ContentProperty

LabeledView.xaml

<?xml version="1.0" encoding="UTF-8" ?>
<ContentView
    x:Class="MyApp.Controls.LabeledView"
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">
    <ContentView.Content>
        <StackLayout>
            <Label x:Name="label" />
            <ContentView x:Name="mainContent" />
        </StackLayout>
    </ContentView.Content>
</ContentView>

LabeledView.xaml.cs

using System.Runtime.CompilerServices;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace MyApp.Controls
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    [ContentProperty(nameof(MainContent))]
    public partial class LabeledView : ContentView
    {
        public static readonly BindableProperty MainContentProperty
            = BindableProperty.Create(nameof(MainContent), typeof(View), typeof(LabeledView), default(View));

        public static readonly BindableProperty LabelTextProperty
            = BindableProperty.Create(nameof(LabelText), typeof(string), typeof(LabeledView), default(string));

        public static readonly BindableProperty LabelHorizontalOptionsProperty
            = BindableProperty.Create(nameof(LabelHorizontalOptions), typeof(LayoutOptions), typeof(LabeledView), LayoutOptions.Fill);

        public static readonly BindableProperty LabelColorProperty
            = BindableProperty.Create(nameof(LabelColor), typeof(Color), typeof(LabeledView), default(Color));

        public static readonly BindableProperty LabelSizeProperty
            = BindableProperty.Create(nameof(LabelSize), typeof(NamedSize), typeof(LabeledView), default(NamedSize));

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

        public string LabelText
        {
            get => (string)GetValue(LabelTextProperty);
            set => SetValue(LabelTextProperty, value);
        }

        public LayoutOptions LabelHorizontalOptions
        {
            get => (LayoutOptions)GetValue(LabelHorizontalOptionsProperty);
            set => SetValue(LabelHorizontalOptionsProperty, value);
        }

        public Color LabelColor
        {
            get => (Color)GetValue(LabelColorProperty);
            set => SetValue(LabelColorProperty, value);
        }

        public NamedSize LabelSize
        {
            get => (NamedSize)GetValue(LabelSizeProperty);
            set => SetValue(LabelSizeProperty, value);
        }

        public LabeledView()
        {
            InitializeComponent();

            label.HorizontalOptions = HorizontalOptions;
            label.TextColor = LabelColor;
            label.FontSize = GetFontSize();
        }

        protected override void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            base.OnPropertyChanged(propertyName);

            if (propertyName == nameof(MainContent))
            {
                mainContent.Content = MainContent;
            }
            if (propertyName == nameof(LabelText))
            {
                label.Text = LabelText;
            }
            if (propertyName == nameof(LabelHorizontalOptions))
            {
                label.HorizontalOptions = LabelHorizontalOptions;
            }
            if (propertyName == nameof(LabelColor))
            {
                label.TextColor = LabelColor;
            }
            if(propertyName == nameof(LabelSize))
            {
                label.FontSize = GetFontSize();
            }
        }

        private double GetFontSize()
        {
            FontSizeConverter myFontSizeConverter = new FontSizeConverter();
            return (double)myFontSizeConverter.ConvertFromInvariantString(LabelSize.ToString());
        }
    }
}

MyPage.xaml.cs

<ContentPage
    x:Class="MyPage"
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:controls="clr-namespace:MyApp.Controls"
    xmlns:d="http://xamarin.com/schemas/2014/forms/design"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Shell.NavBarIsVisible="False"
    mc:Ignorable="d">
    <ContentPage.Content>
        <StackLayout Padding="10,0,10,0" VerticalOptions="Center">
            <controls:LabeledView
                LabelColor="Red"
                LabelHorizontalOptions="Start"
                LabelText="testowy label">
                <Entry />
            </controls:LabeledView>

        </StackLayout>
    </ContentPage.Content>
</ContentPage>