Qt QMediaPlayer seek/setPosition之后，QGraphicsVideoItem不再进行屏幕更新

我得到了QT5.6示例“视频播放器”。它可以处理本地文件。 我修改了“打开文件”功能以使用internet URL而不是本地文件。事实上，我使用youtube URL。像这样：

void VideoPlayer::openFile()
{
//QString fileName = QFileDialog::getOpenFileName(this, tr("Open Movie"),QDir::homePath());
QString fileName = "https://r4---sn-n8v7znls.googlevideo.com/videoplayback?initcwndbps=1427500&dur=3369.354&sver=3&sparams=dur%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Cratebypass%2Crequiressl%2Csource%2Cupn%2Cexpire&lmt=1416581653227418&requiressl=yes&ip=80.68.2.81&mime=video%2Fmp4&source=youtube&mm=31&mn=sn-n8v7znls&pl=23&id=o-AODorl7SvqZSnDSvxck5hqt4t3DHXmuquI-oLniRvtIu&ms=au&mt=1464897333&mv=m&expire=1464919182&upn=vf3wyJFr-XM&key=yt6&ipbits=0&ratebypass=yes&itag=18&fexp=9412859%2C9414702%2C9416126%2C9416891%2C9419451%2C9422596%2C9423965%2C9425351%2C9426985%2C9428398%2C9431012%2C9432822%2C9433096%2C9433221%2C9433946%2C9434790%2C9435527%2C9435876%2C9436917%2C9437067%2C9437180%2C9437553&signature=A22462E28C81BAFEA1D411B53F4AA7624179608D.BB0B47C09F09A93C3751D49EEE61AF6059A635FB";
if (!fileName.isEmpty()) {
    mediaPlayer.setMedia(QUrl(fileName));
    playButton->setEnabled(true);
    mediaPlayer.play();
    qDebug() << "playback started!!";
}

好像在玩？但是没有视频输出，没有音频输出。。没有什么。 如果是internet mp4 URL源，如何处理QMediaPlayer的seek/setPosition

void VideoPlayer::positionChanged(qint64 position)
{
positionSlider->setValue(position);
qDebug()
        << " pos " << position
        << " sta " << mediaPlayer.mediaStatus();
}

pos  2363199  sta  QMediaPlayer::BufferedMedia
pos  2364049  sta  QMediaPlayer::BufferedMedia
pos  2364049  sta  QMediaPlayer::BufferedMedia
pos  2365050  sta  QMediaPlayer::BufferedMedia
pos  2365983  sta  QMediaPlayer::BufferedMedia
pos  2365983  sta  QMediaPlayer::BufferedMedia