Silverlight Research Summary
So, I made it through Step 2: Read of my Learning Process 2.0 for Silverlight. Here is the product of that process.
Prologue: There is a really cool demo of Silverlight capabilities here.
What is Silverlight?
Remember “Write Once, Run Anywhere?” This lofty goal has been promised before <cough Java cough>. It’s still a desirable thing, to be sure: just ask any developer that still has to code webpages to look decent on IE6.
A new generation of technologies is beginning to deliver on the multi-platform promise of old. Silverlight is Microsoft’s entry into that fray. The Silverlight runtime is a subset of the .NET Base Class Library, runs on PC and Mac, and runs in all the major browsers including IE, Firefox, Safari, Opera, Chrome, and more.
Silverlight was born as “WPF/E”, which stands for Windows Presentation Foundation/Everywhere. WPF and Silverlight are both Microsoft’s latest User Interface technology offerings. Both are based on eXtensible Application Markup Language, aka XAML. The difference between WPF and Silverlight is in delivery: WPF is a desktop technology. Silverlight is a Rich Internet Application framework for delivery of rich user interfaces via the web.
No discussion of Silverlight is complete without mentioning Flash. Yes, Silverlight competes with Flash. Directly.
- Both Flash and Silverlight run in a sandboxed environment on the client.
- Both offer vector graphics and media (audio/video) capability.
- As of Silverlight 3.0, both support an out-of-browser running mode (Adobe AIR allows Flash/Flex programs to run locally outside a browser.)
The primary difference between them is that Flash uses Actionscript, and Silverlight is based on .NET, which means Silverlight applications may be developed in any .NET-compliant language. These include VB.NET, C#, and with the Dynamic Languages support in Silverlight, IronPython and IronRuby.
Browser Integration
A Silverlight application may be embedded into a webpage using the Silverlight ActiveX control and the <object> notation, or by using javascript. A javascript bridge (HTML Bridge) between Silverlight applications and the hosting page allows javascript to call into the Silverlight app, and managed Silverlight code to call out to javascript functions declared in the hosting page.
XAML
XAML stands for eXtensible Application Markup Language. It is an XML-based language for describing visual elements of a User Interface.
Think back to the days of VB6: the UI was all tangled up in the code. Then came .NET, and for some time the UI was all tangled up in auto-generated “designer” code. Now comes WPF, Silverlight, and XAML. Using XAML, the UI definition is completely separated from the programming logic. XAML pages declaratively describe an object graph. Parsing a Silverlight XAML page results in the instantiation of that graph and the rendering of the user interface it describes.
Dynamic Language Support
Dynamic Languages in Silverlight allow IronPython and IronRuby programs to manipulate and interact with the user interface in the same way as C# or VB.NET. This opens up Silverlight to Python and Ruby developers. The DLR Console Sample demonstrates another capability: interacting with a running Silverlight application in real time using code.
Layout System
The Silverlight layout system determines how visible elements of an application are positioned relative to the application host and each other. Several layout controls are included with Silverlight, including Canvas, Grid, and StackPanel, to name a few.
Controls are laid out by the engine using inherited Measure and Arrange methods that cooperate to determine how much visible area a control wants, and how much it gets as a result of the overall layout of all visible elements.
Controls
Silverlight 2.0 introduced a library of ready-made controls, such as TextBlock, TextBox, ComboBox, etc. MSDN has a handy directory of controls.
Silverlight controls are “look-less” in that the code that defines their behavior contains absolutely nothing about how they appear visually. The visual appearance of controls, including custom controls, is defined entirely in XAML. The .NET class that implements the control is referred to as a “control contract.”
Just as with .NET Winforms and ASP.NET controls, WPF/Silverlight controls can be customized or created from scratch. Customization of controls in Silverlight is quite different, however. Winforms and ASP.NET controls can be subclassed and then customized. Silverlight control customization involves replacing the ControlTemplate of a control. This allows a developer to completely replace the visual behavior of an existing control.
Common content may be shared across controls using a DataTemplate. This supports adding a common icon to a custom “submit” button, for example.
Networking
Communicating with the cloud in Silverlight presents several security challenges. Silverlight applications cannot freely communicate with just any service over any port; there are restrictions to prevent security threats such as malware, viruses, and spyware. Depending on configuration, communication is either handled for the Silverlight application by the browser, or the Silverlight client itself handles HTTP/HTTPS communications. The latter configuration is useful for RESTful communications or when the developer wants to handle cookies manually.
The standard class for network communcation is WebClient. WebClient has asynchronous methods for writing and reading to a network stream. The HttpWebRequest and HttpWebResponse classes contain methods for more granular communications, including the ability to manipulate HTTP headers. Silverlight also supports Sockets for low-level TCP communications.
The security model Silverlight uses allows minimally-restricted communication between the hosting domain and a Silverlight application. If a Silverlight app wants resources (i.e. files, read/write to a service) from another domain, however, a cross-domain policy file must be present on that domain. Flash/Flex/AIR applications use a similar security model, and Silverlight can read Flash cross-domain policy files. The policy file is an XML-formatted file that indicates what services and resources are accessible, and to whom.
Data
This was a source of confusion for me when I first started with Silveright. It’s important to remember that Silverlight apps run in the browser. This makes them more like Winforms apps than ASP.NET apps. Silverlight apps get their data over the wire just like any other desktop application. What does a Winforms app do when it needs data from the cloud? It uses a webservice.
Silverlight apps can get their data over the wire via RESTful or SOAP-based services like WCF. Microsoft provides some help for data access in the form of ADO.NET Data Services (formerly Astoria) and .NET RIA Services.
Here I am going to punt and guide you to the best authority I’ve seen on the difference between these options: Shawn Wildermuth.
Media and Animation
Silverlight’s Animation system is centered on the Storyboard control and the Animation controls. “From-To” animations including DoubleAnimation, PointAnimation, and ColorAnimation are used to animate double, point, and color-type properties of a control. These animations have keyframe-based companions (DoubleAnimationUsingKeyFrames, PointAnimationUsingKeyframes, you get the idea) that use keyframing to perform the animation. Keyframing is a technique by which frames are interpolated between key points that are set in an animation.
Bonus: DependencyObject and DependencyProperty
DependencyObjects and DependencyProperties were not in the mind map, but they are important enough that they probably should have been, so I’ll go into them briefly here.
Silverlight controls need lots of dependencies on each other for things like animation and layout. This requires more than the common model of using private fields to back public properties. DependencyProperties are like super-properties. When the values of a DependencyProperty are getted (is that a word?) or setted (again?) the internal implementation of DependencyObject (DependencyProperties’ base class) allows other things to happen, like notifying an ObservableCollection that a member has been updated (this is databinding), or animating a property of a control from outside the scope of that control.
