c# - ViewModel 中计时器的更好解决方案？

Question

我在 ViewModel 中有一个用于图形组件的 DispatcherTimer，用于定期更新它(滚动它)。

最近我发现这是一个巨大的资源泄漏，因为每次我导航到图形 View 时都会新创建 ViewModel，并且 DispatcherTimer 会阻止 GC 破坏我的 ViewModel，因为 Tick-Event 对它有很强的引用。

我用一个围绕 DispatcherTimer 的包装器解决了这个问题，它使用 FastSmartWeakEvent来自 Codeproject/Daniel Grunwald，以避免对 VM 的强引用，并在没有更多监听器时自行销毁:

public class WeakDispatcherTimer
{
    /// <summary>
    /// the actual timer
    /// </summary>
    private DispatcherTimer _timer;



    public WeakDispatcherTimer(TimeSpan interval, DispatcherPriority priority, EventHandler callback, Dispatcher dispatcher)
    {
        Tick += callback;

        _timer = new DispatcherTimer(interval, priority, Timer_Elapsed, dispatcher);
    }


    public void Start()
    {
        _timer.Start();
    }


    private void Timer_Elapsed(object sender, EventArgs e)
    {
        _tickEvent.Raise(sender, e);

        if (_tickEvent.EventListenerCount == 0) // all listeners have been garbage collected
        {
            // kill the timer once the last listener is gone
            _timer.Stop(); // this un-registers the timer from the dispatcher
            _timer.Tick -= Timer_Elapsed; // this should make it possible to garbage-collect this wrapper
        }
    }


    public event EventHandler Tick
    {
        add { _tickEvent.Add(value); }
        remove { _tickEvent.Remove(value); }
    }
    FastSmartWeakEvent<EventHandler> _tickEvent = new FastSmartWeakEvent<EventHandler>(); 
}

我就是这样用的。这与之前没有“弱”的完全一样:

internal class MyViewModel : ViewModelBase
{
    public MyViewModel()
    {
        if (!IsInDesignMode)
        {
            WeakDispatcherTimer repaintTimer = new WeakDispatcherTimer(TimeSpan.FromMilliseconds(300), DispatcherPriority.Render, RepaintTimer_Elapsed, Application.Current.Dispatcher);
            repaintTimer.Start();
        }
    }

    private void RepaintTimer_Elapsed(object sender, EventArgs e)
    {
        ...
    }
}

它似乎运行良好，但这真的是最好/最简单的解决方案还是我遗漏了什么？

我在谷歌上完全找不到任何东西，不敢相信我是唯一一个在 ViewModel 中使用计时器来更新某些东西并导致资源泄漏的人......这感觉不对!

更新

由于图形组件 (SciChart) 提供了一种附加修饰符(行为)的方法，因此我编写了一个 SciChartRollingModifier，这基本上就是 AlexSeleznyov 在他的回答中所建议的。使用 Behavior 也是可能的，但这更简单!

如果其他人需要滚动 SciChart 线图，请按以下步骤操作:

public class SciChartRollingModifier : ChartModifierBase
{
    DispatcherTimer _renderTimer;

    private DateTime _oldNewestPoint;



    public SciChartRollingModifier()
    {
        _renderTimer = new DispatcherTimer(RenderInterval, DispatcherPriority.Render, RenderTimer_Elapsed, Application.Current.Dispatcher);
    }




    /// <summary>
    /// Updates the render interval one it's set by the property (e.g. with a binding or in XAML)
    /// </summary>
    private static void RenderInterval_PropertyChangedCallback(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
    {
        SciChartRollingModifier modifier = dependencyObject as SciChartRollingModifier;

        if (modifier == null)
            return;

        modifier._renderTimer.Interval = modifier.RenderInterval;
    }



    /// <summary>
    /// this method actually moves the graph and triggers a repaint by changing the visible range
    /// </summary>
    private void RenderTimer_Elapsed(object sender, EventArgs e)
    {
        DateRange maxRange = (DateRange)XAxis.GetMaximumRange();
        var newestPoint = maxRange.Max;

        if (newestPoint != _oldNewestPoint) // prevent the graph from repainting if nothing changed
            XAxis.VisibleRange = new DateRange(newestPoint - TimeSpan, newestPoint);

        _oldNewestPoint = newestPoint;
    }





    #region Dependency Properties

    public static readonly DependencyProperty TimeSpanProperty = DependencyProperty.Register(
        "TimeSpan", typeof (TimeSpan), typeof (SciChartRollingModifier), new PropertyMetadata(TimeSpan.FromMinutes(1)));

    /// <summary>
    /// This is the timespan the graph always shows in rolling mode. Default is 1min.
    /// </summary>
    public TimeSpan TimeSpan
    {
        get { return (TimeSpan) GetValue(TimeSpanProperty); }
        set { SetValue(TimeSpanProperty, value); }
    }


    public static readonly DependencyProperty RenderIntervalProperty = DependencyProperty.Register(
        "RenderInterval", typeof (TimeSpan), typeof (SciChartRollingModifier), new PropertyMetadata(System.TimeSpan.FromMilliseconds(300), RenderInterval_PropertyChangedCallback));


    /// <summary>
    /// This is the repaint interval. In this interval the graph moves a bit and repaints. Default is 300ms.
    /// </summary>
    public TimeSpan RenderInterval
    {
        get { return (TimeSpan) GetValue(RenderIntervalProperty); }
        set { SetValue(RenderIntervalProperty, value); }
    }

    #endregion




    #region Overrides of ChartModifierBase

    protected override void OnIsEnabledChanged()
    {
        base.OnIsEnabledChanged();

        // start/stop the timer only of the modifier is already attached
        if (IsAttached)
            _renderTimer.IsEnabled = IsEnabled;
    }

    #endregion


    #region Overrides of ApiElementBase

    public override void OnAttached()
    {
        base.OnAttached();

        if (IsEnabled)
            _renderTimer.Start();
    }

    public override void OnDetached()
    {
        base.OnDetached();

        _renderTimer.Stop();
    }

    #endregion
}

Answer 1

我可能无法准确理解您的需求，但在我看来，您向 ViewModel 中添加的功能似乎超出了它的处理能力。在 View 模型中使用计时器会使单元测试变得更加困难。

我会将这些步骤提取到一个单独的组件中，该组件会通知 ViewModel 计时器间隔已用完。而且，如果实现为 Interactivity Behavior ，这个单独的组件会确切地知道 View 何时创建/销毁(通过 OnAttached/OnDetached 方法)，并且反过来可以启动/停止计时器。

这里的另一个好处是您可以轻松地对该 ViewModel 进行单元测试。