue
     cell.checkMarkButton.tag = indexPath.row;

     return cell;
}

Now set up the method for the clicking of the checkmark button

- (void)checkPressed:(id)sender {
    UIButton *checkmark = (UIButton *)sender;

    // This will give you the row of the checked button
    int checkedCellRow = checkmark.tag;

    NSIndexPath *indexPath = [NSIndexPath indexPathForRow:checkedCellRow inSection:0];

    // Now you can grab a reference to that cell if you need to
    MyCell *cell = [self.tableView cellForRowAtIndexPath:indexPath];
}

This way, you keep all of the controller-related stuff in your controller class (i.e. how to handle the checkmark button being pressed), and you don't need to deal with this whackiness of referencing the view controller of your cell's table.

EDIT: I guess I should also help answer your questions...First of all, if you're saying that in your refreshVisibleViewForCell method, you're getting a nil value for self.myTableView, are you sure it is hooked up properly in IB? Even if it's hooked up, click the little x to unhook it and hook it up again to be sure. Also make sure you've @synthesized your myTableView property. Without seeing more code, an IB issue is my best guess as to why you're getting a nil value for tableView. A nil value here will result in a nil indexPath and selectedCell, also. As for your big question, you can access properties of objects within your view controller. Those properties can, of course, be objects. So in your example, if you have a tag property on selectedCell, you can access it from anywhere that you have a valid reference to selectedCell. If selectedCell is nil, the property will be nil. @class is better suited for header files. For instance, if you wanted to make your custom cell a property of your view controller, you might say:

#import <UIKit/UIKit.h>

@class MyCell;

@interface RootViewController : UIViewController

@property (nonatomic, strong) MyCell *cell;

@end

Then, in your implementation file, you would actually import MyCell.h. Giving the @class forward declaration just keeps you from having to import all of the details about the MyCell class in your header file. The header doesn't need to know about all of the properties and methods of MyCell, just that you intend on using it in the implementation file. So you @class in the header, #import in the implementation.

in RootViewController.h:

@interface RootViewController : UITableViewController <UITableViewDelegate>

in RootViewController.m:

- (void) refreshVisibleViewForCellTagged:(NSInteger)cellTag {
NSIndexPath *indexPath = [self.tableView indexPathForSelectedRow];
MyCell *selectedCell = (MyCell*)[self.myTableView cellForRowAtIndexPath:indexPath]; // nil

etc...

I'm not seeing declarations of myTableView in your RootViewController. But if your RootViewController implements UITableViewController, you can use self.tableView to access the tableview. You don't need to keep a reference to it by yourself.

@RachelD, if your RootView is more complicated than just a UITableViewController consider using a separate class, such as RootTableViewController. Then in your RootView xib, create IBOutlet for RootTableViewController to reference it. Like this:

// RootTableViewController definition
@interface RootTableViewController : UITableViewController
{
}

// RootViewController definition
@interface RootViewController : UIViewController
{
    RootTableViewController *table_c;
}

@property (nonatomic, retain) IBOutlet RootTableViewController *table_c;

Note that you need to drag an "Object" into the "Objects" section (for RootViewController) in the interface builder, and type RootTableViewController in the Custom Class section for this object. Right click this object, make sure its IBOutlet, view, 2 delegates are correctly set.

The reason why your myTableView is nil is because it's not properly initialized. I mean, if you don't use UITableViewController you are responsible for assigning it manually via interface builder or something.