The title of this post could also have been "Move over Hyperlink, here comes Actionlink" or "Creating interactive text in Silverlight." But alas, there can be only one.

Hyperlinks are very useful. However, they are also limited because their action is fixed: browse to a URL. This may have been adequate at the start of the Internet, but nowadays, in web applications, the one thing we do not want to happen is a complete change of context. In applications we typically like a hyperlink selection to initiate an action that updates a part of the screen. For instance, if my application has a map displayed with some text next to it, the map would react to a selection of a hyperlink in the text, e.g. by zooming in on a location and displaying additional locational information in a popup. In this way, the text becomes interactive text.

It is quite common that one company creates and maintains websites for many client companies. To keep maintenance cost low, it is important that the content of these websites can be updated by the client companies themselves, without the need to involve a software engineer. To accommodate this scenario, we want the author of the interactive text to configure all hyperlinks (without writing any code).

In a Silverlight RichTextArea, the default action of a Hyperlink is the same as a traditional hyperlink, but it can be changed: if the Command property has a value then upon a click event this command is called with the value of the CommandParameter as parameter. How can we let the author of the text specify a command for each hyperlink in the text, and how can we let an application react properly to a hyperlink selection event?

We are talking about any command here. Obviously, the application would recognize only a specific set of commands, with well defined parameters, but the approach we take here is generic in the sense that it pertains to the RichTextArea and any command.

So what do we require? We wish that:

1. As a text author, I can configure the action of a hyperlink in a (rich) text without writing code;
2. As a text author, I can persist the action of a hyperlink with the text;
3. As a reader of persisted text, I can click a hyperlink and the configured action will happen;
4. As an application developer, I can configure a control to use my application specific commands.

In an excellent introduction to the RichTextArea, John Papa shows (among other things) how to persist a text created using this control. To meet our requirements, we can create a subclass of RichTextArea that uses John's code and allows plugging in two command specific components: one to prompt for a command definition, and one to execute the command. Since both of these plugins are application specific, our RichTextArea subclass should not assume anything about them except their interface.

  public interface IDefineCommand  
  {
    void Prompt(string content,                     // the link content
                Action<string, object> callback);   // the method called to convey the link definition
  }
  
  public interface IPerformCommand : ICommand {}

The IDefineCommand plugin receives the content of the link (the text visible to the reader) and displays some kind of control that allows the author to define the link. When that's done, this (possibly changed) content string is conveyed back to the RichTextArea, together with an object that defines the command to execute when the link is clicked by the reader of the published text.
The IPerformCommand plugin simply implements System.Windows.Input.ICommand.

Let's use MEF to load the proper plugins. In the example solution there is a project that contains rudimentary implementations of these. The IDefineCommand plugin simply prompts for a command string (cf. a command line or query string), and the IPerformCommand plugin displays a MessageBox showing this command string. An actual application using this extended RichTextArea would have its own set of commands, each having their own parameters, and hence would provide more user friendly application specific plugins. Nonetheless, in any case a command can be persisted as a string and hence the two interfaces defined above suffice.

For a Visual Studio 2010 solution, see my article on The Code Project.