This is the simplest way I could figure out how to make a view-based NSOutlineView

OutlineView.m

@interface AppDelegate : NSObject <NSApplicationDelegate, NSOutlineViewDataSource, NSOutlineViewDelegate>
@end

@implementation AppDelegate {
  NSWindow *_w;
  NSDictionary *_data;
}

static NSString *const kChildren = @"children";
static NSString *const kTitle = @"title";

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{

  _data = @{kTitle : @"a",
            kChildren : @[
                @{
                  kTitle: @"b",
                  kChildren : @[@{kTitle: @"c"}, @{kTitle: @"d"}, @{kTitle: @"e"}]
                  },
                @{
                  kTitle: @"f",
                  kChildren : @[@{kTitle: @"g"}, @{kTitle: @"h"}]
                  }
                ]
            };


  CGRect contentRect = CGRectMake(100, 100, 500, 400);
  _w = [[NSWindow alloc] initWithContentRect:contentRect
                                   styleMask:NSTitledWindowMask|NSClosableWindowMask
                                     backing:NSBackingStoreBuffered
                                       defer:NO];
  
  CGRect interior = (CGRect){.size = contentRect.size};


  NSScrollView *scrollView = [[NSScrollView alloc] initWithFrame:interior];
  NSOutlineView *outlineView = [[NSOutlineView alloc] initWithFrame:interior];
  outlineView.dataSource = self;
  outlineView.delegate = self;

  NSTableColumn *tc = [[NSTableColumn alloc] initWithIdentifier:@"blah"];
  [outlineView addTableColumn:tc];
  tc.width = interior.size.width;
  [outlineView setOutlineTableColumn:tc];

  scrollView.documentView = outlineView;

  _w.contentView = scrollView;
  [_w makeKeyAndOrderFront:nil];
}

- (NSInteger)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(NSDictionary *)item
{
  return item ? [item[kChildren] count] : 1;
}

- (id)outlineView:(NSOutlineView *)outlineView child:(NSInteger)index ofItem:(id)item
{
  return item ? [item[kChildren] objectAtIndex:index] : _data;
}

- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item
{
  return [item[kChildren] count] > 0;
}

- (NSView *)outlineView:(NSOutlineView *)outlineView viewForTableColumn:(NSTableColumn *)tableColumn item:(id)item
{
  NSTextField *t = [[NSTextField alloc] initWithFrame:CGRectZero];
  t.editable = NO;
  t.bordered = NO;
  t.backgroundColor = [NSColor clearColor];
  t.stringValue = item[kTitle];
  return t;
}


@end