Why are there two instances of the ViewController and how to fix it? - objective-c

Question

First I'll give you a short overview.
I'm ...
creating a new cocoa project
customizing the AppDelegate (see listing 1)
adding a "Custom View" to my MainMenu.xib
creating a new Cocoa Class (NSViewController + XIB) in the project, calling it MyTableViewController.*
adding a "Table View" to the recently added ViewController, like described in LINK
the code of my MyTableViewController can be seen in listing 2
Now to my problem.
The table and it's content is shown.
But if I select an item of the table and press a button (connected to (IBAction)action:(id)sender) on this subview, <NSIndexSet: 0x60000022c100>(no indexes) is shown in the output (see: selectedColumnIndexes).
After experimenting a while, I found out that there are two instances of the MyTableViewController class.
Can someone please explain me why there are two instances and help me to fix this problem.
Thx
listing 1:
// FILE: AppDelegate.h
#import <Cocoa/Cocoa.h>
#class MyTableViewController;
#interface AppDelegate : NSObject <NSApplicationDelegate>
#property (nonatomic, assign) NSViewController * currentViewController;
#property (nonatomic, strong) MyTableViewController * myTableViewController;
#property (weak) IBOutlet NSView *myview;
#end
// FILE: AppDelegate.m
#import "AppDelegate.h"
#import "MyTableViewController.h"
#interface AppDelegate ()
#property (weak) IBOutlet NSWindow *window;
#end
#implementation AppDelegate
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
[self changeViewController];
}
- (void)applicationWillTerminate:(NSNotification *)aNotification {
// Insert code here to tear down your application
}
- (BOOL)applicationShouldTerminateAfterLastWindowClosed:(NSApplication *)theApplication {
return YES;
}
- (void)changeViewController
{
if ([self.currentViewController view] != nil) {
[[self.currentViewController view] removeFromSuperview];
}
switch (0) {
case 0:
default:
if (self.myTableViewController == nil) {
_myTableViewController = [[MyTableViewController alloc] initWithNibName:#"MyTableViewController" bundle:nil];
}
self.currentViewController = self.myTableViewController;
NSLog(#"EndView");
break;
}
[self.myview addSubview:[self.currentViewController view]];
[[self.currentViewController view] setFrame:[self.myview bounds]];
[self.currentViewController setRepresentedObject:[NSNumber numberWithUnsignedInteger:[[[self.currentViewController view] subviews] count]]];
[self didChangeValueForKey:#"viewController"];
NSLog(#"ViewController changed");
}
#end
listing 2:
// FILE: MyTableViewController.h
#import <Cocoa/Cocoa.h>
#interface MyTableViewController : NSViewController
#property (weak) IBOutlet NSTableView *tview;
- (IBAction)action:(id)sender;
#end
// FILE: MyTableViewController.m
#import "MyTableViewController.h"
#import "AppDelegate.h"
#interface MyTableViewController ()
#end
#implementation MyTableViewController
- (NSUInteger)numberOfRowsInTableView:(NSTableView *)tableViewObj {
return 2;
}
- (id)tableView:(NSTableView *)tableView objectValueForTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row {
if ([tableView tableColumns][0] == tableColumn) {
return #"bla";
} else if ([tableView tableColumns][1] == tableColumn) {
return #"blub";
}
NSLog(#"dropped through tableColumn identifiers");
return NULL;
}
- (IBAction)action:(id)sender {
// selectedColumnIndexes
NSLog(#"%#", [self.tview selectedColumnIndexes]);
}
#end

Answer 1

Two instances of a class, when you only expect/want one, can be caused by failing to appreciate that objects in your xib files are themselves actual automatically-generated instances of that class.
Have you dragged a blue cube into either of your xib files and set it's class to your view controller subclass? If you have, then this will account for one of the objects that you're seeing - Apple's machinery creates it on your behalf. The second object is the one created by you, in code, in your changeViewController method.
I get the impression that you're simply trying to create a window, which contains an NSTableView, which in turn gets its data from your own NSViewController subclass. Is this correct? If it is then you should do away with the second xib file, and instead just use the xib created for you when you created the project.
In Brief
Drag a blue object cube into the Interface Builder dock and set it's subclass to MyTableViewController using the Identity Inspector.
With your view controller blue-cube selected go to the Connections Inspector and drag from the view option to your table view - you must make sure you're dragging to the table view, not the scroll view or clip view that encloses it.
Select the table view (again, make sure it really is the table view you've selected), and go to it's Connections Inspector. Drag from the data source and delegate options to your blue view-controller cube.
Implement the relevant data-soruce methods
Hint: If you aren't sure which inspector is which, open the right sidebar in Xcode and select a xib file from the left file-viewer sidebar. Sit the cursor over each of the icons at the top of the right sidebar and the tool tip will tell you which is which.
Update
A good way of identifying specific objects, and something that can assist with debugging, is to set their identifier. In the attributes inspector, this is the restoration ID. Do this for your NSTableView instance, and for your NSTableColumn instances. Then, in your data-source methods do some logging with them - for instance does the table view passed as the first argument for this methods have the expected identifier, what about the table columns?