Activity Designer چیست؟

<sap:ActivityDesigner x:Class="ActivityLibrary.SendEmailActivityDesigner" 
                      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
                      xmlns:sap="clr-namespace:System.Activities.Presentation;assembly=System.Activities.Presentation"
                      xmlns:sapv="clr-namespace:System.Activities.Presentation.View;assembly=System.Activities.Presentation" 
                      xmlns:sapc="clr-namespace:System.Activities.Presentation.Converters;assembly=System.Activities.Presentation"
                      xmlns:s="clr-namespace:System;assembly=mscorlib">
    <sap:ActivityDesigner.Resources>
        <sapc:ArgumentToExpressionConverter x:Key="ArgumentToExpressionConverter"/>
        <DataTemplate x:Key="Collapsed">
            <StackPanel>
                <TextBlock>This is the collapsed view</TextBlock>
            </StackPanel>
        </DataTemplate>
        <DataTemplate x:Key="Expanded">
            <StackPanel>
                <Label Content="To"></Label>
                <sapv:ExpressionTextBox 
                    HintText="To:"
                    OwnerActivity="{Binding Path=ModelItem}"
                    Expression="{Binding Path=ModelItem.to,Mode=TwoWay,Converter={StaticResource ArgumentToExpressionConverter },ConverterParameter=In}"
                    ExpressionType="s:String"/>
                <Label Content="Subject:"></Label>
                <sapv:ExpressionTextBox 
                    HintText="Subject"
                    OwnerActivity="{Binding Path=ModelItem}"
                    Expression="{Binding Path=ModelItem.subject,Mode=TwoWay,Converter={StaticResource ArgumentToExpressionConverter },ConverterParameter=In}"
                    ExpressionType="s:String"/>
                <Label Content="Body:"></Label>
                <sapv:ExpressionTextBox
                    HintText="Body"
                    OwnerActivity="{Binding Path=ModelItem}"
                    Expression="{Binding Path=ModelItem.body, Mode=TwoWay,Converter={StaticResource ArgumentToExpressionConverter },ConverterParameter=In}"
                    ExpressionType="s:String" Height="100" />
            </StackPanel>
        </DataTemplate>
        <Style x:Key="ExpandOrCollapsedStyle" TargetType="{x:Type ContentPresenter}">
            <Setter Property="ContentTemplate"
                    Value="{DynamicResource Collapsed}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Path=ShowExpanded}" Value="true">
                    <Setter Property="ContentTemplate" Value="{DynamicResource Expanded}"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </sap:ActivityDesigner.Resources>
    <Grid>
        <ContentPresenter Style="{DynamicResource ExpandOrCollapsedStyle}" Content="{Binding}" />
    </Grid>
</sap:ActivityDesigner>

using ActivityLibrary;
using System.Activities;
using System.ComponentModel;

[Designer(typeof(SendEmailActivityDesigner))]
public sealed class SendEmailActivity : CodeActivity
{
    public InArgument from { get; set; }
    public InArgument host { get; set; }
    public InArgument userName { get; set; }
    public InArgument password { get; set; }
    public InArgument to { get; set; }
    public InArgument subject { get; set; }
    public InArgument body { get; set; }
    public OutArgument result { get; set; }
    
    protected override void Execute(CodeActivityContext context)
    {
        var mailMessage = new System.Net.Mail.MailMessage();
        mailMessage.To.Add(to.Get(context).ToString());
        mailMessage.Subject = subject.Get(context).ToString();
        mailMessage.Body = body.Get(context);
        mailMessage.From = new System.Net.Mail.MailAddress(from.Get(context));
        var smtp = new System.Net.Mail.SmtpClient();
        smtp.Host = host.Get(context);
        smtp.Credentials = new System.Net.NetworkCredential(
        userName.Get(context), password.Get(context));
        smtp.EnableSsl = true;
        smtp.Send(mailMessage);
        result.Set(context, "Sent Email Successfully!");
    }
}