将 NSProgressIndicator 添加到停靠栏图标

我正在创建一个应用程序，它应该在停靠图标中显示进度条。目前我有这个，但它不起作用：

  NSProgressIndicator *progressIndicator = [[NSProgressIndicator alloc] initWithFrame:NSMakeRect(0.0f, 0.0f, 10.0f, 20.0f)];
  [progressIndicator setStyle:NSProgressIndicatorBarStyle];
  [progressIndicator setIndeterminate:NO];
  [[[[NSApplication sharedApplication] dockTile] contentView] addSubview:progressIndicator];
  [progressIndicator release];

还是我必须自己在码头上画它？谁能帮我？谢谢。

最后我不得不使用以下代码，因为 contentView 为空：

    docTile = [[NSApplication sharedApplication] dockTile];
    NSImageView *iv = [[NSImageView alloc] init];
    [iv setImage:[[NSApplication sharedApplication] applicationIconImage]];
    [docTile setContentView:iv];

    progressIndicator = [[NSProgressIndicator alloc]
                                              initWithFrame:NSMakeRect(0.0f, 0.0f, docTile.size.width, 10.)];
    [progressIndicator setStyle:NSProgressIndicatorBarStyle];
    [progressIndicator setIndeterminate:NO];
    [iv addSubview:progressIndicator];

    [progressIndicator setBezeled:YES];
    [progressIndicator setMinValue:0];
    [progressIndicator setMaxValue:1];
    [progressIndicator release];

    [self setProgress:[NSNumber numberWithFloat:-1]];
}

- (void)setProgress:(NSNumber *)fraction {
    if ( [fraction doubleValue] >= 0 ) {
        [progressIndicator setDoubleValue:[fraction doubleValue]];
        [progressIndicator setHidden:NO];
    }
    else
        [progressIndicator setHidden:YES];
    [docTile display];
}