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Windows & .NET Watch: Fun with WPF applications


By Larry O'Brien


 Larry O'Brien
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January 1, 2010 —  (Page 1 of 2)
Silverlight 4 is now available in beta form. I decided to take it for a test drive by implementing some calendars for amateur astronomy. Specifically, I wanted to use Silverlight to calculate and graph the positions of the four Galilean moons of Jupiter, including interesting events like occultations, shadow transits and the position of the Great Red Spot.

The task highlights the advanced display model of WPF/XAML, with a combination of standard controls for setting the date and time parameters, dynamically repositioned and scaled lines for scaling the calendar, and totally dynamic paths for the satellite’s positions. Visual Studio 2010 (now in Beta 2) includes a complete visual design surface for Silverlight, allowing me to rough out a design declaratively (including visual prototypes for the curves I wanted to draw).

The Model-View-ViewModel pattern has come to be the de facto standard design for WPF applications. In this pattern, the “Model” retains its standard meaning as those elements relating to the domain. The “View” is also quite familiar as the user interface, but is more passive than the view in the traditional “Model-View-Controller” triad; in MVVM, the View typically relies heavily on the data-binding facilities of the .NET Framework to attach to the “ViewModel,” which houses the UI’s interaction logic. In this way, the View is almost like a concrete implementation of the ViewModel’s abstraction (“almost” because the ViewModel is not a pure abstraction, and the two are not related by class inheritance but by data-binding).

Because the View is so loosely coupled to the ViewModel (at least visibly; the infrastructure for data-binding is complex but hardly ever requires attention), view “stuff” is easy to test. This was especially useful in my application, where one view is animated, and another boils down to a bunch of sine waves superimposed on a ruler-like calendar that needs to be accurate when fully zoomed in.

Just to make things interesting, my Model initially disagreed with other calculators. I cannot deny jumping to the conclusion that I had a time zone problem (in any application that references time in any way, there will at some point be bugs relating to time zones), but tests saved me from countless hours trying to find the problem by judging, “OK, it looks like Io and Ganymede cross on the 17th around 10 UTC. Is that right?” (It turned out that the orbital constants I used were based upon a non-standard epoch. Live by the easy Internet search, die by the easy Internet search.)


Related Search Term(s): Silverlight, Visual Studio

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