绑定不会刷新标签的内容

Question

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();
            }
        }
    }

Answer 1

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