EventToCommandBehavior

The EventToCommandBehavior is a Xamarin.Forms behavior that allows you to connect a .NET event exposed by a visual in XAML to an ICommand property on a ViewModel or other binding source.

It will automatically pull the BindingContext from the associated control unless you specifically set the BindingContext yourself.

Parameters

You must supply two values:

EventName : the name of the .NET event to connect to.
Command : the ICommand implementation to forward the event to.

By default, no parameter is passed to the command (e.g. null). However, two additional properties can be set to control this behavior.

CommandParameter : a specific bindable property which provides a value to pass as the command parameter.
EventArgsConverter : an IValueConverter implementation which is passed the sender and EventArgs from the event and returns some object to use as the command parameter.

EventArgsConverter overrides CommandParameter - so if the former is set, the latter will not be used.

There is a built-in value converter EventArgsConverter which can pull out a single property or field from a class and return it. For example to the retrieve the Item from the ItemTappedEventArgs. Alternatively, you can create your own IValueConverter as shown below.

Example

In the following example, the MyCommand command would be located on the BindingContext of the associated Label.

<Page xmlns:inf="clr-namespace:XamarinUniversity.Infrastructure;assembly=XamU.Infrastructure" ...>
...
<Label Text="{Binding Text}" ...>
   <Label.Behaviors>
      <inf:EventToCommandBehavior
           EventName = "SizeChanged" Command="{Binding MyCommand}"
           EventArgsConverter="{StaticResource converter}"/>
   </Label.Behaviors>
</Label>

The converter would look like this:

// Convert SizeChanged event into `Size` object
class SizeChangedValueConverter : IValueConverter
{
    public object Convert (object value, Type targetType, object parameter, CultureInfo culture)
    {
        // value = sender
        // parameter = EventArgs
        View v = (View)value;
        return new Size { Height = v.Height, Width = v.Width };
    }

    public object ConvertBack (object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException ();
    }
}

Changing the BindingContext

By default, the EventToCommandBehavior pulls the binding context from it's associated object. However, you might need to change that - you can use the RelativeBindingContext class to do this (and keep it synchronized).

<Page xmlns:inf="clr-namespace:XamarinUniversity.Infrastructure;assembly=XamU.Infrastructure" 
      x:Name="Root" ...>
...
<Label Text="{Binding Text}" ...>
   <Label.Behaviors>
      <inf:EventToCommandBehavior BindingContext="{RelativeBindingContext Root}" ... />
   </Label.Behaviors>
</Label>

In this example, the binding context would be set to whatever the Page binding context is.

Uh oh!

EventToCommandBehavior