I am excited about Silverlight 1.1 – very excited! Whilst all the graphics and animations will be a wonder for creating great user experiences, I eagerly awaiting the chance to run managed code in the browser.
In my mind, and partially in code, I have already begun to formulate a new way to design applications. Of course, by new I really mean stealing a bunch of existing good ideas and gluing them together!
The first key idea is putting the users first – always. These auto-magic, drag-n-drop data binding tools are great for quick and dirty CRUD applications, but I am yet to see one produce an application that has great usability. We need to start by putting ourselves in the user’s position and thinking about why they are using our software in the first place. It is all about finding the simplest, cleanest, more beautiful way to enable the user to get their job done. Slapping another data grid on a page probably is the worst approach!
I am a new convert to the Model-View-Presenter way of writing client applications. Although, I think, the order of coding should be Presenter-View-Model (it does not have quite the same ring to it however). In addition, by View I mean the view implementation, not the UI design. I always sketch the view design on paper/white board first. I suppose using a tool like Microsoft Expression Blend would be OK as long as you are not precious about the XAML it creates. I am never afraid to throw out code and start afresh, so why should UI mark-up be any different? Use whatever tools you find easiest to knock up UI prototypes and shove them under the user’s nose. The key here is to know roughly what buttons, textboxes, etc, the UI will contain (we can leave all the fancy colouring in to some guy in a turtle neck). Once this is the case, we can begin the most important part of the coding: The User Interaction Logic (UIL).
The UIL is the Presenter. The UIL says stuff like when the user clicks this button, read this data from the UI, tell the underlying model to do something and then tell the view to update. Whilst for some applications this level of control may seem like overkill, I argue that having to slow down sometimes is OK. This approach means you have to think about what the user will be doing in your application.
The best way to develop this code is by being test-driven. Create unit tests that describe the behaviour of the Presenter in terms of how it interacts with View and Model object-oriented interfaces. Each user story then becomes testable.
Key tools I plan to use here are:
Rhino Mocks – to create mock objects of Views and Models
Boo + my Rhino Mocks DSL – to make the unit tests easy to read and write
SharpDevelop – because Boo is not readily usable in Visual Studio yet
Visual Studio 2008 – to write the implementation of the Presenter in C#
As awesome as the language Boo is, I just cannot stand SharpDevelop for more than simple projects like the unit tests. Refactoring, intellisense and code-completion are just way to well done in Visual Studio to ignore it. I have enough memory and screen space to run VS and SD side-by-side.
As the Presenter is written, the interfaces for the View and Model are being fleshed out. The refactoring tool adding the methods automatically to the interfaces usually does this.
The choice now is whether to develop the Model or the View next. The View is a very dumb object that simply maps data to and from the UI and raises events when the user does stuff (clicks, key presses, etc). Since Silverlight does not have data binding yet, this may lead to somewhat tedious code. Bear in mind also that the Presenter is meant to be doing tasks such as converting strings into numbers and back. However, I reckon a tool can generate most of the standard mapping code.
The declarative nature of XAML means it should be easy to add attributes to button elements that describe the event they should raise when clicked. A tool could scan this data and write the code for me.
If data binding was ever added to Silverlight, I would consider creating pure “data” objects that the Presenter tells the View to bind too. No logic would be contained within the data objects.
The Model is where things become more interesting from a software architecture point of view. Silverlight runs in the client’s web browser. So the Presenter is running client-side, as is the Model on which is depends. The Model is maintaining state about the client application. For example, which check boxes are checked, which customer is selected? However, the Model also will need to communicate with a database running back on the server to read and write data.
To solve this problem, I plan to use my tier splitting C# macro. I will write the Model class as one coherent class. By coherent I mean all the data is there and all the logic regarding the data is there. I do not believe manually splitting the class and messing about with proxies and web services makes any business sense. I am not developing some kind of SOA application that is to be called by numerous different clients. I am trying to get my Model to save its valuable data on the database that is only accessible from the web server from whence it came. This is a reasonable tight coupling between the client and server versions of the Model. The coupling is so tight; I argue that I do not even want to see it!
In the case where the data does come from a remote web service, we still have to return to the application’s home server since Silverlight disallows cross-domain calls. It makes sense to put any database calls/remote web services calls into the Model. The Model knows what data it needs, so why bother moving that knowledge outside of the class only to marshal all the data back in again.
With LINQ being available we do not even have to pollute the Model with database specific SQL.
The tier splitting macro will handle generating all the WCF services and proxies. I just have to decorate the Model’s remote methods with [RunAtServer].
Thorough unit testing of the Model is required. It should be possible to create a mock D-LINQ implementation. I can then make assertions about how the Model interacts with the data layer.
In summary, I want to have a Silverlight UI that is but a marionette to a Presenter running client-side. This presenter has complete unit testing to describe the different user interactions. The Presenter uses a Model object to store all data. This Model exists on both the client and server side. However, the split implementation is effectively invisible to the developer.
The application of adequate tooling (both 3rd party and custom made) will make this an excellent way to approach writing applications. As Silverlight 1.1 becomes more complete, I will continue to expand this topic.
The result should be a happier developer and, more importantly, a happier user.