PyGST中访问UVC摄像头extra-controls及查询控件当前值的问题

我在PyQt应用里用PyGST显示UVC摄像头画面，能通过v4l2src的直接属性访问亮度、对比度这类控件，但想访问v4l2-ctl里有的对焦这类额外控件。按了解这类控件可以通过v4l2src的extra-controls（Python中为extra_controls）属性访问，但启动管道后这个属性在运行时是空的，请问哪里遗漏了？

最小示例代码 #

import sys
from PyQt5 import QtWidgets as qtw
from PyQt5 import QtCore as qtc
from PyQt5 import QtGui as qtg
from PyQt5 import uic
import subprocess
import re

#GStreamer libraries
import gi
gi.require_version('Gst', '1.0')
gi.require_version('GstVideo', '1.0')
from gi.repository import Gst, GObject, GstVideo
Gst.init(None)

class WebcamTest(qtw.QMainWindow):
    def __init__(self):
        super().__init__()

        self.resize(640,480)
        self.gst_video = VideoWidget()
        self.setCentralWidget(self.gst_video)
        self.gst_video.prepare()

class VideoWidget(qtw.QWidget):
    def __init__(self):
        super(VideoWidget, self).__init__()
        self.setAttribute(qtc.Qt.WA_NativeWindow)
        self.windowId = self.winId()
        qtw.qApp.sync()
        self.pipeline = None

    def prepare(self):
        pipeline = "v4l2src device=/dev/video0 name=source ! image/jpeg, width=640, height=480, framerate=30/1, format=MJPG ! jpegdec ! videoconvert ! xvimagesink"

        self.pipeline = Gst.parse_launch(pipeline)
        self.source = self.pipeline.get_child_by_name('source')

        bus = self.pipeline.get_bus()
        bus.add_signal_watch()
        bus.enable_sync_message_emission()
        bus.connect('sync-message::element', self.on_sync_message)
        self.pipeline.set_state(Gst.State.PLAYING)
        print(f'source properties: {self.source.list_properties()}')
        print(f'brightness={self.source.props.brightness}')
        print(f'extra-controls={self.source.props.extra_controls}')

    def on_sync_message(self, bus, msg):
        message_name = msg.get_structure().get_name()
        # qtc.qDebug(message_name)
        if message_name == 'prepare-window-handle':
            win_id = self.windowId
            assert win_id
            imagesink = msg.src
            imagesink.set_window_handle(win_id)

    def dispose(self):
        if (self.pipeline):
            self.pipeline.set_state(Gst.State.NULL)
        self.pipeline = None
    
    def isPlaying(self):
        return self.pipeline.current_state == Gst.State.PLAYING

if __name__ == '__main__':
    app = qtw.QApplication(sys.argv)
    main_ui = WebcamTest()
    main_ui.move(200,100)
    main_ui.show()
    app.exec_()

摄像头可用控件列表 #

$ v4l2-ctl -l
                     brightness 0x00980900 (int)    : min=-64 max=64 step=1 default=0 value=0
                       contrast 0x00980901 (int)    : min=0 max=64 step=1 default=32 value=32
                     saturation 0x00980902 (int)    : min=0 max=128 step=1 default=64 value=64
                            hue 0x00980903 (int)    : min=-40 max=40 step=1 default=0 value=0
 white_balance_temperature_auto 0x0098090c (bool)   : default=1 value=1
    white_balance_red_component 0x0098090e (int)    : min=1 max=500 step=1 default=100 value=100 flags=inactive
   white_balance_blue_component 0x0098090f (int)    : min=1 max=500 step=1 default=100 value=100 flags=inactive
                          gamma 0x00980910 (int)    : min=72 max=500 step=1 default=100 value=100
                           gain 0x00980913 (int)    : min=0 max=100 step=1 default=0 value=0
           power_line_frequency 0x00980918 (menu)   : min=0 max=2 default=1 value=1
                       hue_auto 0x00980919 (bool)   : default=0 value=0
      white_balance_temperature 0x0098091a (int)    : min=2800 max=6500 step=1 default=4600 value=4600 flags=inactive
                      sharpness 0x0098091b (int)    : min=0 max=6 step=1 default=3 value=3
         backlight_compensation 0x0098091c (int)    : min=0 max=2 step=1 default=1 value=1
                  exposure_auto 0x009a0901 (menu)   : min=0 max=3 default=3 value=3
              exposure_absolute 0x009a0902 (int)    : min=1 max=5000 step=1 default=157 value=157 flags=inactive
         exposure_auto_priority 0x009a0903 (bool)   : default=0 value=1
                 focus_absolute 0x009a090a (int)    : min=1 max=1023 step=1 default=1 value=688
                     focus_auto 0x009a090c (bool)   : default=0 value=0
                zoom_continuous 0x009a090f (int)    : min=0 max=0 step=0 default=0 value=0 flags=write-only
                        privacy 0x009a0910 (bool)   : default=0 value=0
                  iris_absolute 0x009a0911 (int)    : min=0 max=0 step=0 default=0 value=0
                  iris_relative 0x009a0912 (int)    : min=0 max=0 step=0 default=0 value=0 flags=write-only
                      pan_speed 0x009a0920 (int)    : min=0 max=0 step=0 default=0 value=0
                     tilt_speed 0x009a0921 (int)    : min=0 max=0 step=0 default=0 value=0

补充：已找到设置方法，但仍需查询控件值 #

已经找到设置额外控件的方法，比如下面的代码可以激活自动对焦：

extra_controls = Gst.Structure.new_from_string('i,focus_auto=1')
self.source.set_property('extra_controls', extra_controls)

但现在还是不知道怎么像v4l2-ctl -l那样查询这些额外控件的当前值。

内容的提问来源于stack exchange，提问作者Wolfy