Brian Genisio's House of Bilz

More Adventures in MVVM

One of the biggest points of friction for me when implementing the MVVM pattern happens when I need to bind commands to events.  When using Prism, I get the Button.Click command binding out of the box, but every other event needs to be wired up individually.  Doing this requires a LOT of boilerplate code that is very easy to get wrong.  In my last post, I published some code to alleviate that pain.  Still, it requires you to write a new behavior and attachment for every event that you want to bind an event to.

For a while now, I have toyed with the idea of just binding commands to events directly.  I ran into a lot of bumps in the road.  For instance, every event handler has a different event argument type.  This requires all of the handlers to be dynamic.  I also couldn’t create an inline command binding – I will surely want to bind more than one event per control – so I need to create a collection of bindings.  Creating arrays of structures created its own troubles – binding only works with FrameworkElements within the visual tree.  This required me to write my own crude binding within my generic behavior.

What follows is very loosely based off of the Chinch MVVM framework.  I tested this code in Silverlight and WPF and it works really well!

Assume I have a ViewModel that looks like this:

public class MainPageViewModel : INotifyPropertyChanged
{
    ...
    public ICommand MouseLeaveCommand { get; private set; }
    public ICommand MouseEnterCommand { get; private set; }
    public ICommand ClickCommand { get; private set; }
    ...
}

I can then bind the commands to events on a control (Button, for instance):

<Button Content="Click Me">
    <Behaviors:Events.Commands>
        <Behaviors:EventCommandCollection>
            <Behaviors:EventCommand CommandName="MouseEnterCommand" EventName="MouseEnter" />
            <Behaviors:EventCommand CommandName="MouseLeaveCommand" EventName="MouseLeave" />
            <Behaviors:EventCommand CommandName="ClickCommand" EventName="Click" />
        </Behaviors:EventCommandCollection>
    </Behaviors:Events.Commands>
</Button>

I no longer need to write any extra code whenever I want to attach commands to my events!  There are a few caveats to this code:

1. The XAML requires the EventCommandCollection to be declared in the XAML.  I struggled to figure out how to eliminate this but gave up.  Someone smarter than me might be able to tell me what I am doing wrong.
2. This code does not consider command properties.  Every command assumes a null parameter.  If you need parameters (like data context), then you’ll have to do something differently (either use the old-school mechanism or extend this code to handle some special event types).
3. You don’t bind directly to the command.  Instead, you declare the name of the command (Notice CommandName is not bound).  The behavior binds for you using a primitive mechanism.

Here is the command behavior that does all the work:

public class Events
{
    private static readonly DependencyProperty EventBehaviorsProperty =
        DependencyProperty.RegisterAttached(
        "EventBehaviors",
        typeof(EventBehaviorCollection),
        typeof(Control),
        null);

    private static readonly DependencyProperty InternalDataContextProperty =
        DependencyProperty.RegisterAttached(
        "InternalDataContext",
        typeof(Object),
        typeof(Control),
        new PropertyMetadata(null, DataContextChanged));

    private static void DataContextChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
        var target = dependencyObject as Control;
        if (target == null) return;

        foreach (var behavior in GetOrCreateBehavior(target))
            behavior.Bind();
    }

    public static readonly DependencyProperty CommandsProperty =
        DependencyProperty.RegisterAttached(
        "Commands",
        typeof(EventCommandCollection),
        typeof(Events),
        new PropertyMetadata(null, CommandsChanged));

    public static EventCommandCollection GetCommands(DependencyObject dependencyObject)
    {
        return dependencyObject.GetValue(CommandsProperty) as EventCommandCollection;
    }

    public static void SetCommands(DependencyObject dependencyObject, EventCommandCollection eventCommands)
    {
        dependencyObject.SetValue(CommandsProperty, eventCommands);
    }

    private static void CommandsChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
        var target = dependencyObject as Control;
        if (target == null) return;

        var behaviors = GetOrCreateBehavior(target);
        foreach (var eventCommand in e.NewValue as EventCommandCollection)
        {
            var behavior = new EventBehavior(target);
            behavior.Bind(eventCommand);
            behaviors.Add(behavior);
        }

    }

    private static EventBehaviorCollection GetOrCreateBehavior(FrameworkElement target)
    {
        var behavior = target.GetValue(EventBehaviorsProperty) as EventBehaviorCollection;
        if (behavior == null)
        {
            behavior = new EventBehaviorCollection();
            target.SetValue(EventBehaviorsProperty, behavior);
            target.SetBinding(InternalDataContextProperty, new Binding());
        }

        return behavior;
    }
}

public class EventCommand
{
    public string CommandName { get; set; }
    public string EventName { get; set; }
}

public class EventCommandCollection : List<EventCommand>
{
}

public class EventBehavior : CommandBehaviorBase<Control>
{
    private EventCommand _bindingInfo;

    public EventBehavior(Control control)
        : base(control)
    {

    }

    public void Bind(EventCommand bindingInfo)
    {
        ValidateBindingInfo(bindingInfo);

        _bindingInfo = bindingInfo;

        Bind();
    }

    private void ValidateBindingInfo(EventCommand bindingInfo)
    {
        if(bindingInfo == null) throw new ArgumentException("bindingInfo");
        if (string.IsNullOrEmpty(bindingInfo.CommandName)) throw new ArgumentException("bindingInfo.CommandName");
        if (string.IsNullOrEmpty(bindingInfo.EventName)) throw new ArgumentException("bindingInfo.EventName");
    }

    public void Bind()
    {
        ValidateBindingInfo(_bindingInfo);
        HookPropertyChanged();
        HookEvent();
        SetCommand();
    }

    public void HookPropertyChanged()
    {
        var dataContext = TargetObject.DataContext as INotifyPropertyChanged;
        if (dataContext == null) return;

        dataContext.PropertyChanged -= DataContextPropertyChanged;
        dataContext.PropertyChanged += DataContextPropertyChanged;
    }

    private void DataContextPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == _bindingInfo.CommandName)
            SetCommand();
    }

    private void SetCommand()
    {
        var dataContext = TargetObject.DataContext;
        if (dataContext == null) return;

        var propertyInfo = dataContext.GetType().GetProperty(_bindingInfo.CommandName);
        if (propertyInfo == null) throw new ArgumentException("commandName");

        Command = propertyInfo.GetValue(dataContext, null) as ICommand;
    }

    private void HookEvent()
    {
        var eventInfo = TargetObject.GetType().GetEvent(
            _bindingInfo.EventName, BindingFlags.Public | BindingFlags.Instance);
        if (eventInfo == null) throw new ArgumentException("eventName");

        eventInfo.RemoveEventHandler(TargetObject, GetEventMethod(eventInfo));
        eventInfo.AddEventHandler(TargetObject, GetEventMethod(eventInfo));
    }

    private Delegate _method;
    private Delegate GetEventMethod(EventInfo eventInfo)
    {
        if (eventInfo == null) throw new ArgumentNullException("eventInfo");
        if (eventInfo.EventHandlerType == null) throw new ArgumentException("EventHandlerType is null");

        if (_method == null)
        {
            _method = Delegate.CreateDelegate(
                eventInfo.EventHandlerType, this,
                GetType().GetMethod("OnEventRaised",
                BindingFlags.NonPublic | BindingFlags.Instance));
        }

        return _method;
    }

    private void OnEventRaised(object sender, EventArgs e)
    {
        ExecuteCommand();
    }
}

public class EventBehaviorCollection : List<EventBehavior>
{ }