A common base page for using IsBusy property to show a busy indicator.

BaseContentPage.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"
             x:Class="LoadingPage"
             x:Name="ContentPage">
    <ContentPage.Content>
        <AbsoluteLayout>
            <ContentView Content="{Binding Source={x:Reference ContentPage},Path=MainContent}"
                         HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" 
                         AbsoluteLayout.LayoutBounds="0,0,1,1" AbsoluteLayout.LayoutFlags="All">
            </ContentView>
            <!-- Your Busy Indicator (Check out syncfusion's busy indicator) -->
            <ContentView Content="{Binding Source={x:Reference ContentPage}, Path=LoadingView}" 
                         IsVisible="{Binding Source={x:Reference ContentPage},Path=IsBusy}" 
                                    AbsoluteLayout.LayoutBounds="0,0,1,1" AbsoluteLayout.LayoutFlags="All" />
        </AbsoluteLayout>
    </ContentPage.Content>
</ContentPage>

BaseContentPage.xaml.cs

[ContentProperty(nameof(MainContent))]
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class LoadingPage : ContentPage
{
    public LoadingPage()
    {
        InitializeComponent();
    }

    public static readonly BindableProperty MainContentProperty =
        BindableProperty.Create(nameof(MainContent), typeof(View), typeof(LoadingPage));

    public static readonly BindableProperty LoadingContentProperty =
        BindableProperty.Create(nameof(LoadingView), typeof(View), typeof(LoadingPage));

    public View LoadingView
    {
        get => (View) GetValue(LoadingContentProperty);
        set => SetValue(LoadingContentProperty, value);
    }

    public View MainContent
    {
        get { return (View) GetValue(MainContentProperty); }
        set { SetValue(MainContentProperty, value); }
    }

    protected override void OnBindingContextChanged()
    {
        base.OnBindingContextChanged();
        if (MainContent == null)
        {
            return;
        }
        SetInheritedBindingContext(MainContent, BindingContext);
    }
}

The other pages in the application would inherit from BaseContentPage instead of Content page.

Instead of inheriting from BasePage we can also create a controltemplate like shown here http://surfy57.free.fr/?p=42