I have a little project with a simulator. In the end the winner should stand in the label. The backcode works fine but the binding doesn't refresh after I set "Winner" to the new string. I let somethings out because they aren't important for the problem.

Here's my xaml:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="Risiko.MainPage"
             xmlns:local="clr-namespace:Risiko"
             x:DataType="local:MainViewModel">

        <StackLayout>
            <Frame BackgroundColor="Red" Margin="10,10,5,10" CornerRadius="5">
                <StackLayout>
                    <Label Text="Angreifer:" HorizontalTextAlignment="Center" TextColor="White" FontSize="36"/>
                    <StackLayout>
                        <Entry Text="{Binding Enemie}" HorizontalOptions="FillAndExpand" Keyboard="Numeric" TextColor="White"/>
                        <Label Text="Soldaten" HorizontalOptions="EndAndExpand" TextColor="White"/>
                    </StackLayout>
                </StackLayout>
            </Frame>
            <Frame BackgroundColor="Blue" Margin="10,5,10,10" CornerRadius="5">
                <StackLayout>
                    <Label Text="Verteildiger:i" HorizontalTextAlignment="Center" TextColor="White" FontSize="36"/>
                    <StackLayout>
                        <Entry Text="{Binding Defender}" HorizontalOptions="FillAndExpand" Keyboard="Numeric" TextColor="White"/>
                        <Label Text="Soldaten" HorizontalOptions="EndAndExpand" TextColor="White"/>
                    </StackLayout>
                </StackLayout>
            </Frame>
            <Button Text="Start" Command="{Binding Start}" Background="green"/>
            <Frame BackgroundColor="Gray" Margin="10,5,10,10" CornerRadius="5">
                <Label Text="{Binding Winner, Mode=TwoWay}" HorizontalOptions="FillAndExpand" TextColor="White"/>
            </Frame>
        </StackLayout>
</ContentPage>

Here's my Viewmodel:

  using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Text;
using Xamarin.Forms;

namespace Risiko
{
    public  class MainViewModel
    {
        private string _defender;
        private string _winner;
        private string _enemie;
        private int[] enemieArray = new int[4];
        private int[] defenderArray = new int[3];
        Random random = new Random();
        private int max;
        private int max2;
        public Command Start { get; }

    public string Defender
    {
        get => _defender;
        set => SetProperty(ref _defender, value);
    }
    public string Enemie
    {
        get => _enemie;
        set => SetProperty(ref _enemie, value);
    }
    public string Winner
    {
        get => _winner;
        set => SetProperty(ref _winner, value);
    }

    public MainViewModel()
    {
        Start = new Command(OnStart);
    }

    private void OnStart()
    {            
        if(defenderArray[0] <= 0)
        {
            Winner  = "Gewonnen hat der Angreifer mit nur noch "   enemieArray[0]   " übrigen Truppen.";
        }
        else
        {
            Winner = "Gewonnen hat der Verteildiger mit nur noch "   defenderArray[0]   " übrigen Truppen.";
        }
    }

    
    #region INotifyPropertyChanged
    protected bool SetProperty<T>(ref T backingStore, T value,
        [CallerMemberName] string propertyName = "",
        Action onChanged = null)
    {
        if (EqualityComparer<T>.Default.Equals(backingStore, value))
            return false;

        backingStore = value;
        onChanged?.Invoke();
        OnPropertyChanged(propertyName);
        return true;
    }

    
    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
    {
        var changed = PropertyChanged;
        if (changed == null)
            return;

  

      changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion
    }

}

And my backcode cs

  namespace Risiko
    {
        public partial class MainPage : ContentPage
        {
            public MainPage()
            {
                InitializeComponent();
                BindingContext  = new MainViewModel();
            }
        }
    }

CodePudding user response：

the binding mechanism checks to see if the class assigned as the BindingContext implements INotifyPropertyChanged. This is what makes binding work. Just calling PropertyChanged alone won't do anything. You need to add

public class MainViewModel : INotifyPropertyChanged