MainPage.cs

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
             xmlns:local="clr-namespace:DataTriggersFormsSample" 
             x:Class="DataTriggersFormsSample.MainPage">
    <StackLayout VerticalOptions="StartAndExpand"
                 Padding="20,60,20,20">

        <Button Text="Click me"
                BackgroundColor="DarkCyan"
                TextColor="White">
          <Button.Triggers>
                 <EventTrigger Event="Clicked">
                    <local:ExpandButtonTriggerAction />
                </EventTrigger>
           </Button.Triggers>
        </Button>
        
    </StackLayout>
    
</ContentPage>