WPF从重写方法更新UI不起作用

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我正在尝试从ViewModel更新WPF UI。

风景：

    <UserControl.DataContext>
        <local:ConcreteObserver />
    </UserControl.DataContext>
    <Grid>
        <TextBlock
            HorizontalAlignment="Center"
            VerticalAlignment="Center"
            Text="{Binding Key, UpdateSourceTrigger=PropertyChanged}" />
        <Button
            VerticalAlignment="Top"
            HorizontalAlignment="Right"
            Content="Click"
            Command="{Binding TestDelegateCommand}" />
    </Grid>

ViewModel：

ConcreteObserver : Observer<Mouse>, INotifyPropertyChanged
    {

        private string _key;

        public DelegateCommand TestDelegateCommand { get; set; }

        public string Key
        {
            get { return _key; }
            set { OnPropertyChanged(nameof(Key)); }
        }


        public ConcreteObserver ()
        {
            TestDelegateCommand = new DelegateCommand(UpdateGui);
        }

        private void UpdateGui()
        {
            Key = "Test refresh";
        }

        public override void Update(TestObject subject)
        {
            Key = "Test Update";

            if (subject is TestObject)
            {
                subject.MouseAction += OnMouse;
                subject.Start();

            }
        }

        private void OnMouse(object sender, RowMouseDataEventArgs e)
        {
            Key += "1";
        }

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

单击鼠标时触发了一个事件，该事件运行正常。

The problem that the UI is not updated if the Key is changed within the update method and nothing is displayed on the GUI if the Key property changes.

I set a breakpoint and watched the change from the Key property and everything works fine but the GUI doesn't recognize the change.

我用一个按钮对其进行了测试，单击该按钮时会显示更改

有人可以向我解释原因。

谢谢