Xamarin Forms 中的 UI 中的属性值不会将值更新为标签 [英] Property value doesn't update value to Label in UI in Xamarin Forms

问题描述

我有一个按钮，想在 ViewModel 中单击按钮时更新 UI 标签值.我实现了 INotifyPropertyChanged 但它不起作用.Xamarin Forms 中的 UI 中的属性值不会更新到标签的值

I have a button and want to update UI Label value on button click from ViewModel. I implemented INotifyPropertyChanged but it is not working. Property value doesn't update value to Label in UI in Xamarin Forms

MyViewModel

public class MyViewModel : INotifyPropertyChanged
    { 
        public ICommand selectDurationCommand;
        public ICommand SelectDurationCommand
        {
            get { return selectDurationCommand; }
            set
            {
                selectDurationCommand = value;
                OnPropertyChanged();
            }
        }
        public string _fare{ get; set; } 
        public string Fare
        {
            get { return _fare; }
            set
            {
                _fare = value;
                OnPropertyChanged();
            }
        }      
        public event PropertyChangedEventHandler PropertyChanged;
        public MyViewModel()
        {
           selectDurationCommand = new Command((object s) => get_fare(s));
            _fare = "$00.00";
        }
       public void get_fare(object btn)
        {
            var b = (Button)btn;
            _fare="$03.00";
        }
 protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
                handler(this, new PropertyChangedEventArgs(propertyName));
        }
}

FareDetails.xaml

<cl:BasePage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="familyinfo.FareDetails" xmlns:cl="clr-namespace:familyinfo;assembly=familyinfo" x:Name="PDuration">
<Label FontSize="22" TextColor="Black" VerticalOptions="FillAndExpand" Font="Roboto-Medium" Text="{Binding Fare}" VerticalTextAlignment="Center" HorizontalTextAlignment="Center" />
<Button Command="{Binding Source={x:Reference PDuration}, Path=BindingContext.SelectDurationCommand}" CommandParameter="{x:Reference min15Button}" x:Name="min15Button" HeightRequest="30" HorizontalOptions="FillAndExpand" BorderRadius="8" Text="15 MIN" TextColor="#ffffff" BackgroundColor="#f2415c" />
</cl:BasePage>

FareDetails.xaml.cs

public partial class FareDetails : BasePage
    {
        MyViewModel _MyViewModel { get; set; }
        public FareDetails()
        {
            InitializeComponent();
            _MyViewModel = (MyViewModel)this.BindingContext;
        }
}

推荐答案

要引发 PropertyChanged 通知，您需要将值设置为 Property 而不是私有字段.

For the PropertyChanged notification to be raised you need to set the value to the Property not to the private field.

将您的命令方法更改为:

Change your Command method to this:

public void get_fare(object btn)
{
     var b = (Button)btn;
     Fare="$03.00";
}

注意:您的 _fare 可以安全地成为私有字段.

Note: your _fare can safely be a private field.

private string _fare;
public string Fare
{
    get { return _fare; }
    set
    {
        _fare = value;
        OnPropertyChanged();
    }
}  

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