Adding yourself as an observer to an array than an NSArrayController is bound to? - objective-c

Question

I have class called AppController which contains a mutable array of Person objects called people. Each person simply has an NSString and a float.
I also have an NSArrayController whose contentArray is bound to the people array in AppController. Then I have the usual setup of a table view bound to the array controller to show a list of all the person objects.
In my AppController class, I tried to register as an observer of the people array to add undo support. Here's how I do it:
#implementation AppController
#synthesize people;
- (id)init
{
self = [super init];
if (self != nil) {
people = [[NSMutableArray alloc] init];
undoManager = [[NSUndoManager alloc] init];
[self addObserver:self
forKeyPath:#"people"
options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
context:NULL];
}
return self;
}
- (void)undo:(id)sender
{
[undoManager undo];
}
- (void)redo:(id)sender
{
[undoManager redo];
}
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
int kindOfChange = [[change objectForKey:NSKeyValueChangeKindKey] intValue];
int index = [[change objectForKey:NSKeyValueChangeIndexesKey] firstIndex];
Person *p;
if (kindOfChange == NSKeyValueChangeInsertion) {
p = [change objectForKey:NSKeyValueChangeNewKey];
[[undoManager prepareWithInvocationTarget:self] removePersonAtIndex:index];
} else {
p = [change objectForKey:NSKeyValueChangeOldKey];
[[undoManager prepareWithInvocationTarget:self] addPerson:p atIndex:index];
}
}
- (void)addPerson:(Person *)person atIndex:(int)index
{
[[self mutableArrayValueForKey:#"people"] addObject:person];
}
- (void)removePersonAtIndex:(int)index
{
[[self mutableArrayValueForKey:#"people"] removeObjectAtIndex:index];
}
#end
With this code, I can successfully add a person object and then undo it. I cannot, however, undo deleting a person object. I always get this exception when I undo a delete operation:
[<NSCFArray 0x3009a0> addObserver:forKeyPath:options:context:] is not supported. Key path: personName
It seems like I'm not the only one, though: http://www.cocoabuilder.com/archive/cocoa/84489-modifying-the-array-of-an-nsarraycontroller.html#84496
Any ideas?
UPDATE:
Here's the backtrace:
#0 0x7fff8161b41e in -[NSArray(NSKeyValueObserverRegistration) addObserver:forKeyPath:options:context:]
#1 0x7fff8822fc47 in -[_NSModelObservingTracker _registerOrUnregister:observerNotificationsForModelObject:]
#2 0x7fff883b2bb9 in -[_NSModelObservingTracker startObservingModelObjectAtReferenceIndex:]
#3 0x7fff883acc41 in -[_NSModelObservingTracker setObservingToModelObjectsRange:]
#4 0x7fff883aca5a in -[NSTableBinder tableView:updateVisibleRowInformation:]
#5 0x7fff883ac98e in -[_NSBindingAdaptor tableView:updateVisibleRowInformation:]
#6 0x7fff882b4c7b in -[NSTableView drawRect:]
#7 0x7fff882ab081 in -[NSView _drawRect:clip:]
#8 0x7fff882a9cf4 in -[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:]
#9 0x7fff882aa05e in -[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:]
#10 0x7fff882aa05e in -[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:]
#11 0x7fff882a83c6 in -[NSView _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:]
#12 0x7fff882a9292 in -[NSView _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:]
#13 0x7fff882a9292 in -[NSView _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:]
#14 0x7fff882a7ee8 in -[NSThemeFrame _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:]
#15 0x7fff882a479a in -[NSView _displayRectIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:]
#16 0x7fff8821dff6 in -[NSView displayIfNeeded]
#17 0x7fff88218ea2 in _handleWindowNeedsDisplay
#18 0x7fff81da9077 in __CFRunLoopDoObservers
#19 0x7fff81d84ef4 in __CFRunLoopRun
#20 0x7fff81d8484f in CFRunLoopRunSpecific
#21 0x7fff8836ab31 in _NSUnhighlightCarbonMenu
#22 0x7fff8834d0c1 in -[NSMenu performKeyEquivalent:]
#23 0x7fff8834be69 in -[NSApplication _handleKeyEquivalent:]
#24 0x7fff8821caa1 in -[NSApplication sendEvent:]
#25 0x7fff881b3922 in -[NSApplication run]
#26 0x7fff881ac5f8 in NSApplicationMain
#27 0x100001469 in main at main.m:13

Answer 1

What's happening is that [change objectForKey: NSKeyValueChangeNewKey] returns NSArray *, not Person *. You should call -lastObject on the array returned to get the actual Person object (assuming that the array contains only one object).