Objective C 绘制透明窗口的方法

    [drawStrokeWindow setLevel:NSFloatingWindowLevel];
    [drawStrokeWindow setOpaque:NO];
    [drawStrokeWindow setBackgroundColor:[NSColor clearColor]];
    [drawStrokeWindow setStyleMask:NSBorderlessWindowMask];

　　不是直接设置窗口的透明度