Ayende @ Rahien

Oren Eini aka Ayende Rahien CEO of Hibernating Rhinos LTD, which develops RavenDB, a NoSQL Open Source Document Database.

You can reach me by:

oren@ravendb.net

+972 52-548-6969

, @ Q j

Posts: 6,858 | Comments: 49,164

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time to read 3 min | 413 words

Continuing in my exploration of the Effectus code base, which originated from my Building a Desktop To-Do Application with NHibernate article on MSDN Magazine, I wanted to talk today about separating features into individual pieces in the application.

Each feature in the Effectus application is built as an isolated unit, that has little to do with any other features. This is very clear when you look at how they are organized:

image 

Sidenote: In each feature, the component parts are named the same (Presenter, View, Model). This is intentional, since it make it that much harder to mix stuff from other features.

Every feature has a pretty rigid structure, defined by the application, and trying to reuse stuff between features is frowned upon and avoided, even if doing so can reduce duplication. As a good example, the Model for both CreateNew and Edit features are identical, but they are two different classes with no attempt to merge it up.

The reason for this is quite simple, treating each feature as an isolated unit bring us great benefits, we can have different developers working on different features without the chance of stepping on each other toes, it is very hard for one feature to break another, deploying new features is easier to handle, etc. Trying to apply DRY and consolidate common pieces of code would introduce dependencies between features, and that is a bad idea.

But what about communication between features? What about when one feature needs to call another?

As it turned out, it is quite easy to turn a lot of the inter feature communication into an indirect calls, using pub/sub. We gain the independence from dependencies using pub/sub, while maintaining the feel of a well integrated product for the user. For the rare cases of one feature calling another, I create a simple Call(“FeatureName”) mechanism (in the code, this is Presenters.Show(name) method) that all us to just explicitly invoke another feature, maybe with some initial arguments. A more complete implementation will handle even that using pub/sub, probably with an integrator class that would dispatch the appropriate event to start and invoke the feature(s) that want to handle it.

You can read more about the architecture principles behind this system in: Application structure: Concepts & Features

time to read 4 min | 689 words

Continuing in my exploration of the Effectus code base, which originated from my Building a Desktop To-Do Application with NHibernate article on MSDN Magazine, I wanted to talk today about how to build an application infrastructure.

First, to be clear, I am not talking about infrastructure pieces such as caching, data access, etc. Those are generic concerns and you should be able to just get them from an off the shelve library. When I am talking about an application infrastructure, I am talking about the infrastructure code that you are going to need to write for a particular application. You are always going to have to do that, even if it is something as simple as just wiring the container together, or registering session handling, etc.

From my point of view, the point of the application infrastructure is to make writing the actual application features as simple as possible. Ideally, you want to make the infrastructure pieces handle everything that isn’t purely related to the feature you are implementing. The reason that an application infrastructure can do this while a generic infrastructure cannot is that an application infrastructure can make a lot of assumptions about the way the application is built.

Let us take the Main screen feature in Effectus, shall we, it will let us look at how this is implemented:

public Observable<int> CurrentPage { get; set; }


public
Fact CanMovePrev { get { return new Fact(CurrentPage, () => CurrentPage > 0); } } public Fact CanMoveNext { get { return new Fact(CurrentPage, () => CurrentPage + 1 < NumberOfPages); } } public void OnCreateNew() { Presenters.Show("CreateNew"); } public void OnActionsChoosen(ToDoAction action) { Presenters.Show("Edit", action.Id); } public void OnLoaded() { LoadPage(0); } public void OnMoveNext() { LoadPage(CurrentPage + 1); } public void OnMovePrev() { LoadPage(CurrentPage - 1); }

Looking the code, we can see that there are actually a lot of infrastructure playing a part here.

  • Automatic binding of On[Button Name]() methods to the button command.
    • Within this, enabling / disabling automatically using Can[Button Name] properties
  • Automatic binding of On[List name]Choosen(Item) to the selected item changed behavior on the list.
  • Automatic updates of property states using Fact.
  • Automatic updates of property values, using Observable.

There isn’t much to say about the automatic binding between UI actions and presenter methods, except that I think it is clear how this behavior reduce the level of infrastructure level code that you would have to write, test and maintain. But both Fact and Observable deserve a bit more explanation.

In the LoadPage() method, not shown here, we are updating the CurrentPage property, this will trigger invalidation of the all the facts bound to the value (CanMoveNext, CanMovePrev), which will force their re-evaluation. Since the infrastructure knows how to wire this up to the appropriate commands on the UI, this means that this will enable/disable the given controls automatically. What is more, look at the code. It is clear, concise and pretty easy to reason about and use.

I’ll leave the actual exploration of the implementation to you, it is fairly simple, but the idea is important. The Effectus application actually have more infrastructure code than it have code to implement its features. But that is a very small application. Just to give you an idea, the # of lines of code devoted to infrastructure in Effectus is about 600 LOC! In most applications, you probably wouldn’t even notice the amount of code that the infrastructure takes, but the effects are quite clear.

time to read 2 min | 357 words

I waited for a while after my article was posted, to see if anyone caught on to some of the things that I did in the sample code that weren’t related to NHibernate usage. It seems that no one caught on to that, so I’ll try pointing them out explicitly.

The basic format of a feature in Effectus is this:

image

Each feature have a Presenter (business logic for the feature), a View Model, which is directly bound to the View and is the main way that Presenter has to communicate with the View. The main responsibility of the Model is to be bound to the UI, and allow the presenter to update it, but it is not quite enough.

The UI also need to raise events and handle user input. A common example is handling button clicks. You can’t really map them into Model behavior, at least not easily and in a way that make sense to the developers. Instead, I defined a set of conventions. As you can see in the picture, based on the name of the element, we match is with the appropriate execute method and the way to decide if the command can execute. Under the cover, we generate the appropriate ICommand instance, bound to the Presenter behavior.

image

If you look at the code, you can see that the conventions extend even to handling the selected item changed in a list box, including passing the newly selected instance back to the presenter. Those conventions are project specific, based on what the project need, and they result in a code base that is very easy to read and follow. Moreover, they result in a codebase that is relatively free from infrastructure concerns.

I am going to have a few more posts about Effectus, can you figure out what else I put in that code base?

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