ViewModel中计时器的更好解决方案？

Question

我在ViewModel的图形模型中有一个DispatcherTimer，用于定期更新（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视图时都会重新创建ViewModel，并且DispatcherTimer阻止了GC破坏我的ViewModel，因为Tick-Event拥有强大的引用。

我与包装解决了这个围绕它使用的DispatcherTimer FastSmartWeakEvent从CodeProject /丹尼尔·格伦沃尔德以避免强烈的参考VM和破坏自己一旦有没有更多的听众：

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>(); 
}

这就是我的用法。没有之前的“弱”，这是完全相同的：

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)
    {
        ...
    }
}

看来效果很好，但这真的是最好/最简单的解决方案，还是我错过了什么？

我在Google上一无所获，真不敢相信我是唯一在ViewModel中使用计时器来更新某些内容并造成资源泄漏的人……感觉不对！

更新

由于图形组件（SciChart）提供了一种附加修饰符（Behaviours）的方法，因此我写了一个SciChartRollingModifier，基本上就是AlexSeleznyov在他的回答中建议的内容。有了“行为”，这也是可能的，但这甚至更简单！

如果其他任何人都需要滚动的SciChart线形图，请执行以下操作：

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

我可能不完全了解您所追求的，但是对我来说，您似乎在ViewModel中增加了更多功能，无法处理。在视图模型中使用计时器会使单元测试更加困难。

我将那些步骤提取到一个单独的组件中，该组件将通知ViewModel计时器间隔已过去。并且，如果以交互行为的形式实现，则该单独的组件将确切地知道何时创建/销毁View（通过OnAttached / OnDetached方法），进而可以启动/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。