How to create a custom composite activity

Designer:

<sap:ActivityDesigner x:Class="CompositeActivity.CompositeDesigner"
    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">
        <StackPanel>
            <TextBlock>This is the Expanded View</TextBlock>
            <sap:WorkflowItemsPresenter HintText="Drop Activities Here"
                                    Items="{Binding Path=ModelItem.Activities}">
                <sap:WorkflowItemsPresenter.SpacerTemplate>
                    <DataTemplate>
                        <Ellipse Width="10" Height="10" Fill="Black"/>
                    </DataTemplate>
                </sap:WorkflowItemsPresenter.SpacerTemplate>
                <sap:WorkflowItemsPresenter.ItemsPanel>
                    <ItemsPanelTemplate>
                        <StackPanel Orientation="Horizontal"/>
                    </ItemsPanelTemplate>
                </sap:WorkflowItemsPresenter.ItemsPanel>
            </sap:WorkflowItemsPresenter>
        </StackPanel>
</sap:ActivityDesigner>

Code:

    [Designer(typeof(CompositeDesigner))]
    public class MyCompsiteActivity : NativeActivity
    {
        // Define an activity input argument of type string
        public InArgument<string> Text { get; set; }
        private Variable<string> EmployeeID = new Variable<string>() { Name = "EmployeeID" };

        public Variable<int> Index = new Variable<int>();    
        public Collection<Activity> Activities { get; set; }
        public Collection<Variable> Variables { get; set; }

        public MyCompsiteActivity()
        {          
            Activities = new Collection<Activity>();
            Variables = new Collection<Variable>();
        }

        protected override void CacheMetadata(NativeActivityMetadata metadata)
        {
            base.CacheMetadata(metadata);

            metadata.SetChildrenCollection(Activities);

            foreach (Variable variable in this.Variables)
            {
                //declare a public variable
                metadata.AddVariable(variable);
            }
            //declare a implementation variable
            metadata.AddImplementationVariable(this.Index);

            metadata.AddVariable(this.EmployeeID);

        }

        protected override void Execute(NativeActivityContext context)
        {
            // Obtain the runtime value of the Text input argument
            string text = context.GetValue(this.Text);

            //this.EmployeeID.Set(context, 12);
            //Activity '1.1: MyCompsiteActivity' cannot access this variable because it is declared at the scope of activity '1.1: MyCompsiteActivity'.  An activity can only access its own implementation variables.

            context.DataContext.GetProperties()["EmployeeID"].SetValue(context.DataContext,"123");

            this.Index.Set(context, 0);
            ScheduleActivities(context);

        }

        void ScheduleActivities(NativeActivityContext context)
        {
            int currentIndex = this.Index.Get(context);

            if (currentIndex < this.Activities.Count)
            {
                context.ScheduleActivity(this.Activities[currentIndex], onActivityCompleted);
                this.Index.Set(context, currentIndex+1);
            }
        }
        void onActivityCompleted(NativeActivityContext context, ActivityInstance instance)
        {
            ScheduleActivities(context);
        }


    }

Note:

There must be a Public get method to make the designer can be edited

The "public variable"  can't access directly because an activity can only access its own implementation variables. Hence, we using:

context.DataContext.GetProperties()["EmployeeID"].SetValue(context.DataContext,"123");

Workflow Foundation 4.0 Activity Data Model (III)

http://blogs.msdn.com/b/flow/archive/2010/04/27/workflow-foundation-4-0-activity-data-model-iii.aspx