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Creating a custom dependency property

From: Silverlight 5 Essential Training

Video: Creating a custom dependency property

Most properties on Silverlight Elements are Dependency Properties. When you create your own elements you should follow the same pattern and create Custom Dependency Properties. Let's review. The Dependency Properties System exists to provide services to your Silverlight Elements. For instance the services like Animation, Data Binding, Control Templates or Styles. Your elements can also partake in these services. If you were to implement your control properties only as .net properties, then sooner or later you will discover that you have created a UI element that cannot be animated or styled and that would be a tragedy.

Creating a custom dependency property

Most properties on Silverlight Elements are Dependency Properties. When you create your own elements you should follow the same pattern and create Custom Dependency Properties. Let's review. The Dependency Properties System exists to provide services to your Silverlight Elements. For instance the services like Animation, Data Binding, Control Templates or Styles. Your elements can also partake in these services. If you were to implement your control properties only as .net properties, then sooner or later you will discover that you have created a UI element that cannot be animated or styled and that would be a tragedy.

To participate and create a dependency property, you have to register with the Dependency Property System. That means you have to write what I call ugly registration code. You will see what I mean about this ugliness in a few seconds. Microsoft has a naming convention which you can follow if you like. Microsoft always applies a property suffix to all dependency properties. So the Border Stroke property becomes the BorderStrokeProperty. Now when I say it they sound exactly the same. But if you look at this slide, you can see that they are different. On the bottom of this slide is a short example of how you would register a property.

I call a static member on the dependency class, called register, and I pass in a number of properties. The point string represents the name of the property, the second argument, type of double, represents the data type of the property, the third parameter represents the class that I'm assigning the property to and the fourth property currently set to null represents additional information that I want to pass into the registration system. Let me show you what I mean inside Visual Studio. I am inside a project called CreatingCustomDependencyProperty. I have created my own custom user control down here called the StarShape which looks like this.

I decided that I will like to expose a property in this called points, the number of points on the star. So I went to my code behind and I registered a point's property. Now in my MainPage when I create a star shape I can come down here and set the Points property equal to 5 and the Points property equal to 6. Something is wrong however because one of these stars does not have six points. I'll show you how to fix it. Go here and look at my Dependency registration code again.

This first line represents a field, a member of my type called PointsProperty. I am using Microsoft's naming convention here, it's of data type DependencyProperty. I've marked it as a static member of my type, so it's accessible throughout Silverlight and I've marked it as a readonly. What that means is that I can only instantiate and set this type in the static constructor of my class, which is right here. Inside my static constructor, I am using that same registration code I showed you on the slide a few minutes ago.

To make my code .net friendly, I'm also going to expose a regular .net property called Points. Notice that all I am doing in my getter is calling the dependency property GetValue and then my setter, calling the SetValue. Don't write any other code in here because it might get bypassed by the Silverlight dependency system. Why isn't my points changing? Because I need to register a callback with the dependency system that says, when the point value changes, run some code. I have written a section of code down here which has this method OnPointsChanged and inside this OnPointsChanged virtual method I am looking for the new value.

If the NewValue is or equal to 6, then I show a six point star and I hide my five point star, and if it's any other number other than six, then I show my five point star. I am going to comment out this line of code, and I am going to uncomment these two lines of code here, Ctrl+K, Ctrl+U. Notice what I am doing here. I am setting up my Registration. I am also passing in a new value here in this last argument.

I am doing two things here. I am creating something called a PropertyMetadata and I am assigning the default value, 5 points, and I am also registering my call back. I am saying the PointsChanges where you are going to call back whenever the point number changes. Now when I run the application or I compile it and return back to my main page, I have a 6 point star and now when I double-click on this button and uncomment this line of code, Star1.Points = 6 at runtime it will change my five-point star to a six-point star.

As you have seen, custom dependency properties are tremendously important, especially if you create custom user controls.

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This video is part of

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Silverlight 5 Essential Training

106 video lessons · 5392 viewers

Walt Ritscher
Author

 
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  1. 1m 41s
    1. Welcome
      53s
    2. Using the exercise files
      48s
  2. 35m 0s
    1. Overview of Silverlight
      3m 29s
    2. Setting up a developer computer
      2m 46s
    3. Installing the Silverlight Toolkit
      2m 21s
    4. Exploring the toolkit samples
      2m 35s
    5. Using Visual Studio 2010 to create a Silverlight project
      5m 10s
    6. Using Expression Blend to create a Silverlight project
      3m 13s
    7. Getting to know the Visual Studio interface
      8m 15s
    8. Working in the Expression Blend interface
      7m 11s
  3. 29m 46s
    1. Understanding the Visual Studio project structure
      3m 38s
    2. Creating a Silverlight page
      3m 17s
    3. Compiling your first application
      5m 0s
    4. Using other assemblies in an application
      5m 45s
    5. Deploying a Silverlight application
      3m 43s
    6. Understanding the startup process for an application
      3m 13s
    7. Understanding how users get Silverlight on their computers
      5m 10s
  4. 16m 59s
    1. Exploring the relationship between XAML and .NET
      3m 57s
    2. Using C#
      7m 16s
    3. Exploring the code-behind file
      5m 46s
  5. 43m 22s
    1. Working with XAML elements and Property attributes
      4m 56s
    2. Investigating XAML namespaces
      7m 31s
    3. Converting XAML properties with TypeConverters
      5m 1s
    4. Working with Property elements
      4m 24s
    5. Assigning runtime data with XAML markup extensions
      4m 21s
    6. Digging into the dependency property system
      6m 12s
    7. Creating a custom dependency property
      4m 42s
    8. Understanding attached properties
      6m 15s
  6. 9m 35s
    1. Debugging your code
      5m 17s
    2. Special Silverlight debugging techniques
      4m 18s
  7. 36m 0s
    1. Understanding layout
      4m 55s
    2. Using DockPanel and WrapPanel
      4m 2s
    3. Exploring sophisticated layouts with the Grid
      6m 40s
    4. Absolute positioning with the Canvas panel
      5m 20s
    5. Scrolling content with the ScrollViewer
      3m 28s
    6. Adjusting content alignment, margins, and sizing
      5m 6s
    7. Using the TabControl
      2m 17s
    8. Manipulating elements with transforms
      4m 12s
  8. 20m 32s
    1. Painting the user interface (UI) with SolidColorBrush
      6m 7s
    2. Getting colorful with gradient brushes
      3m 15s
    3. Decorating elements with ImageBrush
      4m 56s
    4. Creating effects with VideoBrush
      6m 14s
  9. 25m 42s
    1. Understanding routed events
      3m 12s
    2. Wiring up event handlers in Silverlight
      6m 4s
    3. Understanding event bubbling
      4m 39s
    4. Exploring mouse events
      7m 43s
    5. Exploring keyboard events
      4m 4s
  10. 32m 31s
    1. Displaying text on the screen
      3m 24s
    2. Gathering text input from the user
      5m 30s
    3. Showing complex text with RichTextBox
      6m 7s
    4. Understanding text overflow and text linking
      3m 14s
    5. Searching content with the AutoComplete type-ahead control
      6m 45s
    6. Using and embedding Silverlight fonts
      7m 31s
  11. 26m 47s
    1. Understanding content controls
      5m 7s
    2. Understanding button controls
      6m 16s
    3. Using the BusyIndicator
      4m 40s
    4. Showing tooltips
      4m 34s
    5. Changing content size with ViewBox
      1m 39s
    6. Exploring more controls
      4m 31s
  12. 55m 36s
    1. Connecting elements with binding
      9m 54s
    2. Using business data in a binding
      9m 21s
    3. Listing data with ItemsControls
      6m 39s
    4. Digging into the DataGrid
      8m 28s
    5. Using the DataForm
      3m 49s
    6. Storing data on the client
      7m 10s
    7. Debugging XAML bindings
      5m 41s
    8. Charting data
      4m 34s
  13. 24m 36s
    1. Creating an Out-of-Browser (OOB) application
      6m 16s
    2. Controlling aspects of an OOB application
      2m 49s
    3. Alerting the user with notification windows
      7m 13s
    4. Hosting HTML content in Silverlight applications
      8m 18s
  14. 30m 2s
    1. Exploring Silverlight trust levels
      3m 51s
    2. Creating an elevated trust application
      5m 57s
    3. Reaping the benefits of XAP signing
      6m 5s
    4. Making cross-domain calls
      5m 1s
    5. Getting started with COM Interop using the Speech API
      4m 21s
    6. Using COM Interop to interact with Microsoft Office
      4m 47s
  15. 16m 33s
    1. Showing child windows
      3m 48s
    2. Using a child window as a dialog window
      5m 7s
    3. Creating native windows
      2m 53s
    4. Maximizing a window with full screen mode
      4m 45s
  16. 13m 20s
    1. Dynamically loading UI content with user controls
      4m 46s
    2. Using the Navigation Framework
      8m 34s
  17. 9m 28s
    1. Using resource files
      3m 53s
    2. Using images in an application
      5m 35s
  18. 16m 32s
    1. Playing audio
      4m 27s
    2. Playing sound effects
      5m 4s
    3. Viewing video content with MediaElement
      4m 58s
    4. Changing video playback speed with TrickPlay
      2m 3s
  19. 14m 40s
    1. Capturing video with a web camera
      4m 16s
    2. Working with a microphone
      3m 47s
    3. Creating print output
      6m 37s
  20. 22m 35s
    1. Understanding Silverlight animations
      4m 35s
    2. Creating animations
      7m 52s
    3. Controlling animations
      4m 18s
    4. Simulating physics with animation easings
      5m 50s
  21. 27m 47s
    1. Placing XAML resources within a FrameworkElement
      6m 47s
    2. Centralizing settings in styles
      4m 8s
    3. Creating an alternate control UI with ControlTemplates
      6m 29s
    4. Dressing up your data with DataTemplates
      4m 33s
    5. Explaining the VisualStateManager: a simple way to manage control states
      5m 50s
  22. 10m 55s
    1. Handling global errors in a Silverlight application
      5m 46s
    2. Showing an application loading screen
      5m 9s
  23. 10m 48s
    1. Integrating with XNA
      7m 40s
    2. Exploring data analysis with PivotViewer
      3m 8s
  24. 1m 6s
    1. Goodbye
      1m 6s

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