Why the EXC_BAD_ACCESS in this simple NSOutlineView data source?

Question

This is basically the "hello world" of NSOutlineView with a data source. All I do is drag an NSOutlineView into the MainMenu.xib window, connect it to an outlet, and then try to set it's datasource to a simple implementation. When I run it, I get Thread 1: EXC_BAD_ACCESS, but the stack trace in the debugger is helping me.

(lldb) bt
* thread #1: tid = 0x158e35, 0x00007fff8948d097 libobjc.A.dylib`objc_msgSend + 23, queue = 'com.apple.main-thread, stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
frame #0: 0x00007fff8948d097 libobjc.A.dylib`objc_msgSend + 23
frame #1: 0x00007fff8839e590 AppKit`loadItemEntryLazyInfoIfNecessary + 120
frame #2: 0x00007fff883d3e5f AppKit`-[NSOutlineView _rowEntryForRow:requiredRowEntryLoadMask:] + 77
frame #3: 0x00007fff883d37d1 AppKit`-[NSOutlineView frameOfCellAtColumn:row:] + 323
frame #4: 0x00007fff885d5548 AppKit`-[NSTableView drawRow:clipRect:] + 1154
frame #5: 0x00007fff885d4f7d AppKit`-[NSTableView drawRowIndexes:clipRect:] + 776
...

When I set break points, I only see one of my methods getting called. (outlineView:numberOfChildrenOfItem:) Here's my code. Any ideas?

It creates three simple "root" items, and returns a string for the values. That never get's called anyway, according to breakpoints.

In app delegate:

@implementation TmTrkAppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)notification
{
    _outlineView.dataSource = [[TmTrkOutlineViewDataSource alloc] init];
}

@end

Data source:

@interface TmTrkOutlineViewDataSource : NSObject <NSOutlineViewDataSource>

@end
...

@interface TmTrkTagItem : NSObject {

}

@end

@implementation TmTrkTagItem {

}

@end

@implementation TmTrkOutlineViewDataSource {
    NSMutableArray *_roots;
}

- (id)init {
    printf("init\n");
    self = [super init];
    if (self) {
        _roots = [[NSMutableArray alloc] init];
        for (int i=0; i<3; i++) {
            id item = [TmTrkTagItem new];
            [_roots addObject:item];
        }
        printf("Created _roots %d.\n", (int)_roots.count);
    }
    return self;
}

- (id)outlineView:(NSOutlineView *)outlineView child:(NSInteger)index ofItem:(id)item {
    if (!item) {
        printf("child: %d ofItem: %s\n", (int)index, [item description].UTF8String);
        return _roots[index];
    }
    return nil;
}

- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item {
    if (!item) return YES;
    else return NO;
}

- (NSInteger)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(id)item {
    if (!item) return _roots.count;
    else return 0;
}

- (id)outlineView:(NSOutlineView *)outlineView objectValueForTableColumn:(NSTableColumn *)tableColumn byItem:(id)item {
    NSString *val = @"Test Value";
    return val;
}

@end

This is a new project, in Xcode 5.0.2, with ARC enabled.

Answer 1

Your data source might be getting released right after you create it since your app delegate doesn't maintain a strong reference to it, and NSOutlineView's dataSource is also a weak reference.