Objective c NSOutlineView不显示数据源

我在一个简单的OSX项目中实现了一个NSOutlineView。我正在使用项目生成的NSViewController对象托管NSOutlineView并设置_outlineView.dataSource=self；其中self是NSViewController。我已经实现了所需的NSOutlineViewDataSource方法，所有这些方法都按预期调用，但我没有显示我的简单数据，而是观察以下内容（如有任何提示，将不胜感激）：

以下是我的NSViewController实现：

#import "ViewController.h"
@implementation ViewController- (void)viewDidLoad 
{
    [super viewDidLoad];
    dataSource = @[@"One", @"Two", @"Three", @"Three", @"Four"];
    outlineView.dataSource = self;
}

- (void)setRepresentedObject:(id)representedObject
{
    [super setRepresentedObject:representedObject];
}
- (id)outlineView:(NSOutlineView *)outlineView child:(NSInteger)index ofItem:(id)item
{
    return [dataSource objectAtIndex:index];
}
- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item
{
    return NO;
}

- (NSInteger)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(id)item
{
    return [dataSource count];
}
- (id)outlineView:(NSOutlineView *)outlineView objectValueForTableColumn:     (NSTableColumn *)tableColumn byItem:(id)item
{
    return item;
}
@end

---

你少了几个部分。看这里；