Custom control with public command

sample-usage.xaml

<ContentPage x:name="page">
  <ListView>
    <ListView.ItemTemplate>
      <DataTemplate>
        <ViewCell>
          <CustomControl Command="{Binding BindingContext.ShowItemCommand, Source={Reference page}}" />
        </ViewCell>
      </DataTemplate>
    </ListView.ItemTemplate>
  </ListView>
</ContentPage>

xamarin.forms-custom-controls.xaml

<ContentView x:Name="view">
  <Button Text="Click me!" Command="{Binding Command, Source={Reference view}" />
</ContentView>

xamarin.forms-custom-controls.xaml.cs

public partial class CustomControl : ContentView {
  public static readonly BindableProperty CommandProperty =
  BindableProperty.Create("ICommand", typeof(ICommand), typeof(CustomControl), null, null);

  public ICommand Command
  {
    get { return GetValue(CommandProperty); }
    set { SetValue(CommandProperty, value); }
  }
}