Model-View-Presenter
In MVP, the Presenter contains the UI business logic for the View. All invocations from the View delegate directly to the Presenter. The Presenter is also decoupled directly from the View and talks to it through an interface. This is to allow mocking of the View in a unit test. One common attribute of MVP is that there has to be a lot of two-way dispatching. For example, when someone clicks the "Save" button, the event handler delegates to the Presenter's "OnSave" method. Once the save is completed, the Presenter will then call back the View through its interface so that the View can display that the save has completed.
MVP tends to be a very natural pattern for achieving separated presentation in WebForms. The reason is that the View is always created first by the ASP.NET runtime. You can find out more about both variants.
Two primary variations
Passive View: The View is as dumb as possible and contains almost zero logic. A Presenter is a middle man that talks to the View and the Model. The View and Model are completely shielded from one another. The Model may raise events, but the Presenter subscribes to them for updating the View. In Passive View there is no direct data binding, instead, the View exposes setter properties that the Presenter uses to set the data. All state is managed in the Presenter and not the View.
- Pro: maximum testability surface; clean separation of the View and Model
- Con: more work (for example all the setter properties) as you are doing all the data binding yourself.
Supervising Controller: The Presenter handles user gestures. The View binds to the Model directly through data binding. In this case, it's the Presenter's job to pass off the Model to the View so that it can bind to it. The Presenter will also contain logic for gestures like pressing a button, navigation, etc.
- Pro: by leveraging data binding the amount of code is reduced.
- Con: there's a less testable surface (because of data binding), and there's less encapsulation in the View since it talks directly to the Model.
Model-View-Controller
In the MVC, the Controller is responsible for determining which View to display in response to any action including when the application loads. This differs from MVP where actions route through the View to the Presenter. In MVC, every action in the View correlates with a call to a Controller along with an action. In the web, each action involves a call to a URL on the other side of which there is a Controller who responds. Once that Controller has completed its processing, it will return the correct View. The sequence continues in that manner throughout the life of the application:
Action in the View
-> Call to Controller
-> Controller Logic
-> Controller returns the View.
One other big difference about MVC is that the View does not directly bind to the Model. The view simply renders and is completely stateless. In implementations of MVC, the View usually will not have any logic in the code behind. This is contrary to MVP where it is absolutely necessary because, if the View does not delegate to the Presenter, it will never get called.
Presentation Model
One other pattern to look at is the Presentation Model pattern. In this pattern, there is no Presenter. Instead, the View binds directly to a Presentation Model. The Presentation Model is a Model crafted specifically for the View. This means this Model can expose properties that one would never put on a domain model as it would be a violation of separation-of-concerns. In this case, the Presentation Model binds to the domain model and may subscribe to events coming from that Model. The View then subscribes to events coming from the Presentation Model and updates itself accordingly. The Presentation Model can expose commands which the view uses for invoking actions. The advantage of this approach is that you can essentially remove the code-behind altogether as the PM completely encapsulates all of the behavior for the view. This pattern is a very strong candidate for use in WPF applications and is also called Model-View-ViewModel.
There is a MSDN article about the Presentation Model and a section in the Composite Application Guidance for WPF (former Prism) about Separated Presentation Patterns
What makes you say that either a singleton or a static method isn't thread-safe? Usually both should be implemented to be thread-safe.
The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces (or derive from useful base classes, although that's less common, in my experience), so you can pass around the singleton as if it were "just another" implementation.
Best Answer
The factory should be the one-stop shop to construct domain objects. Any other part of the code that needs to do this should use the factory.
Typically, there are at least three sources of data that are used as input into a factory for domain object construction: input from the UI, the results of queries from persistence, and domain-meaningful requests. So to answer your specific question, the repository would use the factory.
Here is an example. I am using Holub's Builder pattern here. Edit: disregard the use of this pattern. I've started realizing that it doesn't mix too well with DDD factories.
Here is what the OrderFactory might look like:
The logic for the featured Product and the generation of the Itinerary go in the OrderFactory.
Now here is how the factory might be used in each instance.
In OrderRepository:
In your application layer:
Within your domain layer, a unit test perhaps:
Domain.
In my example, all dependencies flow from top to bottom. I used the Dependency Inversion Principle (PDF link) to avoid the problem you speak of.
When you have logic that doesn't fit into any single domain object OR you have an algorithm that involves orchestrating multiple domain objects, use a service. The service would encapsulate any logic that doesn't fit in anything else and delegate to domain objects where it does fit.
In the example I scribbled here, I imagine that coming up with an Itinerary for the Order would involve multiple domain objects. The OrderFactory could delegate to such a service.
BTW, the hierarchy you described should probably be UI > App > Domain Services > Domain > Infrastructure (Data)
Applying Domain Driven Design and Patterns by Jimmy Nilsson is a great compliment to Eric Evans' Domain-Driven Design. It has lots of code examples, though I don't know if there is an emphasis on layering. Layering can be tricky and is almost a topic separate from DDD.
In the Evans book, there is a very small example of layering you might want to check out. Layering is an enterprise pattern, and Martin Fowler wrote Patterns of Enterprise Application Architecture, which you might find useful too.