.net MAUI Picker SelectedItem 在集合内时不会导致项目显示

Question

选择器位于集合内部并按行呈现，选择起作用并触发命令，但在恢复表单时它不显示所选项目，它显示空项目而不是绑定到 xaml 内的 selectedItem 的地址属性。

这是xaml代码：

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:sys="clr-namespace:System;assembly=mscorlib"
             xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
             xmlns:appCtrl="clr-namespace:WS.ORD.Apps.POS.Controls"
             xmlns:appVM="clr-namespace:WS.ORD.Apps.POS.ViewModels"
             xmlns:appModel="clr-namespace:WS.ORD.Apps.POS.Models"
             x:Class="WS.ORD.Apps.POS.Views.PrinterSettingsPage"
             x:Name="page">

    <ContentPage.BindingContext>
        <appVM:PrinterSettingsVM />
    </ContentPage.BindingContext>

    <Shell.BackButtonBehavior>
        <BackButtonBehavior IsVisible="False" />
    </Shell.BackButtonBehavior>

    <ContentPage.Content>
        <VerticalStackLayout BindableLayout.ItemsSource="{Binding Printers}" Spacing="30" Margin="100,50">
            <BindableLayout.ItemTemplate>
                <DataTemplate x:DataType="appModel:PrinterModel">
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto" />
                        </Grid.RowDefinitions>

                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="*" />
                            <ColumnDefinition Width="*" />
                        </Grid.ColumnDefinitions>

                        <Label Grid.Row="0" Grid.Column="0" Text="{Binding Name}" VerticalOptions="Center" />

                        <Picker Grid.Row="0" Grid.Column="1"
                         ItemsSource="{Binding Source={RelativeSource AncestorType={x:Type appVM:PrinterSettingsVM}},Path=Devices}"
                         ItemDisplayBinding="{Binding .}"
                         HorizontalOptions="Fill"
                         VerticalOptions="Center"
                         SelectedItem="{Binding Address, Mode=TwoWay}"
                         Margin="0,0,0,10">
                            <Picker.Behaviors>
                                <toolkit:EventToCommandBehavior EventName="SelectedIndexChanged" Command="{Binding Source={x:Reference page}, Path=BindingContext.PrinterSelectCommand}" CommandParameter="{Binding .}" />
                            </Picker.Behaviors>
                        </Picker>
                    </Grid>
                </DataTemplate>
            </BindableLayout.ItemTemplate>
        </VerticalStackLayout>
    </ContentPage.Content>
</ContentPage>

这是隐藏代码：

  public partial class PrinterSettingsPage : ContentPage
  {
      public PrinterSettingsPage()
      {
          InitializeComponent();
      }
  }

这是视图模型：

namespace WS.ORD.Apps.POS.ViewModels
{
    internal class PrinterSettingsVM : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        public ICommand PrinterSelectCommand { get; private set; }


        public PrinterSettingsVM()
        {
            this.Devices[0] = "device1";
            this.Devices[1] = "device2";
            this.Devices[2] = "device3";


            this.Printers = new List<PrinterModel>()
            {
                new PrinterModel { Id=Guid.NewGuid(), Name="Printer 2", Address="device1" },
                new PrinterModel { Id=Guid.NewGuid(), Name="Printer 2", Address="device2" }
            };
        }


        private string[] _devices = new string[3];
        public string[] Devices
        {
            get { return this._devices; }
            set
            {
                this._devices = value;

                if (this.PropertyChanged != null)
                {
                    this.PropertyChanged(this, new PropertyChangedEventArgs(nameof(this.Devices)));
                }
            }
        }


        private IEnumerable<PrinterModel> _printers;
        public IEnumerable<PrinterModel> Printers
        {
            get { return this._printers; }
            set
            {
                this._printers = value;

                if (this.PropertyChanged != null)
                {
                    this.PropertyChanged(this, new PropertyChangedEventArgs(nameof(this.Printers)));
                }
            }
        }
    }
}

最后是模型：

namespace WS.ORD.Apps.POS.Models
{
    public class PrinterModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        public Guid Id { get; set; }
        public string Name { get; set; }


        private string _address;
        public string Address
        {
            get { return this._address; }
            set
            {
                this._address = value;

                if (this.PropertyChanged != null)
                {
                    this.PropertyChanged(this, new PropertyChangedEventArgs(nameof(this.Address)));
                }
            }
        }
    }
}

Answer 1

在非常简单的情况下，当这个应用程序启动时，预期的行为是 查看选择的选择器，但没有。

您的应用程序关闭后，所有变量都会被释放，因此应用程序重启后数据将不会恢复。

但是您可以使用

local databases

、

Preferences

或

SecureStorage

来保存数据。并且您可以在重新启动应用程序后恢复它。

1.

NET MAUI local databases

SQLite 数据库引擎允许 .NET MAUI 应用程序在共享代码中加载和保存数据对象。您可以将 SQLite.NET 集成到 .NET MAUI 应用程序中，以在本地数据库中存储和检索信息。

2.

Preferences

Preferences

使用 String 键存储。首选项的值必须是以下数据类型之一：

Boolean
Double
Int32
Single
Int64
String
DateTime

3.

ISecureStorage

ISecureStorage 有助于安全地存储简单的键/值对。

将给定密钥的值保存在安全存储中：

 await SecureStorage.Default.SetAsync("oauth_token", "secret-oauth-token-value");

要从安全存储中检索值：

string oauthToken = await SecureStorage.Default.GetAsync("oauth_token");

if (oauthToken == null)
{
    // No value is associated with the key "oauth_token"
}