NSOutlineView (Source List) EXC_BAD_ACCESS error with ARC - cocoa

Question

I've an ARC enabled project and within IB I've created a window that holds the source list component which I believe is just a configured NSOutlineView. I'm using the magical delegate method:
- (id)outlineView:(NSOutlineView *)outlineView viewForTableColumn:(NSTableColumn *)tableColumn item:(id)item
for which I cannot find any documentation for at all. Once this method is implemented the root node in my outline view will appear, upon which my entire model gets deallocated. Then when I try and expand the root node the app immediately crashes as model no longer exists.
If I don't use this method, my model remains, the source list works but none of the cells appear (understandably). I'm really not doing any thing fancy here at all.
I've never run into this sort of issue with ARC before, but it's late so there is a chance I've done something dumb and just can't see it. Here's the full code:
#implementation RLListController
- (void)awakeFromNib
{
RLPerson *stan = [[RLPerson alloc] initWithName:#"Stan"];
RLPerson *eric = [[RLPerson alloc] initWithName:#"Eric"];
RLPerson *ken = [[RLPerson alloc] initWithName:#"Ken"];
RLPerson *andrew = [[RLPerson alloc] initWithName:#"Andrew"];
RLPerson *daniel = [[RLPerson alloc] initWithName:#"Daniel"];
RLPerson *aksel = [[RLPerson alloc] initWithName:#"Aksel"];
[stan addChild:eric];
[stan addChild:ken];
[stan addChild:andrew];
[ken addChild:daniel];
[daniel addChild:aksel];
self.people = [#[stan] mutableCopy];
}
#pragma mark - Source List dataSource
- (NSInteger)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(id)item
{
RLPerson *person = item;
return (item != nil) ? [person.children count] : [self.people count];
}
- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item
{
RLPerson *person = item;
return (item != nil) ? [person.children count] > 0 : YES;
}
- (id)outlineView:(NSOutlineView *)outlineView child:(NSInteger)index ofItem:(id)item
{
RLPerson *person = item;
return (item != nil) ? [person.children objectAtIndex:index] : [self.people objectAtIndex:index];
}
- (id)outlineView:(NSOutlineView *)outlineView objectValueForTableColumn:(NSTableColumn *)tableColumn byItem:(id)item
{
RLPerson *person = item;
return person.name;
}
- (id)outlineView:(NSOutlineView *)outlineView viewForTableColumn:(NSTableColumn *)tableColumn item:(id)item
{
RLPerson *person = item;
NSTableCellView *cell = [outlineView makeViewWithIdentifier:#"DataCell" owner:self];
cell.objectValue = person;
[cell.textField setStringValue:person.name];
return cell;
}
#end
#implementation RLPerson
- (id)initWithName:(NSString *)name
{
self = [super init];
if(self)
{
_name = [name copy];
_children = [[NSMutableArray alloc] initWithCapacity:0];
}
return self;
}
- (void)addChild:(RLPerson *)child
{
[_children addObject:child];
}
- (void)dealloc
{
NSLog(#"dealloc");
}
#end

Answer 1

I've just figured out a similar crash in my code. I'll describe what the cause was for me... I'm pretty sure the same applies here, but I haven't tested your code.
Be aware that awakeFromNib can be called multiple times if you have multiple NIBs. I believe this is the case if you have NSTableCellView objects embedded within the NSOutlineView within your XIB file, which are extracted when you call makeViewWithIdentifier:owner: within outlineView:viewForTableColumn:item:.
Because you are creating your model objects (stan etc) within awakeFromNib, they are being recreated during these multiple calls. With each call, ARC is cleaning up the previous model objects, but NSOutlineView is still referencing them, hence the later crash when NSOutlineView tries to ask them for more information.
The fix is to move the model object creation out of awakeFromNib, perhaps into an init method instead.
Update:
Some other small points... it also took me a while to find the documentation for the magic outlineView:viewForTableColumn:item: method. For some reason, it is part of the NSOutlineViewDelegate protocol, not NSOutlineViewDataSource. I believe that if you implement this method, you don't need an implementation of outlineView:objectValueForTableColumn:byItem:.