How do I get window number from CFTypeRef in cocoa?

CFTypeRef focusedWindow;
AXUIElementCopyAttributeValue(
        frontMostApp, kAXFocusedWindowAttribute, (CFTypeRef *)&focusedWindow
);

in this case, How do I get window number from focusedWindow?

Window number is l开发者_如何学运维ike [[NSApp mainWindow] windowNumber]

It's an AXUIElement, and unfortunately, you can't get its window number AFAIK.