Exemplo de Loading - Código xaml.

DemoLoadingPage.xaml

<?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:DemoLoading"
             x:Class="DemoLoading.DemoLoadingPage" >
    <StackLayout >
        <Button x:Name="botao" Clicked="Show" Text="Login" TextColor="White" BackgroundColor="#3b5998" Margin="10,30,10,30"/>

        <ActivityIndicator x:Name="actInd"
                             IsRunning="{Binding IsLoading}"
                             IsVisible="{Binding IsLoading}" Color="Red"/>

    </StackLayout>
</ContentPage>