具有数据绑定的默认选择器项

如何解决具有数据绑定的默认选择器项

我正在尝试在 Xamarin.Forms 中创建一个下拉列表，允许用户选择他们选择的货币符号。我希望 CurrencyList 的第一个元素是默认选择的符号。

目前，默认情况下没有选择任何元素 - 即使我在 Page1.cs 文件中使用了 CurrencyPicker.Selectedindex = 1;。

Page1.xaml：

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:local="clr-namespace:appproject.viewmodels"
             x:Class="appproject.Views.Page1">

    <ContentPage.BindingContext>
        <local:Currencyviewmodel></local:Currencyviewmodel>
    </ContentPage.BindingContext>


    <Grid Margin="0,0">
        Grid.ColumnDeFinitions>
            <ColumnDeFinition Width="0.1*" />
            <ColumnDeFinition Width="0.0*" />
            <ColumnDeFinition Width="0.4*" />
            <ColumnDeFinition Width="0.5*" />
        </Grid.ColumnDeFinitions>
        <Grid.RowDeFinitions>
            <RowDeFinition Height="50" />
        </Grid.RowDeFinitions>

       <Picker ItemdisplayBinding="{Binding CurrencySymbol}" x:Name="CurrencyPicker" ItemsSource="{Binding CurrencyList}" Grid.Column="0" Grid.Row="0" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"/>
       <Entry Grid.Column="2" Grid.Row="0" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"/>
    </Grid>

Page1.cs：

using System;
using System.Collections.Generic;
using System.Text;

namespace appproject.Views
{
    class Page1{
        CurrencyPicker.Selectedindex = 1;
    }
}

货币.cs：

using System;
using System.Collections.Generic;
using System.Text;

namespace appproject.Models
{
public class Currency
{
    public int CurrencyID { get; set; }
    public string CurrencySymbol { get; set; }
}
}

Currencyviewmodels.cs：

using appproject.Models;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Text;

namespace appproject.viewmodels
{
class Currencyviewmodel
{
    public IList<Currency> CurrencyList { get; set; }

    public Currencyviewmodel() 
    {
        try
        {
            CurrencyList = new ObservableCollection<Currency>
            {
                new Currency { CurrencyID = 1,CurrencySymbol = "£" },new Currency { CurrencyID = 2,CurrencySymbol = "$" },new Currency { CurrencyID = 3,CurrencySymbol = "$" }
            };

        }
        catch (Exception)
        {

        }       
    }

 }
}

如何使第一个元素（货币符号 £）成为默认元素？谢谢。

解决方法

首先让你的picker xaml代码像这样

  <Label Grid.Row="2" Grid.Column="0" Text="Currency Symbol" Margin="5,0" Style=" 
      {StaticResource baseLabelBold}" />
      <Picker Grid.Row="2" Grid.Column="2" Grid.ColumnSpan="3"
      ItemDisplayBinding="{Binding CurrencySymbol}" x:Name="CurrencyPicker" ItemsSource=" 
      {Binding CurrencyList}"
      SelectedItem="{Binding SelectedCurrency,Mode=TwoWay}">
      </Picker>

像这样在 View Model 上取两个属性 Currency list 和 SelectedCurrency

private ObservableCollection<CurrencyList> _currencylist;
        public ObservableCollection<CurrencyList> CurrencyList
        {
            get { return _currencylist; }
            set
            {
                _currencylist = value;
                base.RaisePropertyChanged(nameof(CurrencyList));
            }
        }


 private CurrencyList _SelectedCurrency;
        public CurrencyList SelectedCurrency
        {
            get { return _SelectedCurrency; }
            set
            {
                _SelectedCurrency = value;
            
                base.RaisePropertyChanged(nameof(SelectedCurrency));
            }
        }

现在创建一个类似 CurrencyList_Completed() 的方法，在您的集合中插入数据并将所选货币作为默认值

protected async Task CurrencyList_Completed()
        {
            _currencylist = new ObservableCollection<CurrencyList>();
            _currencylist.Add(new CurrencyList() { CurrencyID = 1,CurrencySymbol = "£" });
            _currencylist.Add(new CurrencyList() { CurrencyID = 2,CurrencySymbol = "$" });
            _currencylist.Add(new CurrencyList() { CurrencyID = 3,CurrencySymbol = "$" });
            CurrencyList = _currencylist.ToObservableCollection();
            SelectedCurrency = CurrencyList.Where(x => x.CurrencyID == 1).FirstOrDefault();

        }

在初始化您的 ViewModel 时调用此方法

protected override async Task Initialize(NavigationParameters parameters)
        {
            if (parameters.GetNavigationMode() == NavigationMode.New)
            {
               
                CurrencyList_Completed();
            }

        }

现在您可以获得预期的输出。请参阅附加的视频 Video