Xamarin data binding how to bind the value times two - xamarin

Question

I'm learning Xamarin Data Binding and learning the below code:
<?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:DataBinding"
x:Class="DataBinding.MainPage">
<StackLayout>
<Label x:Name="label" Text="twisting" FontSize="70" HorizontalOptions="Center" VerticalOptions="CenterAndExpand"
BindingContext="{x:Reference Name=slider}"
Rotation="{Binding Path=Value}">
</Label>
<Slider x:Name="slider" BackgroundColor="Yellow" Maximum="360" VerticalOptions="CenterAndExpand">
</Slider>
</StackLayout>
</ContentPage>
I want to double the twisting speed so I modify the code:
<Label x:Name="label" Text="twisting" FontSize="70"
HorizontalOptions="Center" VerticalOptions="CenterAndExpand"
BindingContext="{x:Reference Name=slider}"
Rotation="{Binding Path=Value*2}">
</Label>
But it doesn't work at all. I know the below solution works but I want a solution relating to the above:
<Slider x:Name="slider" BackgroundColor="Yellow" Maximum="720" VerticalOptions="CenterAndExpand">
</Slider>
How to do it?
UPDATE1
I customize a converter as the below code:
using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using Xamarin.Forms;
namespace DataBinding
{
public class MyValueConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var speed = value as int?;
return speed * 2;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return null;
}
}
}
and modify the xaml as the below:
<?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:DataBinding"
x:Class="DataBinding.MainPage">
<StackLayout>
<Label x:Name="label" Text="twisting" FontSize="70" HorizontalOptions="Center" VerticalOptions="CenterAndExpand"
BindingContext="{x:Reference Name=slider}"
Rotation="{Binding Path, Converter={StaticResource myCo
nverter}}">
</Label>
<Slider x:Name="slider" BackgroundColor="Yellow" Maximum="360" VerticalOptions="CenterAndExpand">
</Slider>
</StackLayout>
<ContentPage.Resources>
<ResourceDictionary>
<local:MyValueConverter x:Key="myConverter"></local:MyValueConverter>
</ResourceDictionary>
</ContentPage.Resources>
</ContentPage>
But my converter doesn't work properly and what is more, I made a breakpoint and found that the Convert function even not getting triggered.
UPDATE2
I found out the reason, the label in XAML should be the below:
<Label x:Name="label" Text="twisting" FontSize="70" HorizontalOptions="Center" VerticalOptions="CenterAndExpand"
BindingContext="{x:Reference Name=slider}"
Rotation="{Binding Path=Value, Converter={StaticResource myConverter}}">
</Label>
and the type of value inside the Convert function is double instead of string:
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var tmp = (Double)value*2;
return tmp;
}
Then it works.