如何避免多个UserControl重复定义相同EventTrigger，寻求更优实现方案

你当前的ContentControl方案可以正常运行，但存在多余视觉树开销、代码冗余的问题，下面提供你需要的自定义EventsUserControl实现，以及更优的解耦方案：

方案1：自定义EventsUserControl基类（满足DI关联需求） #

第一步：定义ViewModel约束接口 #

首先定义接口让所有需要绑定这两个事件的ViewModel实现，方便DI容器扫描匹配：

public interface IWindowEventAware
{
    ICommand WindowLoadedCommand { get; }
    ICommand WindowClosedCommand { get; }
}

第二步：实现EventsUserControl基类 #

直接继承UserControl，内置事件绑定逻辑，不需要依赖Interaction触发器：

public class EventsUserControl : UserControl
{
    public EventsUserControl()
    {
        Loaded += OnLoaded;
        // 绑定宿主窗口的Closed事件
        Loaded += (s, e) => 
        {
            if (Window.GetWindow(this) is Window window)
            {
                window.Closed += (o, args) => ExecuteCommand(WindowClosedCommand);
            }
        };
    }

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        ExecuteCommand(WindowLoadedCommand);
    }

    private void ExecuteCommand(ICommand command)
    {
        if (command?.CanExecute(null) == true)
        {
            command.Execute(null);
        }
    }

    public ICommand WindowLoadedCommand
    {
        get => (ICommand)GetValue(WindowLoadedCommandProperty);
        set => SetValue(WindowLoadedCommandProperty, value);
    }
    public static readonly DependencyProperty WindowLoadedCommandProperty =
        DependencyProperty.Register(nameof(WindowLoadedCommand), typeof(ICommand), typeof(EventsUserControl), new PropertyMetadata(null));

    public ICommand WindowClosedCommand
    {
        get => (ICommand)GetValue(WindowClosedCommandProperty);
        set => SetValue(WindowClosedCommandProperty, value);
    }
    public static readonly DependencyProperty WindowClosedCommandProperty =
        DependencyProperty.Register(nameof(WindowClosedCommand), typeof(ICommand), typeof(EventsUserControl), new PropertyMetadata(null));

    // 自动绑定ViewModel实现的接口命令，不需要手动在XAML写绑定
    protected override void OnDataContextChanged(EventArgs e)
    {
        base.OnDataContextChanged(e);
        if (DataContext is IWindowEventAware vm)
        {
            WindowLoadedCommand = vm.WindowLoadedCommand;
            WindowClosedCommand = vm.WindowClosedCommand;
        }
    }
}

第三步：使用方式 #

业务UserControl直接继承该基类即可，不需要套任何容器：

<local:EventsUserControl 
  x:Class="MyNamespace.MyUserControl"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:local="clr-namespace:YourBaseClassNamespace">
    <Grid>
      页面内容...
    </Grid>
</local:EventsUserControl>

DI容器可以直接扫描所有继承自EventsUserControl的控件，要求其绑定的ViewModel实现IWindowEventAware接口即可。

方案2：附加行为（更优的无侵入实现） #

如果不想强制要求UserControl继承基类，可以用附加行为实现，完全解耦，适合所有FrameworkElement使用：

public static class WindowEventsBehavior
{
    public static readonly DependencyProperty EnableWindowEventsProperty = DependencyProperty.RegisterAttached(
        "EnableWindowEvents", typeof(bool), typeof(WindowEventsBehavior), new PropertyMetadata(false, OnEnableWindowEventsChanged));

    private static void OnEnableWindowEventsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not FrameworkElement element) return;
        if ((bool)e.NewValue)
        {
            element.Loaded += ElementOnLoaded;
        }
        else
        {
            element.Loaded -= ElementOnLoaded;
        }
    }

    private static void ElementOnLoaded(object sender, RoutedEventArgs e)
    {
        if (sender is FrameworkElement { DataContext: IWindowEventAware vm } element)
        {
            if (vm.WindowLoadedCommand?.CanExecute(null) == true)
            {
                vm.WindowLoadedCommand.Execute(null);
            }
            // 绑定宿主窗口Closed事件
            if (Window.GetWindow(element) is Window window)
            {
                window.Closed += (o, args) => 
                {
                    if (vm.WindowClosedCommand?.CanExecute(null) == true)
                    {
                        vm.WindowClosedCommand.Execute(null);
                    }
                };
            }
        }
    }

    public static void SetEnableWindowEvents(DependencyObject element, bool value)
    {
        element.SetValue(EnableWindowEventsProperty, value);
    }

    public static bool GetEnableWindowEvents(DependencyObject element)
    {
        return (bool)element.GetValue(EnableWindowEventsProperty);
    }
}

使用时只需要在任意UserControl的根节点加一个附加属性即可：

<UserControl 
  x:Class="MyNamespace.MyUserControl"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:behaviors="clr-namespace:YourBehaviorNamespace"
  behaviors:WindowEventsBehavior.EnableWindowEvents="True">
    <Grid>
      页面内容...
    </Grid>
</UserControl>

方案对比 #

• 自定义基类方案：适合需要强约束、DI统一扫描注册的场景，没有多余视觉树开销，代码维护简单
• 附加行为方案：适合存量项目改造、不想修改继承结构的场景，灵活性最高
• 原有ContentControl方案：多了一层视觉树开销，代码冗余度更高，不如前两个方案

内容的提问来源于stack exchange，提问作者Michael280