How to get streammux message from bus in gstreamer

Hi ,everyone
I am using nvstreammux in gstreamer.
but the message of nvstreammux is Gst.MessageType.ELEMENT ,so how to parse the message from nvstreammux ?
Thanks

def bus_call(bus, message, loop,queue,bin):
    t = message.type
    print(t)
    if t == Gst.MessageType.ELEMENT:
        print(message.src)

output logs

<flags GST_MESSAGE_ELEMENT of type Gst.MessageType> 
<__gi__.GstNvStreamMux object at 0x7f5670768708 (GstNvStreamMux at 0x1ef4230)>