Java文件I / O：为什么我总是得到I / O异常？(Java File I/O: Why do I always get an I/O exception?)

 为了顺利完成，你应该使用一个Timer 。 假设帧速率是24 fps，则意味着每1/24 = 0.0416秒或大约42毫秒是一帧。 因此，如果您的计时器每42ms计时一次，您可以向后移动mePlayer.Position ： 
 
 XAML： 
 
<MediaElement x:Name="mePlayer" Source="C:\Sample.mp4"
              LoadedBehavior="Manual" ScrubbingEnabled="True"/>
 
 码： 
 
    System.Windows.Threading.DispatcherTimer dispatcherTimer;
    int t = 240000; // 4 minutes = 240,000 milliseconds

    public MainWindow()
    {
        InitializeComponent();

        dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
        dispatcherTimer.Tick += new EventHandler(dispatcherTimer_Tick);
        //tick every 42 millisecond = tick 24 times in one second
        dispatcherTimer.Interval = new TimeSpan(0, 0, 0, 0, 42);

    }

    private void dispatcherTimer_Tick(object sender, EventArgs e)
    {
        // Go back 1 frame every 42 milliseconds (or 24 fps)
        t = t - 42;
        mePlayer.Position = TimeSpan.FromMilliseconds(t);
    }
    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        mePlayer.Play();
    }
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        // Pause and go to 4th minute of the video then start playing backward
        mePlayer.Pause();                               
        mePlayer.Position = TimeSpan.FromMinutes(4);
        dispatcherTimer.Start();
    }

To do it smoothly you should use a Timer. Assuming the frame rate is 24 fps, then it means that is one frame every 1/24 = 0.0416 second or approximately 42 millisecond. So if your timer ticks every 42ms you can move mePlayer.Position backward:
 
XAML:
 
<MediaElement x:Name="mePlayer" Source="C:\Sample.mp4"
              LoadedBehavior="Manual" ScrubbingEnabled="True"/>
 
Code:
 
    System.Windows.Threading.DispatcherTimer dispatcherTimer;
    int t = 240000; // 4 minutes = 240,000 milliseconds

    public MainWindow()
    {
        InitializeComponent();

        dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
        dispatcherTimer.Tick += new EventHandler(dispatcherTimer_Tick);
        //tick every 42 millisecond = tick 24 times in one second
        dispatcherTimer.Interval = new TimeSpan(0, 0, 0, 0, 42);

    }

    private void dispatcherTimer_Tick(object sender, EventArgs e)
    {
        // Go back 1 frame every 42 milliseconds (or 24 fps)
        t = t - 42;
        mePlayer.Position = TimeSpan.FromMilliseconds(t);
    }
    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        mePlayer.Play();
    }
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        // Pause and go to 4th minute of the video then start playing backward
        mePlayer.Pause();                               
        mePlayer.Position = TimeSpan.FromMinutes(4);
        dispatcherTimer.Start();
    }