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Creating an Extension Point

WRITE THIS SECTIONFor the final part of the tutorial, we will now use the extension point mechanism of Eclipse to add some behavior to our Turing Machines. An extension point is basically a well-defined point where other plug-ins can register to add functionality. The extension point is basically defined by an XML Schema file that defines an interface; other plug-ins may access this interface using XML code in their plugin.xml file, so-called extensions. Our extension point will provide an interface for classes that define behavior of a Turing Machine, and we will call them head controllers (programs that control the tape head).

Defining a Command Class

We will start by defining a class representing a command that will be passed to a selected head controller.

1. Add a class HeadCommand to the package de.cau.cs.rtprak.login.simple.controller.
2. Add a nested public static enumeration Action with values WRITE, ERASE, and NULL.
3. Add a nested public static enumeration Direction with values LEFT, RIGHT, and NONE.

4. Add the following private fields:

   Code Block
   language java
   private Action action; private Direction direction; private char newChar;

5. Add a constructor to initialize the fields.
6. Add getter methods to access the fields.

Defining the Controller Interface

We will now define an interface that all head controllers will have to implement:

1. Add an interface IHeadController in the package de.cau.cs.rtprak.login.simple.controller.

2. Add the following methods to the interface:

   Code Block
   language java
   /** * Calculate the next command depending on the currently seen character. * @param character the currently seen character * @return the next command specifying which character to write and * which direction to move the head */ HeadCommand nextCommand(char character); /** * Reset the internal state of the head controller. */ void reset();

Defining the Extension Point

We will now define the extension point that head controllers will be registered at.

1. Open the plugin.xml file in the Plugin Manifest Editor and switch to the Extension Points tab.
2. Click the Add button and enter de.cau.cs.rtprak.login.simple.headControllers as the extension point's ID, and Head Controllers as its name. Shorten the schema file's file name to schema/headControllers.exsd. Make sure that Edit extension point schema when done is checked and click Finish.
3. Eclipse will now have opened the new schema file in the Extension Point Schema Editor, a graphical editor similar to the Plugin Manifest Editor that provides a way to define things that might be easier than directly editing the text files.
4. In the new editor, open the Definition tab.
5. Add a new element named controller.
6. Add three new attributes to the controller element:
   
   • First attribute: name id, use required, type string, translatable false.
   • Second attribute: name name, use required, type string, translatable true.
   • Third attribute: name class, use required, type java, implements de.cau.cs.rtprak.login.simple.controller.IHeadController. This is the attribute that will tell us which Java class actually implements the controller that is to be registered at our extension point. To make sure that we know how to speak to the class, we require it to implement the interface we defined for head controllers.
7. Add a sequence to the extension element. Right-click the sequence and click New -> controller. Set the Min Occurrences of the sequence to 0, and set Max Occurrences to be Unbounded.
8. Save the editor and switch back to the Plugin Manifest Editor.
9. On the Runtime tab, add de.cau.cs.rtprak.login.simple.controller to the list of packages exported by the plug-in. This is necessary because plug-ins that want to provide extensions for the extension point must provide a class that implements IHeadController. For this to work, those plug-ins must have access to that interface; thus, we have to export the package containing it.

Accessing the Extension Point

We will now add a class that will be in charge of loading all extensions registered at our new extension point.

1. Add a class HeadControllers to the package de.cau.cs.rtprak.login.simple.controller. Add the following code:

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   language java
   /** * Class that gathers extension data from the 'headControllers' extension point * and publishes this data using the singleton pattern. * @author msp */ public class HeadControllers { /** Identifier of the extension point */ public final static String EXTENSION_POINT_ID = "de.cau.cs.rtprak.groupx.simple.headControllers"; /** The singleton instance of the {@code HeadControllers} class */ public final static HeadControllers INSTANCE = new HeadControllers(); /** list of head controller ids with associated names. */ private List<String[]> controllerNames = new LinkedList<String[]>(); /** map of controller ids to their runtime instances. */ private Map<String, IHeadController> controllerMap = new HashMap<String, IHeadController>(); /** * Creates an instance of this class and gathers extension data. */ HeadControllers() { IConfigurationElement[] elements = Platform.getExtensionRegistry() .getConfigurationElementsFor(EXTENSION_POINT_ID); for (IConfigurationElement element : elements) { if ("controller".equals(element.getName())) { String id = element.getAttribute("id"); String name = element.getAttribute("name"); if (id != null && name != null) { try { IHeadController controller = (IHeadController)element .createExecutableExtension("class"); controllerNames.add(new String[] {id, name}); controllerMap.put(id, controller); } catch (CoreException exception) { StatusManager.getManager().handle(exception, Activator.PLUGIN_ID); } } } } } /** * Returns a list of controller ids and names. The arrays in the list are * all of size 2: the first element is an id, and the second element is the * associated name. The controller name is a user-friendly string to be * displayed in the UI. * @return a list of controller ids and names */ public List<String[]> getControllerNames() { return controllerNames; } /** * Returns the head controller instance for the given id. * @param id identifier of a head controller * @return the associated controller */ public IHeadController getController(final String id) { return controllerMap.get(id); } }

Adding Support for Head Controllers to the View

We will now have to add support for head controllers to our view.

1. Open the TapeViewPart class and add the private fields checkedControllerAction of type IAction and currentController of type IHeadController.

2. Add a list of registered head controllers to the view's menu (which can be opened using the small white triangle) in the createPartControl() method:

   Code Block
   language java
   IMenuManager menuManager = getViewSite().getActionBars().getMenuManager(); for (String[] controllerName : HeadControllers.INSTANCE.getControllerNames()) { final String id = controllerName[0]; String name = controllerName[1]; Action action = new Action(name, IAction.AS_RADIO_BUTTON) { public void run() { if (checkedControllerAction != null) { checkedControllerAction.setChecked(false); } this.setChecked(true); checkedControllerAction = this; currentController = HeadControllers.INSTANCE.getController(id); } }; if (checkedControllerAction == null) { action.run(); } menuManager.add(action); }

3. Implement the following method in the TuringTape class:

   Code Block
   language java
   public void execute(final IHeadController controller)

   The method shall have the following properties:

   
   
   • Determine the character at the current head position using getCharacter(getHeadPosition()).
   • Call controller.nextCommand() with the current character as parameter.
   • Depending on the action in the returned head command, either write the returned new character to the current position in text (WRITE), or write the blank symbol (ERASE), or do nothing. If the current position exceeds the end of the text, append enough blank characters up to the current position, then append the new character.
   • Depending on the direction in the returned head command, either move the head to the left (but no further than position 0), or to the right, or do nothing.
4. Copy the files step.gif and reset.gif to the icons folder.
5. Add an action to the toolbar of the Tape view with text Step and icon step.png which does the following:
   
   • Check whether the current head controller is not null, than call tape.execute(currentController).
   • Refresh the table viewer with its refresh() method.

   • Note: actions don't need images, but only image descriptors. Thus, to set the action's icon to step.png, you can use something like the following:

     Code Block
     language java
     Activator.imageDescriptorFromPlugin(Activator.PLUGIN_ID, "path_to_icon");

6. Add another action with text Reset and icon reset.png which does the following:
   
   • Check whether the current head controller is not null, then call the reset() method on currentController.
   • Set the current head position to 1.
   • Refresh the table viewer with its refresh() method.