Windows Phone 7 applications can either be Silverlight based or XNA Framework based. Often, it is obvious which framework to use for a particular type of application. Typically, games will either use Silverlight or XNA, depending on the desired functionality they need. All other types of applications typically will be Silverlight based although from a technical point of view it is possible to create ‘non-game’ applications using XNA as well.
Silverlight uses an event driven model for your application. Events are raised by the operating system (often as a result of some user action) and an application can either react on the event or ignore the event. Inside an application you will ignore the majority of events that will be raised, but act on a number of specific events to achieve the desired functionality of the application. A Silverlight based application is often battery friendly, especially if it only reacts on user actions. In this type of scenario, the application will be idle for most of the time. However, if repetitive behavior is needed for an application (for instance, a clock application might want to update the time every second), a timer object can be used that periodically raises an event on which your application can react. In this scenario you have to keep in mind that you are draining the battery faster, because your application is repeatedly executing code. If your application wakes up to react on a raised event, it will execute code that is defined in an event handler for a particular event. The event handler will get information from the operating system about the source that raised the event. That source, for instance, can be a User Interface element, an expiring timer or a hardware interrupt. Because the event sender is known, the application can assure that it executes the code, belonging to a specific event.
The XNA Framework uses a loop driven model for your application. Periodically your application will wake up and it is up to the application to determine if something needs to be done. In order to achieve this, every XNA Framework application maintains a ‘game loop’ that executes 60 times per second (when using default values). It is the application’s responsibility to first Update the state of the application, for instance by getting user input, updating variables etc. Once a new application state is determined, the application is responsible to Draw to the Windows Phone 7 screen accordingly. Both Update and Draw will execute periodically, with Update taking priority over Draw. Note that these types of applications will drain the battery whenever the user is playing a game, because the application will wake up frequently, even if there is no work to do in either Update or Draw.
From a developer’s perspective, there are a number of reasons to either develop a Silverlight based Windows Phone 7 application or a XNA Framework based Windows Phone 7 application. Silverlight contains a nice collection of ready to use controls like buttons, text boxes etc. You can modify or extend these controls inside your own applications and you can create new controls. The XNA Framework contains functionality to make it relative easy for you to create 2D sprites that support rotation, scaling, stretching and filtering. There is also extensive support to create 3D graphics, including functionality for lighting and shading to create a great 3D experience. When making use of the XNA Framework however, you are responsible to create all of your artwork that is visible inside your application.
In certain situations, a distinct choice between Silverlight and XNA is not that easy to make. Also, in some scenario’s you might want to combine Silverlight with XNA Framework functionality. Using XNA Framework specific functionality inside a Silverlight application will be the topic of another blog entry. For now, if you want to know more about developing applications for Windows Phone 7 and if you want to read more about the initial choice between Silverlight and the XNA Framework, make sure to read this high level overview in the Windows Phone 7 documentation.