选择器未触发所选项目答案

【问题标题】：Picker not firing to selecteditem选择器未触发所选项目
【发布时间】：2020-07-02 19:46:31
【问题描述】：

我在选择器中选择值时遇到问题。我已经实现了视图模型，并且可以显示选项，但是一旦我选择了该项目，所选选项就不会被触发并且 returnig 值为空。我已经在 baseviewmodel 中通知了，请您帮忙

  class CategoriesViewModel : BaseViewModel
    {
        public CategoriesViewModel()
        {
            _mainSection = FillMainSection();
        }
        private List<MainSection> _mainSection = new List<MainSection>();
        private MainSection _selectedMainSection;
        public List<MainSection> MainSection
        {
            get => _mainSection;
            set => SetProperty(ref _mainSection, value);
        }

        public MainSection SelectedMainSection
        {
            get => _selectedMainSection;
            set
            {
                
                switch (_selectedMainSection.Position)
                {
                    case 0:
                        _selectedMainSection.Value = 0;
                        break;
                    case 1:
                        _selectedMainSection.Value = 1;
                        break;
                }
                SetProperty(ref _selectedMainSection, value);
            }
        }

        public List<MainSection> FillMainSection()
        {
            var newListMainSection = new List<MainSection>()
            {
                new MainSection()
                {
                    Position = 0,
                    Text = Resources.AppResources.LabelPetsCategory,
                    Value = 0
                },
                new MainSection()
                {
                    Position = 1,
                    Text = Resources.AppResources.LabelLostAndFound,
                    Value = 1
                },
              
            };

            return newListMainSection;
        }
    }
}

Xaml 
    <Picker
                                        x:Name="SectionPicker"
                                        Title="{x:Static resources:AppResources.LabelSection}"
                                        Grid.Row="2"
                                        ItemDisplayBinding="{Binding Text}"
                                        ItemsSource="{Binding MainSection}"
                                        SelectedItem="{Binding SelectedMainSection, Mode=OneWay}" 
                                        
                                        />

【问题讨论】：

标签： xamarin.forms

【解决方案1】：

我用你的代码做一个测试。这是正在运行的 gif。

首先我将SelectedItem的绑定模式更改为Mode=TwoWay，然后我添加一个Label来显示来自Picker的选择项。

  <StackLayout>
        <!-- Place new controls here -->
        <Picker
                 x:Name="SectionPicker"
                 Title="title"
                 Grid.Row="2"
                 ItemDisplayBinding="{Binding Text}"
                 ItemsSource="{Binding MainSection}"
                 SelectedItem="{Binding SelectedMainSection, Mode=TwoWay}"              
                                        />
        <Label Text="test" TextColor="OrangeRed" />
        <Label Text="{Binding MainSectionText}" TextColor="OrangeRed" />
    </StackLayout>

这里是我的CategoriesViewModel.cs，请注意SelectedMainSection，Position和Value的值相同，为什么需要使用switch重新设置，如果你使用MVVM，你使用@987654332 @，它会为模型更新，你不需要为这个模型再次更新属性。

   public class CategoriesViewModel : BaseViewModel
    {
        private List<MainSection> _mainSection;
        private MainSection _selectedMainSection;
        public CategoriesViewModel()
        {
            _mainSection = FillMainSection();
          //  _selectedMainSection = new MainSection();
        }
       
        public List<MainSection> MainSection
        {
            get => _mainSection;
            set => SetProperty(ref _mainSection, value);
        }

      
        public MainSection SelectedMainSection
        {
            get => _selectedMainSection;
            set
            {
                SetProperty(ref _selectedMainSection, value);
                MainSectionText = "select item" + _selectedMainSection.Value.ToString();
              //  switch (_selectedMainSection.Position)
               // {
              //      case 0:
              //          _selectedMainSection.Value = 0;
              //          break;
              //      case 1:
              //          _selectedMainSection.Value = 1;
              //          break;
             //   }
               
               
            }
        }
        private string _mainSectionText;
        public string MainSectionText
        {
            get
            {
                return _mainSectionText;
            }
            set
            {
                SetProperty(ref _mainSectionText, value);
            }
        }
        public List<MainSection> FillMainSection()
        {
            var newListMainSection = new List<MainSection>()
            {
                new MainSection()
                {
                    Position = 0,
                    Text = "LabelPetsCategory",
                    Value = 0
                },
                new MainSection()
                {
                    Position = 1,
                    Text = "LabelLostAndFound",
                    Value = 1
                },

            };

            return newListMainSection;
        }
    }

    public class MainSection
    {
        public MainSection()
        {
        }

        public int Position { get; set; }
        public string Text { get; set; }
        public int Value { get; set; }
    }

}

【讨论】：