如何创建自定义控件的事件以支持转换为命令？

Question

我创建的自定义控件有一个事件如下。

public class Editor : Control
{
    ...
    public event EventHandler<AlarmCollection> AlarmFired = null;
    ...
}

当触发 TextChanged 事件时，将调用上述事件（如果不为 null）。

private async void TextArea_TextChanged(object sender, TextChangedEventArgs e)
{
    ...
    this.AlarmFired?.Invoke(this, this.alarmList);
    ...
}

现在我尝试将上述事件绑定到外部的 ViewModel，如下所示。

<DataTemplate DataType="{x:Type documentViewModels:EditorTypeViewModel}">
    <editor:Editor FontSize="15" FontFamily="Arial"
                                 KeywordForeground="LightBlue">

        <i:Interaction.Triggers>
            <i:EventTrigger EventName="AlarmFired">
                <mvvm:EventToCommand Command="{Binding AlarmFiredCommand}"
                                     PassEventArgsToCommand="True"
                                     EventArgsConverter="{localConverters:RemoveObjectConverter}"/>
            </i:EventTrigger>
        </i:Interaction.Triggers>

    </editor:Editor>

</DataTemplate>

EditorTypeViewModel的定义如下所示。

public class EditorTypeViewModel : DocumentViewModel
{
    public event EventHandler<AlarmCollection> AlarmFired = null;

    public EditorTypeViewModel(string title) : base(title)
    {
    }

    private RelayCommand<AlarmCollection> alarmFiredCommand = null;
    public RelayCommand<AlarmCollection> AlarmFiredCommand
    {
        get
        {
            if (this.alarmFiredCommand == null)
                this.alarmFiredCommand = new RelayCommand<AlarmCollection>(this.OnAlarmFired);

            return this.alarmFiredCommand;
        }
    }

    private void OnAlarmFired(AlarmCollection alarmInfos)
    {


        this.AlarmFired?.Invoke(this, alarmInfos);
    }
}

当我执行上述程序时，没有调用与 RelayCommand 连接的 OnAlarmFired 方法。 我试图找出原因，发现了一个可疑点。

一个可疑点是在调用Editor的TextChanged方法时，Editor的AlarmFired事件的值为null。如下图所示。

我认为 AlarmFired 不为空，因为它会与 Command 连接，但事实并非如此。

我尝试做的是将CustomControl的事件绑定到ViewModel的Command并使用它。

例如，ListView 的 MouseDoubleClick 事件可以绑定到 MouseDoubleClickCommand，如下所示。 当 MouseDoubleClick 事件被触发时，MouseDoubleClickCommand 将获得控制权。

<ListView>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="MouseDoubleClick">
            <mvvm:EventToCommand Command="{Binding MouseDoubleClickCommand}"
                                 PassEventArgsToCommand="True"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>

</ListView>

我想创建一个事件来支持像 ListView 的 MouseDoubleClick 这样的命令转换 （我不想在CustomControl中创建Command，因为事件数量很多）

我应该怎么做才能实现这个目标？

感谢您的阅读。

Answer 1

如果你想将你的事件参数传递给你的ViewModel，你应该像这样创建一个新的Behavior：

public class EventToCommandBehavior : Behavior<FrameworkElement>
{
    private Delegate _handler;
    private EventInfo _oldEvent;

    // Event
    public string Event
    {
        get => (string)GetValue(EventProperty);
        set => SetValue(EventProperty, value);
    }
    public static readonly DependencyProperty EventProperty = DependencyProperty.Register("Event", typeof(string), typeof(EventToCommandBehavior), new PropertyMetadata(null, OnEventChanged));

    // Command
    public ICommand Command
    {
        get => (ICommand)GetValue(CommandProperty);
        set => SetValue(CommandProperty, value);
    }
    public static readonly DependencyProperty CommandProperty = DependencyProperty.Register("Command", typeof(ICommand), typeof(EventToCommandBehavior), new PropertyMetadata(null));

    // PassArguments (default: false)
    public bool PassArguments
    {
        get => (bool)GetValue(PassArgumentsProperty);
        set => SetValue(PassArgumentsProperty, value);
    }
    public static readonly DependencyProperty PassArgumentsProperty = DependencyProperty.Register("PassArguments", typeof(bool), typeof(EventToCommandBehavior), new PropertyMetadata(false));


    private static void OnEventChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var beh = (EventToCommandBehavior)d;

        if (beh.AssociatedObject != null) // is not yet attached at initial load
            beh.AttachHandler((string)e.NewValue);
    }

    protected override void OnAttached()
    {
        AttachHandler(Event); // initial set
    }

    /// <summary>
    /// Attaches the handler to the event
    /// </summary>
    private void AttachHandler(string eventName)
    {
        // detach old event
        if (_oldEvent != null)
            _oldEvent.RemoveEventHandler(AssociatedObject, _handler);

        // attach new event
        if (!string.IsNullOrEmpty(eventName))
        {
            var ei = AssociatedObject.GetType().GetEvent(eventName);
            if (ei != null)
            {
                var mi = GetType().GetMethod("ExecuteCommand", BindingFlags.Instance | BindingFlags.NonPublic);
                if (mi != null) _handler = Delegate.CreateDelegate(ei.EventHandlerType, this, mi);
                ei.AddEventHandler(AssociatedObject, _handler);
                _oldEvent = ei; // store to detach in case the Event property changes
            }
            else
                throw new ArgumentException($"The event '{eventName}' was not found on type '{AssociatedObject.GetType().Name}'");
        }
    }

    /// <summary>
    /// Executes the Command
    /// </summary>
    // ReSharper disable once UnusedParameter.Local
    private void ExecuteCommand(object sender, EventArgs e)
    {
        object parameter = PassArguments ? e : null;
        if (Command == null) return;
        if (Command.CanExecute(parameter))
            Command.Execute(parameter);
    }
}

并像这样使用：

<ListView>
      <i:Interaction.Behaviors>
            <behaviors:EventToCommandBehavior Command="{Binding AlarmFiredCommand}" Event="AlarmFired" PassArguments="True" />
      </i:Interaction.Behaviors>
</ListView>

注意：如果要传递参数，应为 PassArguments 属性设置 True