视图模型（WPF）中的UI更新命令

Question

我在MVVM模式上创建一个WPF应用程序，用户单击树的项目（由颜色名称组成的超链接，具有相应前景的名称文本）以更改整个窗口的背景。我通过relay命令执行此操作，但在View Model中我不能接受UI，我正在编写命令。

XAML中带有颜色名称的树：

<TreeView Name="tree" ItemSource="{Binding colorList, Mode=TwoWay}" Background="Transparent">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemSource={Binding Children}>
             <TextBlock><Hyperlink Command={Binding ColorChangerCommand} Foreground={Binding Foreground} TextDecorations="None"><TextBlock Text={Binding Name}/></Hyperlink></TextBlock>
        </HierarchicalDataTemplate>
     </TreeView.ItemTemplate>
 <TreeView>

我的视图模型中的命令：

public RelayCommand ColorChangerCommand{ get; set;}

public TreeViewModel() //Constructor of the View Model
{
   ColorChangerCommand= new RelayCommand(ChangeColor);
}

public void ChangeColor(object sender)
{
  this.Background= (sender as TreeViewItem).Foreground;
}

该命令在简单的代码隐藏中运行良好，但现在不在View Model中。请帮忙？

Answer 1

this.Background指的是视图模型的Background属性，前提是ChangeColor方法属于视图模型类。要更改窗口的背景，需要将其绑定到视图模型的Background属性并引发事件以告知UI更新。这需要您的视图模型来实现INotifyPropertyChanged事件：

public class ViewModel : INotifyPropertyChanged
{
    public RelayCommand ColorChangerCommand { get; set; }

    public TreeViewModel() //Constructor of the View Model
    {
        ColorChangerCommand = new RelayCommand(ChangeColor);
    }

    public void ChangeColor(object sender)
    {
        this.Background = (sender as TreeViewItem).Foreground;
    }

    private Brush background= Brushes.White;
    public Brush Background
    {
        get { return background; }
        set { Background = value; NotifyPropertyChanged(Background); }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

XAML：

<Window .... Background="{Binding Background}" />

您还需要将窗口的DataContext设置为视图模型类的实例，并将Command的Hyperlink属性绑定为：

<Hyperlink Command="{Binding DataContext.ColorChangerCommand, RelativeSource={RelativeSource AncestorType=Window}}" 
           Foreground="{Binding Foreground}" TextDecorations="None">

Answer 2

不要通过ViewModel来做。 UI属于View。使用它的行为。

如果将ForwardedColor绑定到任何其他UI控件，则将更改此控件的bound属性，以便您可以在XAML中轻松管理它。

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
<TextBlock Text="Test" Foreground="Aquamarine">
    <i:Interaction.Behaviors>
        <local:ForwardForegroundOnClick  ForwardedColor="{Binding Background, RelativeSource={RelativeSource AncestorType=Window}, Mode=TwoWay}"/>
    </i:Interaction.Behaviors>
</TextBlock>

public class ForwardForegroundOnClick : Behavior<TextBlock>
{
    public Brush ForwardedColor
    {
        get { return (Brush)GetValue(ForwardedColorProperty); }
        set { SetValue(ForwardedColorProperty, value); }
    }

    // Using a DependencyProperty as the backing store for MyProperty.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ForwardedColorProperty =
        DependencyProperty.Register(nameof(ForwardedColor), typeof(Brush), typeof(ForwardForegroundOnClick), new PropertyMetadata(null));

    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.MouseLeftButtonDown += AssociatedObject_MouseLeftButtonDown;
    }

    private void AssociatedObject_MouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
    {
        ForwardedColor = AssociatedObject.Foreground;
    }

    protected override void OnDetaching()
    {
        AssociatedObject.MouseLeftButtonDown -= AssociatedObject_MouseLeftButtonDown;

        base.OnDetaching();
    }
}