如何解决“值不能为空” ChartTrackBallBehavior

Question

我一直在使用UWP构建一个程序，该程序利用Telerik Charting控件向用户显示传入的数据。在我的最新测试中，未经请求的数据正在接收并在Chart上动态绘制，我遇到了一个未处理的异常（NullReferenceException）。这似乎仅在用户将鼠标悬停在带有显示TrackBall信息的图表（Telerik的ChartTrackBallBehavior）的图表上时发生。如果用户的鼠标在其他地方或没有触发TrackBall信息在图表上显示，则永远不会发生此异常。

到目前为止，我已经成功地跟踪了NullException发生在Telerik的UWP用户界面中ChartTrackBallBehavior.cs中的GetIntersectionTemplate（）函数中。除此之外，我不知道该如何解决该问题，使它不再发生。

这是发生异常时的典型堆栈跟踪：

System.ArgumentNullException: Value cannot be null.
   at Telerik.UI.Xaml.Controls.Chart.ChartTrackBallBehavior.GetIntersectionTemplate(DependencyObject instance)
   at Telerik.UI.Xaml.Controls.Chart.ChartTrackBallBehavior.UpdateIntersectionPoints(ChartDataContext context)
   at Telerik.UI.Xaml.Controls.Chart.ChartTrackBallBehavior.UpdateVisuals()
   at Telerik.UI.Xaml.Controls.Chart.RadChartBase.NotifyUIUpdated()
   at Telerik.UI.Xaml.Controls.Chart.PresenterBase.UpdateUI(ChartLayoutContext context)

在将每个系列添加到图表之前和之后，我都尝试禁用ChartTrackBallBehavior，但无济于事。 我尝试将焦点手动更改为图表以外的其他控件，但无济于事。 我尝试了在不同区域中对Chart.UpdateLayout（）的手动调用，只是导致这些调用在同一位置（ChartTrackBallBehavior.cs）中创建了相同的NullReferenceException。

问题的核心似乎是错误地将其设置为null的“值”。到目前为止，我还无法确定将什么“ Value”设置为null，我只能假定它在GetIntersectionTemplate（）函数调用内达到了NullReferenceException throw（）调用。但是我不知道为什么会这样，或者我能做些什么。

我做了一个最小的项目来复制问题。请注意，它将清除图表上的所有系列，然后在图表上重新绘制所有系列。这样做相当于我自己的项目，似乎与问题本身有关。用户可以随时更改要在图表上显示的系列，从而完成“系列清除和重新添加”过程。

我可能可以改变编码结构，以从另一个方向解决这个问题，但是目前，我想更好地了解是什么原因导致了此问题，并在可能的情况下予以解决，否则我可能需要重写了大部分代码，不幸的是，时间不在我这边。

这是示例代码。请注意，此代码使用的是Telerik.UI.for.UniversalWindowsPlatform版本1.0.1.5。

MainPage.xaml

<Page
    x:Class="ExceptionReplicator.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ExceptionReplicator"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:telerikChart="using:Telerik.UI.Xaml.Controls.Chart"
    xmlns:telerikPrimitives="using:Telerik.UI.Xaml.Controls.Primitives"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Grid>
        <telerikChart:RadCartesianChart x:Name="MainChart" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="10,10,10,10">
            <telerikChart:RadCartesianChart.Grid>
                <telerikChart:CartesianChartGrid MajorLinesVisibility="XY"/>
            </telerikChart:RadCartesianChart.Grid>

            <telerikChart:RadCartesianChart.Behaviors>
                <telerikChart:ChartPanAndZoomBehavior ZoomMode="Both" PanMode="Both"/>
                <telerikChart:ChartTrackBallBehavior x:Name="TrackBallBehaviour" InfoMode="Multiple" ShowIntersectionPoints="True">
                    <telerikChart:ChartTrackBallBehavior.LineStyle>
                        <Style TargetType="Polyline">
                            <Setter Property="Stroke" Value="Tomato"/>
                            <Setter Property="StrokeThickness" Value="2"/>
                            <Setter Property="StrokeDashArray" Value="1,2"/>
                        </Style>
                    </telerikChart:ChartTrackBallBehavior.LineStyle>
                    <telerikChart:ChartTrackBallBehavior.IntersectionTemplate>
                        <DataTemplate>
                            <Ellipse Width="10" Height="10" Fill="Tomato"/>
                        </DataTemplate>
                    </telerikChart:ChartTrackBallBehavior.IntersectionTemplate>

                </telerikChart:ChartTrackBallBehavior>
            </telerikChart:RadCartesianChart.Behaviors>

            <telerikChart:RadCartesianChart.VerticalAxis>
                <telerikChart:LinearAxis x:Name="Vertical" Title="Y Axis" Minimum="0"/>
            </telerikChart:RadCartesianChart.VerticalAxis>
            <telerikChart:RadCartesianChart.HorizontalAxis>
                <telerikChart:LinearAxis x:Name="Horizontal" Title="X Axis"/>
            </telerikChart:RadCartesianChart.HorizontalAxis>

        </telerikChart:RadCartesianChart>
    </Grid>
</Page>

MainPage.xaml.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409

namespace ExceptionReplicator
{
    using System;
    using System.Threading;
    using Telerik.UI.Xaml.Controls.Chart;
    using Windows.UI.Core;

    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
    {
        private Timer DataTimer;        // timer to periodically add data to the chart
        private int LineCount = 1;      // arbitrary line counter to differentiate lines from each other

        // custom class for holding the data to be displayed on the chart
        private class Data
        {
            public int XValue { get; set; }
            public int YValue { get; set; }
        }

        // overarching class that holds all of the data for ONE line/series
        private class DataToChart
        {
            // List of all the data points within this instance of DataToChart
            public List<Data> DataPoints;

            // Constructor to initialise DataPoints
            public DataToChart()
            {
                DataPoints = new List<Data>();
            }
        }

        // Overarching container to hold data for ALL lines/series
        private List<DataToChart> allData = new List<DataToChart>();

        public MainPage()
        {
            this.InitializeComponent();

            // set up the timer to call every 10s to add new data to the chart. warning: this will run infinitely
            DataTimer = new Timer(DataCallback, null, (int)TimeSpan.FromSeconds(10).TotalMilliseconds, Timeout.Infinite);
        }

        // Generic callback to call AddLineToChart() on the other thread to handle the Chart's data
        private void DataCallback(object state)
        {
            var task = Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => AddLineToChart());
        }

        // Code to handle adding a line to the chart
        private void AddLineToChart()
        {
            // Using Random() to create random data
            Random rand = new Random();
            DataToChart dataToChart = new DataToChart();

            for (int i = 0; i < 50; i++)
            {
                dataToChart.DataPoints.Add(new Data
                {
                    XValue = i,
                    YValue = rand.Next(0, 100)
                });
            }

            // Add the data for this line/series to the overarching container
            allData.Add(dataToChart);

            // re-initialise the line count
            LineCount = 1;

            // Currently the code needs to clear the chart and redraw it each time new data is introduced
            MainChart.Series.Clear();

            // For each line/series in the main container
            foreach (DataToChart data in allData)
            {
                // Make a series for the line
                ScatterLineSeries scatterLineSeries = new ScatterLineSeries
                {
                    Name = $"Line {LineCount}",
                    ItemsSource = dataToChart.DataPoints,
                    XValueBinding = new PropertyNameDataPointBinding("XValue"),
                    YValueBinding = new PropertyNameDataPointBinding("YValue"),
                    DisplayName = $"Line {LineCount}",
                    LegendTitle = $"Line {LineCount}",
                };

                // Add the line to the Chart's Series collection
                MainChart.Series.Add(scatterLineSeries);

                // Increment arbitrary counter
                LineCount++;
            }

            // Re-set the timer to fire again in 10s
            DataTimer.Change((int)TimeSpan.FromSeconds(10).TotalMilliseconds, Timeout.Infinite);
        }
    }
}

我需要找到一种解决方案，以确保在引入新数据时不再发生此异常。任何帮助将不胜感激。

在短期内，我将ChartTrackBallBehavior从我的Chart中完全删除（注释掉），直到确定解决方案为止。删除“行为”后，不会发生此异常。

Answer 1

您的代码中存在一些问题。

1. 您是否发现自己的IntersectionTemplate根本没有应用于您的“ ScatterLineSeries”？我检查了Telerik文档TrackBall Behavior。他们将IntersectionTemplate而不是telerikChart:LineSeries放在telerikChart:RadCartesianChart.Behaviors中。因此，您需要在后面的代码中将IntersectionTemplate应用于ScatterLineSeries。
2. 您致电MainChart.Series.Clear();来清除图表，并在每次有新数据时重新绘制它。当有大量数据时，它将导致性能问题。我建议您只将新的ScatterLineSeries添加到图表中，然后将旧数据保留在那里。
3. 您声明了List<DataToChart> allData变量。我建议您使用ObservableCollection Class。此类实现了INotifyPropertyChanged接口，当在集合中添加新数据时，它将通知UI。

结合以上三点，我制作了一个代码示例供您参考：

<Page.Resources>
    <DataTemplate x:Key="ChartTrackIntersectionTemplate">
        <Ellipse Width="10" Height="10" Fill="Tomato" />
    </DataTemplate>
</Page.Resources>
<Grid>
    <telerikChart:RadCartesianChart x:Name="MainChart" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="10,10,10,10">
        <telerikChart:RadCartesianChart.Grid>
            <telerikChart:CartesianChartGrid MajorLinesVisibility="XY" />
        </telerikChart:RadCartesianChart.Grid>

        <telerikChart:RadCartesianChart.Behaviors>
            <telerikChart:ChartPanAndZoomBehavior ZoomMode="Both" PanMode="Both" />
            <telerikChart:ChartTrackBallBehavior x:Name="TrackBallBehaviour" InfoMode="Multiple" ShowIntersectionPoints="True">
                <telerikChart:ChartTrackBallBehavior.LineStyle>
                    <Style TargetType="Polyline">
                        <Setter Property="Stroke" Value="Tomato" />
                        <Setter Property="StrokeThickness" Value="2" />
                        <Setter Property="StrokeDashArray" Value="1,2" />
                    </Style>
                </telerikChart:ChartTrackBallBehavior.LineStyle>
            </telerikChart:ChartTrackBallBehavior>
        </telerikChart:RadCartesianChart.Behaviors>

        <telerikChart:RadCartesianChart.VerticalAxis>
            <telerikChart:LinearAxis x:Name="Vertical" Title="Y Axis" Minimum="0" />
        </telerikChart:RadCartesianChart.VerticalAxis>
        <telerikChart:RadCartesianChart.HorizontalAxis>
            <telerikChart:LinearAxis x:Name="Horizontal" Title="X Axis" />
        </telerikChart:RadCartesianChart.HorizontalAxis>
    </telerikChart:RadCartesianChart>
</Grid>

public sealed partial class MainPage : Page
{
    private Timer DataTimer;        // timer to periodically add data to the chart
    private int LineCount = 1;      // arbitrary line counter to differentiate lines from each other
    private DataTemplate ChartTrackIntersectionTemplate;

    // custom class for holding the data to be displayed on the chart
    private class Data
    {
        public int XValue { get; set; }
        public int YValue { get; set; }
    }

    // overarching class that holds all of the data for ONE line/series
    private class DataToChart
    {
        // List of all the data points within this instance of DataToChart
        public List<Data> DataPoints;

        // Constructor to initialise DataPoints
        public DataToChart()
        {
            DataPoints = new List<Data>();
        }
    }

    // Overarching container to hold data for ALL lines/series
    private ObservableCollection<DataToChart> allData = new ObservableCollection<DataToChart>();

    public MainPage()
    {
        this.InitializeComponent();
        // set up the timer to call every 10s to add new data to the chart. warning: this will run infinitely
        DataTimer = new Timer(DataCallback, null, (int)TimeSpan.FromSeconds(10).TotalMilliseconds, Timeout.Infinite);
        ChartTrackIntersectionTemplate = this.Resources["ChartTrackIntersectionTemplate"] as DataTemplate;
        allData.CollectionChanged += AllData_CollectionChanged;
    }

    private void AllData_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        foreach (DataToChart data in e.NewItems)
        {
            // Make a series for the line
            ScatterLineSeries scatterLineSeries = new ScatterLineSeries
            {
                Name = $"Line {LineCount}",
                ItemsSource = data.DataPoints,
                XValueBinding = new PropertyNameDataPointBinding("XValue"),
                YValueBinding = new PropertyNameDataPointBinding("YValue"),
                DisplayName = $"Line {LineCount}",
                LegendTitle = $"Line {LineCount}",
            };
            ChartTrackBallBehavior.SetIntersectionTemplate(scatterLineSeries, ChartTrackIntersectionTemplate);
            // Add the line to the Chart's Series collection
            MainChart.Series.Add(scatterLineSeries);
            // Increment arbitrary counter
            LineCount++;
        }
    }

    // Generic callback to call AddLineToChart() on the other thread to handle the Chart's data
    private async void DataCallback(object state)
    {
        await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => AddLineToChart());
    }

    // Code to handle adding a line to the chart
    private void AddLineToChart()
    {
        // Using Random() to create random data
        Random rand = new Random();
        DataToChart dataToChart = new DataToChart();

        for (int i = 0; i < 50; i++)
        {
            dataToChart.DataPoints.Add(new Data
            {
                XValue = i,
                YValue = rand.Next(0, 100)
            });
        }

        // Add the data for this line/series to the overarching container
        allData.Add(dataToChart);
        DataTimer.Change((int)TimeSpan.FromSeconds(10).TotalMilliseconds, Timeout.Infinite);
    }
}