c++ - 如果我用它调用函数，QMediaPlayer.play() 不起作用

Question

我正在用 Qt 创建一个音频播放器，我正在实现当我加载一首歌曲时它会自动开始播放，但是当我调用开始播放歌曲的函数并计算持续时间时，歌曲不会开始播放。我使用了调试，当我到达播放功能时，调试以结束步进范围停止。我也尝试发出信号但无济于事，只有按下播放按钮时歌曲才会开始。

这里是代码

void AudioPlayer::loadClicked()
{
    QString filename = QFileDialog::getOpenFileName(this, "Select an audio file", "C:/Users/belli/Music" ,"File Mp3 (*.mp3)");

    if(!filename.isEmpty())
    {
        qDebug("%s", filename.toLatin1().constData());
        player.setMedia(QUrl::fromLocalFile(filename));
        this->volumeSliderMoved();
        QFileInfo fi(filename);
        QString name = fi.fileName();
        ui->label->setText(name);
        this->playClicked();
    }
}

void AudioPlayer::playClicked()
{
    player.play();
    songLength = player.duration();
    ui->horizontalSliderPosition->setRange(0, songLength);
}

void AudioPlayer::stopClicked()
{
    player.stop();
    ui->horizontalSliderPosition->setValue(0);
}

void AudioPlayer::playerPositionChanged(qint64 pos)
{
   ui->horizontalSliderPosition->setValue(pos);
   if(pos == songLength)
       this->stopClicked();
}

void AudioPlayer::volumeSliderMoved()
{
    player.setVolume(ui->horizontalSliderVolume->value());
}

void AudioPlayer::positionSliderMoved()
{
    player.setPosition((quint64) ui->horizontalSliderPosition->value());
}

和连接

ui(new Ui::AudioPlayer)
{
    ui->setupUi(this);
    connect(ui->pushButtonLoad, SIGNAL(clicked(bool)), SLOT(loadClicked()));
    connect(ui->pushButtonPlay, SIGNAL(clicked(bool)), SLOT(playClicked()));
    connect(ui->pushButtonStop, SIGNAL(clicked(bool)), SLOT(stopClicked()));
    connect(ui->horizontalSliderVolume, SIGNAL(valueChanged(int)), SLOT(volumeSliderMoved()));
    connect(ui->horizontalSliderPosition, SIGNAL(sliderReleased()), SLOT(positionSliderMoved()));

    connect(&player, SIGNAL(positionChanged(qint64)), SLOT(playerPositionChanged(qint64)));
}

Answer 1

你错过了 setMedia异步工作。也就是说，它返回时无法保证已从 docs 加载媒体文件。 :

This function returns immediately after recording the specified source of the media. It does not wait for the media to finish loading and does not check for errors. Listen for the mediaStatusChanged() and error() signals to be notified when the media is loaded and when an error occurs during loading.

这意味着当您在调用 player.setMedia(...); 之后调用 this->playClicked(); 时，很有可能媒体文件尚未加载，调用 player.duration()失败(它不会返回您尝试播放的媒体文件的实际持续时间)。因此，您最终的持续时间 slider 的范围为 (0, 0)，这导致稍后调用 player.stop()，因为您认为您已到达媒体文件的末尾。 . .

要解决这个问题，您需要按照文档中的说明进行操作。请确保不要前进到您的 playClicked 插槽，除非您确保您的媒体文件已完全加载。你可以通过替换你的来做到这一点:

this->playClicked();

像这样:

connect(&player, &QMediaPlayer::mediaStatusChanged,
        this, [&](QMediaPlayer::MediaStatus status){
    if(status == QMediaPlayer::LoadedMedia) playClicked();
});