Question

我正在为IOS和Android开发Xamarin Forms应用程序。我有3个xamarin表单页面的列表，例如X.xaml，Y.xaml，Z.xaml。页面X.xaml，Y.xaml有1并单击该按钮它打开下一个下一个Xamarin表单。最后Z.xaml有2个按钮btnBack＆amp; btnConfirm

<Button x:Name="btnBack"></Button>
<Button x:Name="btnConfirm"></Button>

当我点击后退按钮时，我想将其导航到我的上一页，但我不想像导航后退按钮或硬件后退按钮单击那样。为此，我尝试了这个代码，这是行不通的。在这段代码中，我实现了自定义单选按钮代码，并将相同的代码调用到来自LoadProviderData()和LoadProviderTimeData()的不同单选按钮数据，你可以看到代码吧在RadioGroupViewModel.cs课程中可能会产生问题帮助我。

Z.xaml代码

<custom:BindableRadioGroup x:Name="MyRadiouGroup"  ItemsSource="{Binding 
       Path=ProviderList.Values}" 
       SelectedIndex="{Binding SelectedIndex}"
       VerticalOptions="CenterAndExpand">
 </custom:BindableRadioGroup>

Z.xaml.cs代码

public partial class Z: ContentPage
        {
            public Z()
            {
                InitializeComponent();
                this.BindingContext = new RadioGroupViewModel();/
                btnBack.Clicked += btnBack_Clicked;
            }   
            private void btnBack_Clicked(object sender, EventArgs e)
            {
                var existingPages = Navigation.NavigationStack.ToList();
                foreach (var page in existingPages)
                {
                    if(page == this)
                        Navigation.RemovePage(page);
                }
            }
        }

在这段代码中，当我单击Back Button时，它会导航到上一页，即Y.xaml，当我点击Y.xaml上的一个按钮并打开Z.xaml页面时，它会显示空白。

这是我的Y.xaml代码

    <custom:BindableRadioGroup x:Name="MyRadiouGroup"  ItemsSource="{Binding Path=ProviderTimeList.Values}" 
        SelectedIndex="{Binding SelectedIndex}"
        VerticalOptions="CenterAndExpand">
    </custom:BindableRadioGroup>

Y.xaml.cs代码

public partial class Y: ContentPage
      {
        public Y()
         {
            InitializeComponent();
            this.BindingContext = new RadioGroupViewModel(); 
            btnZ.Clicked += btnZ_Clicked;
          } 
         void btnZ_Clicked(object sender, System.EventArgs e)
           {            
              Navigation.PushAsync(new Z());
            }
       }

以下是我的RadioGroupViewModel.cs班级

public class RadioGroupViewModel : BaseViewModel //updated code added
    {
        public RadioGroupViewModel()
        {
            providerList = new Dictionary<int, string>();
            providerTimeList = new Dictionary<int, string>();
            selectedIndex = -1;
            LoadProviderData();
            LoadProviderTimeData();
        }

        //Provider Data
        private void LoadProviderData()
        {
            for (int i = 0; i < 100; i++)
            {
                ProviderList.Add(i, "providerList " + i);
            }
        }

        //Provider Time
        private void LoadProviderTimeData()
        {
            for (int i = 0; i < 50; i++)
            {
                providerTimeList.Add(i, "ProviderTimeList" + i);
            }
        }

        private Dictionary<int, string> providerTimeList;
        public Dictionary<int, string> ProviderTimeList
        {
            get { return providerTimeList; }
            set
            {
                providerTimeList = value;
                OnPropertyChanged(nameof(ProviderTimeList));
            }
        }

        private Dictionary<int, string> providerList;
        public Dictionary<int, string> ProviderList
        {
            get { return providerList; }
            set
            {
                providerList = value;
                OnPropertyChanged(nameof(ProviderList));
            }
        }
}

实际上在Y.xaml和Z.xaml页面中有单选按钮。对于单选按钮自定义已实现此库https://github.com/kirtisagar/XamarinFormsRadioButtonXAML/tree/master/XFormsRadioButton并在此类https://github.com/kirtisagar/XamarinFormsRadioButtonXAML/blob/master/XFormsRadioButton/XFormsRadioButton.iOS/Renderer/RadioButtonRenderer.cs中我在第23行获得null ref异常

对于两个Xaml页面，例如Y.xaml & Z.xaml，我在RadioGroupViewModel.cs的构建LoadProviderData()和Y.xaml中的LoadProviderTimeData()实施了Z.xaml类

从navigated xaml页面内的同一个类构造函数调用相同的方法时，null引用异常

0 个答案: