How do I detect a copy/cut paste event in the re-hosted WF 4.0 designer?

Question

I am using the WF 4.0 re-hosted designer.

When I drag and drop a custom activity I created by inheriting from CodeActivity class, I want to generate a new GUID and assign it to the a property in my custom activity.

For this to work, I had to implement the IActivityTemplateFactory interface and in its Create method, instantiate a new GUID each time.

This works when the user drags and drops a new activity on the designer.

However, if the user were to copy an existing activity and paste it (to create a new activity), the Create method of this interface does not fire.

Now i end up with 2 instances of that activity with the same GUID property.

Is there a way to get around this? Is there another interface or event that i can overload / override to detect a copy/cut paste operation?

Answer 1

IActivityTemplateFactory.Create should only be called when the Activity is created, not when it is moved from one place to another on the design surface.

This is a tough one from the design perspective. Your Activities should be designed in such a way so that they do not "know" about the design surface and do not have any dependencies on copy/paste events in the designer.

The ActivityDesigner is a different matter. It might be a simple fix to have the ActivityDesigner register a watcher with its ModelItem property, then whenever this property changes it automatically sets the Guid on that ModelItem. This is kind of complex, unfortunately. You have to be able to understand how ModelItems work and how to do the DependencyProperty.

Here's how to add a watch to a DP:

DependencyPropertyDescriptor
    .FromProperty(
        ActivityDesigner.ModelItemProperty, 
        typeof(ActivityDesigner))
    .AddValueChanged(this, OnModelItemChanged);

Few notes here. First, this should be called in your designer's constructor. DependencyPropertyDescriptor.FromProperty returns a DPD. If your event handler stops firing, you will have to hold a reference to that DPD. The workflow designer and DPDs don't play nice; I've filed a bug report with MS about it. Third, OnModelItemChanged is a simple event handler (void (object, EventHandler)).

ModelItems wrap the Activities they represent. You MUST use the ModelItem to change properties of the wrapped Activity, even though you can get the Activity out of it. If you don't, the design surface will get out of sync with the Activity. So, you must set the Guid property like this:

this.ModelItem.Properties["Guid"].SetValue(Guid.NewGuid());