Question

大家好日子。我正在创建一个Xamarin.Forms（Portable）应用程序，我想使用OxyPlot创建一个Chart。我试过这段代码，但它有一个指向我的错误 LoadApplication（new App（））; 我的Xamarin.Android部分中的MainActivity.cs说明

“System.NullReferenceException：Object Reference未设置为对象的实例”

您认为这背后的原因是什么？

这些是我的一些代码：

SalesPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="XamarinFormsDemo.Views.SalesPage"
         xmlns:oxy="clr-namespace:OxyPlot.XamarinForms;assembly=OxyPlot.XamarinForms"
         BackgroundImage="bg3.jpg"
         Title="Sales Page">

 <StackLayout>
   <oxy:PlotView Model="{Binding OxyPlotModel}" VerticalOptions="Center" HorizontalOptions="Center" />
</StackLayout>

</ContentPage>

SalesPage.xaml

using OxyPlot;
using OxyPlot.Axes;
using OxyPlot.Series;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;   
using Xamarin.Forms;

namespace XamarinFormsDemo.Views
{
    public partial class SalesPage 
    {


    public SalesPage()
    {
        InitializeComponent();
        var plotModel = new PlotModel { Title = "OxyPlot Demo" };

        plotModel.Axes.Add(new LinearAxis { Position = AxisPosition.Bottom });
        plotModel.Axes.Add(new LinearAxis { Position = AxisPosition.Left, Maximum = 10, Minimum = 0 });

        var series1 = new LineSeries
        {
            MarkerType = OxyPlot.MarkerType.Circle,
            MarkerSize = 4,
            MarkerStroke = OxyPlot.OxyColors.White
        };

        series1.Points.Add(new DataPoint(0.0, 6.0));
        series1.Points.Add(new DataPoint(1.4, 2.1));
        series1.Points.Add(new DataPoint(2.0, 4.2));
        series1.Points.Add(new DataPoint(3.3, 2.3));
        series1.Points.Add(new DataPoint(4.7, 7.4));
        series1.Points.Add(new DataPoint(6.0, 6.2));
        series1.Points.Add(new DataPoint(8.9, 8.9));

        plotModel.Series.Add(series1);
        }
    }
}

MainActivity.cs

using System;
using Android.App;
using Android.Content.PM;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;



using ImageCircle.Forms.Plugin.Droid;

namespace XamarinFormsDemo.Droid
{
    [Activity(Label = "XamarinFormsDemo", Icon = "@drawable/recordsicon",     MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize |     ConfigChanges.Orientation)]
    public class MainActivity :     global::Xamarin.Forms.Platform.Android.FormsApplicationActivity
    {
        protected override void OnCreate(Bundle bundle)
        {
        base.OnCreate(bundle);
        global::Xamarin.Forms.Forms.Init(this, bundle);
        LoadApplication(new App());
        OxyPlot.Xamarin.Forms.Platform.Android.PlotViewRenderer.Init();
       ImageCircleRenderer.Init();
        }
    }
}

Answer 1

您没有在任何地方为页面设置DataContext。至少不在您显示的代码中。因此，绑定到OxyPlotModel属性（似乎也不存在）将不起作用。

如果您想要快速而肮脏的修复只是为了查看您在页面中制作的模型，那么只需将其分配到PlotView，就像这样：

为plotview添加name属性。 x:Name="Graph"
只需将创建的PlotModel分配给Graph的模型属性：Graph.Model = plotModel。

但是，您可能需要在OnAppearing覆盖中执行此操作，因此您的代码可能需要在该方法中向下移动，如：

public override void OnAppearing()
{
    var plotModel = new PlotModel { Title = "OxyPlot Demo" };

    plotModel.Axes.Add(new LinearAxis { Position = AxisPosition.Bottom });
    plotModel.Axes.Add(new LinearAxis { Position = AxisPosition.Left, Maximum = 10, Minimum = 0 });

    var series1 = new LineSeries
    {
        MarkerType = OxyPlot.MarkerType.Circle,
        MarkerSize = 4,
        MarkerStroke = OxyPlot.OxyColors.White
    };

    series1.Points.Add(new DataPoint(0.0, 6.0));
    series1.Points.Add(new DataPoint(1.4, 2.1));
    series1.Points.Add(new DataPoint(2.0, 4.2));
    series1.Points.Add(new DataPoint(3.3, 2.3));
    series1.Points.Add(new DataPoint(4.7, 7.4));
    series1.Points.Add(new DataPoint(6.0, 6.2));
    series1.Points.Add(new DataPoint(8.9, 8.9));

    plotModel.Series.Add(series1);
    Graph.Model = plotModel;
}

更好的方法是使用MVVM模式，通过创建一个像这样的ViewModel：

public class SalesViewModel : INotifyPropertyChanged
{
    private PlotModel _plotModel;

    public PlotModel PlotModel
    {
        get { return _plotModel; }
        set {
            _plotModel = value;
            RaisePropertyChanged();
        }
    }

    // other methods here that create your model

    public event PropertyChangedEventHandler PropertyChanged;

    private void RaisePropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
            handler (this, new PropertyChangedEventArgs (propertyName));
    }
}

然后你可以在某处实例化你的ViewModel，可能在OnAppearing（）中并将其设置为BindingContext：

public override void OnAppearing()
{
    var viewModel = new SalesViewModel();
    this.BindingContext = viewModel;
}

然后您可以使用在页面中创建的绑定：

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    x:Class="XamarinFormsDemo.Views.SalesPage"
    xmlns:oxy="clr-namespace:OxyPlot.XamarinForms;assembly=OxyPlot.XamarinForms"
    BackgroundImage="bg3.jpg"
    Title="Sales Page">

    <StackLayout>
        <oxy:PlotView Model="{Binding PlotModel}" VerticalOptions="Center" HorizontalOptions="Center" />
    </StackLayout>
</ContentPage>

每次更改RaisePropertyChanged()时，请务必致电PlotModel，以便将其反映在视图中：RaisePropertyChanged("PlotModel");

Xamarin.Forms：使用OxyPlot创建图表时出错

1 个答案: