Question

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

最近我发现这是一个巨大的资源泄漏，因为每次导航到图形视图时都会重新创建ViewModel，并且DispatcherTimer阻止GC破坏我的ViewModel，因为Tick-Event对它有强烈的引用。

我使用DispatcherTimer周围的Wrapper解决了这个问题，它使用Codeproject / Daniel Grunwald中的FastSmartWeakEvent来避免强烈引用VM并在没有更多侦听器时自行销毁：

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>(); 
}

这就是我使用它的方式。没有＆＃34;弱＆＃34;这是完全一样的。之前：

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)
    {
        ...
    }
}

它似乎运作良好，但这真的是最好/最简单的解决方案还是我错过了什么？

我在谷歌上发现什么都没有，并且无法相信我是唯一一个在ViewModel中使用计时器来更新内容并且资源泄漏的人......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，我写了一个SciChartRollingModifier，这基本上是AlexSeleznyov在他的回答中建议的。使用行为也是可能的，但这更简单！

如果其他人需要滚动的SciChart LineGraph，这是怎么做的：

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

我可能没有得到你正在追求的东西，但对我来说，看起来你正在为ViewModel提供比它能处理的功能更多的功能。在视图模型中使用计时器会使单元测试更加困难。

我将这些步骤提取到一个单独的组件，该组件将通知ViewModel计时器间隔已过去。而且，如果实现为Interactivity Behavior，这个单独的组件将确切地知道何时创建/销毁View（通过OnAttached / OnDetached方法），并且反过来可以启动/停止计时器。

此处的另一个好处是您可以轻松地对ViewModel进行单元测试。

Answer 2

您可以将View的Closing事件绑定到ViewModel中的Command，并在Stop()上调用DispatchTimer方法。这将允许计时器和ViewModel为CG：ed。

考虑查看

<i:Interaction.Triggers>
    <i:EventTrigger EventName="Closing">
        <command:EventToCommand Command="{Binding CloseCommand}" />
    </i:EventTrigger>
</i:Interaction.Triggers>

和ViewModel

public class MyViewModel : ViewModelBase
{
    public MyViewModel()
    {
        DispatcherTimer timer = new DispatcherTimer(
            TimeSpan.FromSeconds(1),
            DispatcherPriority.Render,
            (sender, args) => Console.WriteLine(@"tick"),
            Application.Current.Dispatcher);
        timer.Start();

        CloseCommand = new RelayCommand(() => timer.Stop());
    }

    public ICommand CloseCommand { get; set; }
}

其他解决方案可能是使计时器静态或在ViewModelLocator或类似位置保持对VM的静态引用。

ViewModel中更好的计时器解决方案？

2 个答案: